JTC1/SC22/WG14
N843
Programming languages -- C
1. Scope
[#1] This International Standard specifies the form and
establishes the interpretation of programs written in the C
programming language.1) It specifies
-- the representation of C programs;
-- the syntax and constraints of the C language;
-- the semantic rules for interpreting C programs;
-- the representation of input data to be processed by C
programs;
-- the representation of output data produced by C
programs;
-- the restrictions and limits imposed by a conforming
implementation of C.
[#2] This International Standard does not specify
-- the mechanism by which C programs are transformed for
use by a data-processing system;
-- the mechanism by which C programs are invoked for use
by a data-processing system;
-- the mechanism by which input data are transformed for
use by a C program;
-- the mechanism by which output data are transformed
after being produced by a C program;
-- the size or complexity of a program and its data that
will exceed the capacity of any specific data-
processing system or the capacity of a particular
processor;
-- all minimal requirements of a data-processing system
that is capable of supporting a conforming
implementation.
____________________
1) This International Standard is designed to promote the
portability of C programs among a variety of data-
processing systems. It is intended for use by
implementors and programmers.
1 General 1
2 Committee Draft -- August 3, 1998 WG14/N843
2. Normative references
[#1] The following normative documents contain provisions
which, through reference in this text, constitute provisions
of this International Standard. For dated references,
subsequent amendments to, or revisions of, any of these
publications do not apply. However, parties to agreements
based on this International Standard are encouraged to
investigate the possibility of applying the most recent
editions of the normative documents indicated below. For
undated references, the latest edition of the normative
document referred to applies. Members of ISO and IEC
maintain registers of currently valid International
Standards.
[#2] ISO/IEC 646:1991, Information technology -- ISO 7-bit |
coded character set for information interchange.
[#3] ISO/IEC 2382-1:1993, Information technology --
Vocabulary -- Part 1: Fundamental terms.
[#4] ISO 4217:1995, Codes for the representation of
currencies and funds.
[#5] ISO 8601:1988, Data elements and interchange formats
-- Information interchange -- Representation of dates and
times.
[#6] ISO/IEC 10646:1993, Information technology -- |
Universal Multiple-Octet Coded Character Set (UCS). |
[#7] IEC 60559:1989, Binary floating-point arithmetic for |
microprocessor systems, second edition (previously |
designated IEC 559:1989).
3. Terms and definitions
[#1] For the purposes of this International Standard, the
following definitions apply. Other terms are defined where
they appear in italic type or on the left side of a syntax
rule. Terms explicitly defined in this International
Standard are not to be presumed to refer implicitly to
similar terms defined elsewhere. Terms not defined in this
International Standard are to be interpreted according to
ISO/IEC 2382-1.
3.1
[#1] alignment
requirement that objects of a particular type be located on
storage boundaries with addresses that are particular
multiples of a byte address
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WG14/N843 Committee Draft -- August 3, 1998 3
3.2
[#1] argument
actual argument
actual parameter (deprecated)
expression in the comma-separated list bounded by the
parentheses in a function call expression, or a sequence of
preprocessing tokens in the comma-separated list bounded by
the parentheses in a function-like macro invocation
3.3
[#1] bit
unit of data storage in the execution environment large
enough to hold an object that may have one of two values
[#2] NOTE It need not be possible to express the address of
each individual bit of an object.
3.4
[#1] byte
addressable unit of data storage large enough to hold any
member of the basic character set of the execution
environment
[#2] NOTE 1 It is possible to express the address of each
individual byte of an object uniquely.
[#3] NOTE 2 A byte is composed of a contiguous sequence of
bits, the number of which is implementation-defined. The
least significant bit is called the low-order bit; the most
significant bit is called the high-order bit.
3.5
[#1] character
bit representation that fits in a byte *
3.6
[#1] constraints
restrictions, both syntactic and semantic, by which the
exposition of language elements is to be interpreted
3.7
[#1] correctly rounded result
a representation in the result format that is nearest in
value, subject to the effective rounding mode, to what the
result would be given unlimited range and precision
3.8
[#1] diagnostic message
message belonging to an implementation-defined subset of the
implementation's message output
3.9
[#1] forward references
references to later subclauses of this International
3.2 General 3.9
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Standard that contain additional information relevant to
this subclause
3.10
[#1] implementation
a particular set of software, running in a particular
translation environment under particular control options,
that performs translation of programs for, and supports
execution of functions in, a particular execution
environment
3.11
[#1] implementation-defined behavior
unspecified behavior where each implementation documents how
the choice is made
[#2] EXAMPLE An example of implementation-defined behavior
is the propagation of the high-order bit when a signed
integer is shifted right.
3.12
[#1] implementation limits
restrictions imposed upon programs by the implementation
3.13
[#1] locale-specific behavior
behavior that depends on local conventions of nationality,
culture, and language that each implementation documents
[#2] EXAMPLE An example of locale-specific behavior is
whether the islower function returns true for characters
other than the 26 lowercase Latin letters.
3.14
[#1] multibyte character
sequence of one or more bytes representing a member of the
extended character set of either the source or the execution
environment
[#2] NOTE The extended character set is a superset of the
basic character set.
3.15
[#1] object
region of data storage in the execution environment, the
contents of which can represent values |
[#2] NOTE When referenced, an object may be interpreted as
having a particular type; see 6.3.2.1.
3.9 General 3.15
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3.16
[#1] parameter
formal parameter
formal argument (deprecated)
object declared as part of a function declaration or
definition that acquires a value on entry to the function,
or an identifier from the comma-separated list bounded by
the parentheses immediately following the macro name in a
function-like macro definition
3.17
[#1] recommended practice
specifications that are strongly recommended as being in
keeping with the intent of the standard, but that may be
impractical for some implementations
3.18
[#1] undefined behavior
behavior, upon use of a nonportable or erroneous program
construct, of erroneous data, or of indeterminately valued
objects, for which this International Standard imposes no
requirements
[#2] NOTE Possible undefined behavior ranges from ignoring |
the situation completely with unpredictable results, to
behaving during translation or program execution in a
documented manner characteristic of the environment (with or
without the issuance of a diagnostic message), to
terminating a translation or execution (with the issuance of
a diagnostic message).
[#3] EXAMPLE An example of undefined behavior is the
behavior on integer overflow.
3.19
[#1] unspecified behavior
behavior where this International Standard provides two or
more possibilities and imposes no requirements on which is
chosen in any instance
[#2] EXAMPLE An example of unspecified behavior is the
order in which the arguments to a function are evaluated.
Forward references: bitwise shift operators (6.5.7),
expressions (6.5), function calls (6.5.2.2), the islower
function (7.4.1.6), localization (7.11).
3.16 General 3.19
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4. Conformance
[#1] In this International Standard, ``shall'' is to be
interpreted as a requirement on an implementation or on a
program; conversely, ``shall not'' is to be interpreted as a
prohibition.
[#2] If a ``shall'' or ``shall not'' requirement that
appears outside of a constraint is violated, the behavior is
undefined. Undefined behavior is otherwise indicated in
this International Standard by the words ``undefined
behavior'' or by the omission of any explicit definition of
behavior. There is no difference in emphasis among these
three; they all describe ``behavior that is undefined''.
[#3] A program that is correct in all other aspects,
operating on correct data, containing unspecified behavior
shall be a correct program and act in accordance with
5.1.2.3.
[#4] The implementation shall not successfully translate a |
preprocessing translation unit containing a #error |
preprocessing directive unless it is part of a group skipped |
by conditional inclusion.
[#5] A strictly conforming program shall use only those
features of the language and library specified in this
International Standard.2) It shall not produce output
dependent on any unspecified, undefined, or implementation-
defined behavior, and shall not exceed any minimum
implementation limit.
[#6] The two forms of conforming implementation are hosted
and freestanding. A conforming hosted implementation shall
accept any strictly conforming program. A conforming
freestanding implementation shall accept any strictly
conforming program that does not use complex types and in
which the use of the features specified in the library
clause (clause 7) is confined to the contents of the
standard headers <float.h>, <iso646.h>, <limits.h>,
<stdarg.h>, <stdbool.h>, <stddef.h>, and <stdint.h>. A
conforming implementation may have extensions (including
additional library functions), provided they do not alter
____________________
2) A strictly conforming program can use conditional |
features (such as those in annex F) provided the use is |
guarded by a #ifdef directive with the appropriate macro. |
For example:
#ifdef __STDC_IEC_559__ /* FE_UPWARD defined */
/* ... */
fesetround(FE_UPWARD);
/* ... */
#endif
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WG14/N843 Committee Draft -- August 3, 1998 7
the behavior of any strictly conforming program.3)
[#7] A conforming program is one that is acceptable to a
conforming implementation.4)
[#8] An implementation shall be accompanied by a document
that defines all implementation-defined and locale-specific
characteristics and all extensions.
Forward references: conditional inclusion (6.10.1), |
characteristics of floating types <float.h> (7.7), |
alternative spellings <iso646.h> (7.9), sizes of integer
types <limits.h> (7.10), variable arguments <stdarg.h>
(7.15), boolean type and values <stdbool.h> (7.16), common |
definitions <stddef.h> (7.17), integer types <stdint.h> |
(7.18).
____________________
3) This implies that a conforming implementation reserves no
identifiers other than those explicitly reserved in this
International Standard.
4) Strictly conforming programs are intended to be maximally
portable among conforming implementations. Conforming
programs may depend upon nonportable features of a
conforming implementation.
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5. Environment
[#1] An implementation translates C source files and
executes C programs in two data-processing-system
environments, which will be called the translation
environment and the execution environment in this
International Standard. Their characteristics define and
constrain the results of executing conforming C programs
constructed according to the syntactic and semantic rules
for conforming implementations.
Forward references: In this clause, only a few of many
possible forward references have been noted.
5.1 Conceptual models
5.1.1 Translation environment
5.1.1.1 Program structure
[#1] A C program need not all be translated at the same
time. The text of the program is kept in units called
source files, (or preprocessing files) in this International |
Standard. A source file together with all the headers and
source files included via the preprocessing directive
#include is known as a preprocessing translation unit. After
preprocessing, a preprocessing translation unit is called a
translation unit. Previously translated translation units
may be preserved individually or in libraries. The separate
translation units of a program communicate by (for example)
calls to functions whose identifiers have external linkage,
manipulation of objects whose identifiers have external
linkage, or manipulation of data files. Translation units
may be separately translated and then later linked to
produce an executable program.
Forward references: conditional inclusion (6.10.1),
linkages of identifiers (6.2.2), source file inclusion
(6.10.2), external definitions (6.9), preprocessing
directives (6.10).
5.1.1.2 Translation phases
[#1] The precedence among the syntax rules of translation is
specified by the following phases.5)
1. Physical source file multibyte characters are mapped
to the source character set (introducing new-line
characters for end-of-line indicators) if necessary. |
____________________
5) Implementations shall behave as if these separate phases
occur, even though many are typically folded together in
practice.
5 Environment 5.1.1.2
WG14/N843 Committee Draft -- August 3, 1998 9
Trigraph sequences are replaced by corresponding
single-character internal representations.
2. Each instance of a backslash character (\) immediately
followed by a new-line character is deleted, splicing
physical source lines to form logical source lines. |
If, as a result, a character sequence that matches the |
syntax of a universal character name is produced, the |
behavior is undefined. Only the last backslash on any
physical source line shall be eligible for being part
of such a splice. A source file that is not empty
shall end in a new-line character, which shall not be
immediately preceded by a backslash character before
any such splicing takes place.
3. The source file is decomposed into preprocessing
tokens6) and sequences of white-space characters
(including comments). A source file shall not end in
a partial preprocessing token or in a partial comment.
Each comment is replaced by one space character. New-
line characters are retained. Whether each nonempty
sequence of white-space characters other than new-line
is retained or replaced by one space character is
implementation-defined.
4. Preprocessing directives are executed, macro
invocations are expanded, and _Pragma unary operator |
expressions are executed. If a character sequence
that matches the syntax of a universal character name
is produced by token concatenation (6.10.3.3), the
behavior is undefined. A #include preprocessing
directive causes the named header or source file to be
processed from phase 1 through phase 4, recursively.
All preprocessing directives are then deleted.
5. Each source character set member, escape sequence, and
universal character name in character constants and
string literals is converted to the corresponding
member of the execution character set; if there is no
corresponding member, it is converted to an
implementation-defined member.
6. Adjacent string literal tokens are concatenated.
7. White-space characters separating tokens are no longer
significant. Each preprocessing token is converted
into a token. The resulting tokens are syntactically
and semantically analyzed and translated as a
____________________
6) As described in 6.4, the process of dividing a source
file's characters into preprocessing tokens is context-
dependent. For example, see the handling of < within a
#include preprocessing directive.
5.1.1.2 Environment 5.1.1.2
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translation unit.
8. All external object and function references are
resolved. Library components are linked to satisfy
external references to functions and objects not
defined in the current translation. All such
translator output is collected into a program image
which contains information needed for execution in its
execution environment.
Forward references: universal character names (6.4.3),
lexical elements (6.4), preprocessing directives (6.10),
trigraph sequences (5.2.1.1), external definitions (6.9).
5.1.1.3 Diagnostics
[#1] A conforming implementation shall produce at least one
diagnostic message (identified in an implementation-defined
manner) if a preprocessing translation unit or translation
unit contains a violation of any syntax rule or constraint,
even if the behavior is also explicitly specified as
undefined or implementation-defined. Diagnostic messages
need not be produced in other circumstances.7)
[#2] EXAMPLE An implementation shall issue a diagnostic for
the translation unit:
char i;
int i;
because in those cases where wording in this International
Standard describes the behavior for a construct as being
both a constraint error and resulting in undefined behavior,
the constraint error shall be diagnosed.
5.1.2 Execution environments
[#1] Two execution environments are defined: freestanding
and hosted. In both cases, program startup occurs when a
designated C function is called by the execution
environment. All objects in static storage shall be
initialized (set to their initial values) before program
startup. The manner and timing of such initialization are
otherwise unspecified. Program termination returns control
to the execution environment.
____________________
7) The intent is that an implementation should identify the
nature of, and where possible localize, each violation.
Of course, an implementation is free to produce any
number of diagnostics as long as a valid program is still
correctly translated. It may also successfully translate
an invalid program.
5.1.1.2 Environment 5.1.2
WG14/N843 Committee Draft -- August 3, 1998 11
Forward references: initialization (6.7.8).
5.1.2.1 Freestanding environment
[#1] In a freestanding environment (in which C program
execution may take place without any benefit of an operating
system), the name and type of the function called at program
startup are implementation-defined. Any library facilities |
available to a freestanding program, other than the minimal |
set required by clause 4, are implementation-defined.
[#2] The effect of program termination in a freestanding
environment is implementation-defined.
5.1.2.2 Hosted environment
[#1] A hosted environment need not be provided, but shall
conform to the following specifications if present.
5.1.2.2.1 Program startup
[#1] The function called at program startup is named main.
The implementation declares no prototype for this function.
It shall be defined with a return type of int and with no
parameters:
int main(void) { /* ... */ }
or with two parameters (referred to here as argc and argv,
though any names may be used, as they are local to the
function in which they are declared):
int main(int argc, char *argv[]) { /* ... */ }
or equivalent;8) or in some other implementation-defined
manner.
[#2] If they are declared, the parameters to the main
function shall obey the following constraints:
-- The value of argc shall be nonnegative.
-- argv[argc] shall be a null pointer.
-- If the value of argc is greater than zero, the array
members argv[0] through argv[argc-1] inclusive shall
contain pointers to strings, which are given
implementation-defined values by the host environment
prior to program startup. The intent is to supply to
____________________
8) Thus, int can be replaced by a typedef name defined as
int, or the type of argv can be written as char ** argv,
and so on.
5.1.2 Environment 5.1.2.2.1
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the program information determined prior to program
startup from elsewhere in the hosted environment. If
the host environment is not capable of supplying
strings with letters in both uppercase and lowercase,
the implementation shall ensure that the strings are
received in lowercase.
-- If the value of argc is greater than zero, the string
pointed to by argv[0] represents the program name;
argv[0][0] shall be the null character if the program
name is not available from the host environment. If
the value of argc is greater than one, the strings
pointed to by argv[1] through argv[argc-1] represent
the program parameters.
-- The parameters argc and argv and the strings pointed to
by the argv array shall be modifiable by the program,
and retain their last-stored values between program
startup and program termination.
5.1.2.2.2 Program execution
[#1] In a hosted environment, a program may use all the
functions, macros, type definitions, and objects described
in the library clause (clause 7).
5.1.2.2.3 Program termination
[#1] If the return type of the main function is a type |
compatible with int, a return from the initial call to the
main function is equivalent to calling the exit function
with the value returned by the main function as its |
argument;9) reaching the } that terminates the main function |
returns a value of 0. If the return type is not compatible |
with int, the termination status returned to the host
environment is unspecified.
Forward references: definition of terms (7.1.1), the exit
function (7.20.4.3).
5.1.2.3 Program execution
[#1] The semantic descriptions in this International
Standard describe the behavior of an abstract machine in
which issues of optimization are irrelevant.
[#2] Accessing a volatile object, modifying an object,
modifying a file, or calling a function that does any of
____________________
9) In accordance with 6.2.4, objects with automatic storage
duration declared in main will no longer have storage
guaranteed to be reserved in the former case even where
they would in the latter.
5.1.2.2.1 Environment 5.1.2.3
WG14/N843 Committee Draft -- August 3, 1998 13
those operations are all side effects,10) which are changes
in the state of the execution environment. Evaluation of an
expression may produce side effects. At certain specified
points in the execution sequence called sequence points, all
side effects of previous evaluations shall be complete and
no side effects of subsequent evaluations shall have taken
place. (A summary of the sequence points is given in annex
C.)
[#3] In the abstract machine, all expressions are evaluated
as specified by the semantics. An actual implementation
need not evaluate part of an expression if it can deduce
that its value is not used and that no needed side effects
are produced (including any caused by calling a function or
accessing a volatile object).
[#4] When the processing of the abstract machine is
interrupted by receipt of a signal, only the values of
objects as of the previous sequence point may be relied on.
Objects that may be modified between the previous sequence
point and the next sequence point need not have received
their correct values yet.
[#5] An instance of each object with automatic storage
duration is associated with each entry into its block. Such
an object exists and retains its last-stored value during
the execution of the block and while the block is suspended
(by a call of a function or receipt of a signal).
[#6] The least requirements on a conforming implementation
are:
-- At sequence points, volatile objects are stable in the |
sense that previous accesses are complete and |
subsequent accesses have not yet occurred.
-- At program termination, all data written into files
shall be identical to the result that execution of the
program according to the abstract semantics would have
produced.
____________________
10)The IEC 60559 standard for binary floating-point
arithmetic requires certain user-accessible status flags
and control modes. Floating-point operations implicitly
set the status flags; modes affect result values of
floating-point operations. Implementations that support
such floating-point state are required to regard changes
to it as side effects -- see annex F for details. The
floating-point environment library <fenv.h> provides a
programming facility for indicating when these side
effects matter, freeing the implementations in other
cases.
5.1.2.3 Environment 5.1.2.3
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-- The input and output dynamics of interactive devices
shall take place as specified in 7.19.3. The intent of
these requirements is that unbuffered or line-buffered
output appear as soon as possible, to ensure that
prompting messages actually appear prior to a program
waiting for input.
[#7] What constitutes an interactive device is
implementation-defined.
[#8] More stringent correspondences between abstract and
actual semantics may be defined by each implementation.
[#9] EXAMPLE 1 An implementation might define a one-to-one
correspondence between abstract and actual semantics: at
every sequence point, the values of the actual objects would
agree with those specified by the abstract semantics. The
keyword volatile would then be redundant.
[#10] Alternatively, an implementation might perform various
optimizations within each translation unit, such that the
actual semantics would agree with the abstract semantics
only when making function calls across translation unit
boundaries. In such an implementation, at the time of each
function entry and function return where the calling
function and the called function are in different
translation units, the values of all externally linked
objects and of all objects accessible via pointers therein
would agree with the abstract semantics. Furthermore, at
the time of each such function entry the values of the
parameters of the called function and of all objects
accessible via pointers therein would agree with the
abstract semantics. In this type of implementation, objects
referred to by interrupt service routines activated by the
signal function would require explicit specification of
volatile storage, as well as other implementation-defined
restrictions.
[#11] EXAMPLE 2 In executing the fragment
char c1, c2;
/* ... */
c1 = c1 + c2;
the ``integer promotions'' require that the abstract machine
promote the value of each variable to int size and then add
the two ints and truncate the sum. Provided the addition of
two chars can be done without overflow, or with overflow
wrapping silently to produce the correct result, the actual
execution need only produce the same result, possibly
omitting the promotions.
5.1.2.3 Environment 5.1.2.3
WG14/N843 Committee Draft -- August 3, 1998 15
[#12] EXAMPLE 3 Similarly, in the fragment
float f1, f2;
double d;
/* ... */
f1 = f2 * d;
the multiplication may be executed using single-precision
arithmetic if the implementation can ascertain that the
result would be the same as if it were executed using
double-precision arithmetic (for example, if d were replaced
by the constant 2.0, which has type double).
[#13] EXAMPLE 4 Implementations employing wide registers
have to take care to honor appropriate semantics. Values
are independent of whether they are represented in a
register or in memory. For example, an implicit spilling of
a register is not permitted to alter the value. Also, an
explicit store and load is required to round to the
precision of the storage type. In particular, casts and
assignments are required to perform their specified
conversion. For the fragment
double d1, d2;
float f;
d1 = f = expression;
d2 = (float) expressions;
the values assigned to d1 and d2 are required to have been
converted to float.
[#14] EXAMPLE 5 Rearrangement for floating-point expressions
is often restricted because of limitations in precision as
well as range. The implementation cannot generally apply
the mathematical associative rules for addition or
multiplication, nor the distributive rule, because of
roundoff error, even in the absence of overflow and
underflow. Likewise, implementations cannot generally
replace decimal constants in order to rearrange expressions.
In the following fragment, rearrangements suggested by
mathematical rules for real numbers are often not valid (see
F.8).
double x, y, z;
/* ... */
x = (x * y) * z; // not equivalent to x *= y * z;
z = (x - y) + y ; // not equivalent to z = x;
z = x + x * y; // not equivalent to z = x * (1.0 + y);
y = x / 5.0; // not equivalent to y = x * 0.2; |
5.1.2.3 Environment 5.1.2.3
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[#15] EXAMPLE 6 To illustrate the grouping behavior of
expressions, in the following fragment
int a, b;
/* ... */
a = a + 32760 + b + 5;
the expression statement behaves exactly the same as
a = (((a + 32760) + b) + 5);
due to the associativity and precedence of these operators.
Thus, the result of the sum (a + 32760) is next added to b,
and that result is then added to 5 which results in the
value assigned to a. On a machine in which overflows
produce an explicit trap and in which the range of values
representable by an int is [-32768, +32767], the
implementation cannot rewrite this expression as
a = ((a + b) + 32765);
since if the values for a and b were, respectively, -32754
and -15, the sum a + b would produce a trap while the
original expression would not; nor can the expression be
rewritten either as
a = ((a + 32765) + b);
or
a = (a + (b + 32765));
since the values for a and b might have been, respectively,
4 and -8 or -17 and 12. However, on a machine in which
overflow silently generates some value and where positive
and negative overflows cancel, the above expression
statement can be rewritten by the implementation in any of
the above ways because the same result will occur.
[#16] EXAMPLE 7 The grouping of an expression does not
completely determine its evaluation. In the following
fragment
#include <stdio.h>
int sum;
char *p;
/* ... */
sum = sum * 10 - '0' + (*p++ = getchar());
the expression statement is grouped as if it were written as
sum = (((sum * 10) - '0') + ((*(p++)) = (getchar())));
but the actual increment of p can occur at any time between
the previous sequence point and the next sequence point (the
5.1.2.3 Environment 5.1.2.3
WG14/N843 Committee Draft -- August 3, 1998 17
;), and the call to getchar can occur at any point prior to
the need of its returned value.
Forward references: compound statement, or block (6.8.2),
expressions (6.5), files (7.19.3), sequence points (6.5,
6.8), the signal function (7.14), type qualifiers (6.7.3).
5.1.2.3 Environment 5.1.2.3
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5.2 Environmental considerations
5.2.1 Character sets
[#1] Two sets of characters and their associated collating
sequences shall be defined: the set in which source files
are written, and the set interpreted in the execution
environment. The values of the members of the execution
character set are implementation-defined; any additional
members beyond those required by this subclause are locale-
specific.
[#2] In a character constant or string literal, members of
the execution character set shall be represented by
corresponding members of the source character set or by
escape sequences consisting of the backslash \ followed by
one or more characters. A byte with all bits set to 0,
called the null character, shall exist in the basic
execution character set; it is used to terminate a character
string.
[#3] Both the basic source and basic execution character
sets shall have at least the following members: the 26
uppercase letters of the Latin alphabet
A B C D E F G H I J K L M
N O P Q R S T U V W X Y Z
the 26 lowercase letters of the Latin alphabet
a b c d e f g h i j k l m
n o p q r s t u v w x y z
the 10 decimal digits
0 1 2 3 4 5 6 7 8 9
the following 29 graphic characters
! " # % & ' ( ) * + , - . / :
; < = > ? [ \ ] ^ _ { | } ~
the space character, and control characters representing
horizontal tab, vertical tab, and form feed. The |
representation of each member of the source and execution |
basic character sets shall fit in a byte. In both the
source and execution basic character sets, the value of each
character after 0 in the above list of decimal digits shall
be one greater than the value of the previous. In source
files, there shall be some way of indicating the end of each
line of text; this International Standard treats such an
end-of-line indicator as if it were a single new-line
character. In the execution character set, there shall be
control characters representing alert, backspace, carriage
5.2 Environment 5.2.1
WG14/N843 Committee Draft -- August 3, 1998 19
return, and new line. If any other characters are
encountered in a source file (except in an identifier, a |
character constant, a string literal, a header name, a
comment, or a preprocessing token that is never converted to
a token), the behavior is undefined.
[#4] The universal character name construct provides a way
to name other characters.
Forward references: universal character names (6.4.3),
character constants (6.4.4.4), preprocessing directives
(6.10), string literals (6.4.5), comments (6.4.9), string
(7.1.1).
5.2.1.1 Trigraph sequences
[#1] All occurrences in a source file of the following
sequences of three characters (called trigraph sequences11))
are replaced with the corresponding single character. |
??= # ??) ] ??! |
??( [ ??' ^ ??> }
??/ \ ??< { ??- ~
No other trigraph sequences exist. Each ? that does not
begin one of the trigraphs listed above is not changed.
[#2] EXAMPLE The following source line
printf("Eh???/n");
becomes (after replacement of the trigraph sequence ??/)
printf("Eh?\n");
5.2.1.2 Multibyte characters
[#1] The source character set may contain multibyte
characters, used to represent members of the extended
character set. The execution character set may also contain
multibyte characters, which need not have the same encoding
as for the source character set. For both character sets,
the following shall hold:
-- The single-byte characters defined in 5.2.1 shall be
present.
____________________
11)The trigraph sequences enable the input of characters
that are not defined in the Invariant Code Set as
described in ISO/IEC 646, which is a subset of the seven- |
bit US ASCII code set. |
5.2.1 Environment 5.2.1.2
20 Committee Draft -- August 3, 1998 WG14/N843
-- The presence, meaning, and representation of any
additional members is locale-specific.
-- A multibyte character set may have a state-dependent |
encoding, wherein each sequence of multibyte characters
begins in an initial shift state and enters other
locale-specific shift states when specific multibyte
characters are encountered in the sequence. While in
the initial shift state, all single-byte characters
retain their usual interpretation and do not alter the
shift state. The interpretation for subsequent bytes
in the sequence is a function of the current shift
state.
-- A byte with all bits zero shall be interpreted as a
null character independent of shift state.
-- A byte with all bits zero shall not occur in the second
or subsequent bytes of a multibyte character.
[#2] For source files, the following shall hold: |
-- An identifier, comment, string literal, character |
constant, or header name shall begin and end in the
initial shift state.
-- An identifier, comment, string literal, character |
constant, or header name shall consist of a sequence of
valid multibyte characters.
5.2.2 Character display semantics
[#1] The active position is that location on a display
device where the next character output by the fputc or |
fputwc function would appear. The intent of writing a
printable character (as defined by the isprint or iswprint |
function) to a display device is to display a graphic
representation of that character at the active position and
then advance the active position to the next position on the
current line. The direction of writing is locale-specific.
If the active position is at the final position of a line
(if there is one), the behavior is unspecified.
[#2] Alphabetic escape sequences representing nongraphic
characters in the execution character set are intended to
produce actions on display devices as follows:
\a (alert) Produces an audible or visible alert. The active
position shall not be changed.
\b (backspace) Moves the active position to the previous
position on the current line. If the active position is
at the initial position of a line, the behavior is
unspecified.
5.2.1.2 Environment 5.2.2
WG14/N843 Committee Draft -- August 3, 1998 21
\f (form feed) Moves the active position to the initial
position at the start of the next logical page.
\n (new line) Moves the active position to the initial
position of the next line.
\r (carriage return) Moves the active position to the
initial position of the current line.
\t (horizontal tab) Moves the active position to the next
horizontal tabulation position on the current line. If
the active position is at or past the last defined
horizontal tabulation position, the behavior is
unspecified.
\v (vertical tab) Moves the active position to the initial
position of the next vertical tabulation position. If
the active position is at or past the last defined
vertical tabulation position, the behavior is
unspecified.
[#3] Each of these escape sequences shall produce a unique
implementation-defined value which can be stored in a single
char object. The external representations in a text file
need not be identical to the internal representations, and
are outside the scope of this International Standard.
Forward references: the isprint function (7.4.1.7), the
fputc function (7.19.7.3), the fputwc functions (7.24.3.3), |
the iswprint function (7.25.2.1.7).
5.2.3 Signals and interrupts
[#1] Functions shall be implemented such that they may be
interrupted at any time by a signal, or may be called by a
signal handler, or both, with no alteration to earlier, but
still active, invocations' control flow (after the
interruption), function return values, or objects with
automatic storage duration. All such objects shall be
maintained outside the function image (the instructions that |
compose the executable representation of a function) on a
per-invocation basis.
5.2.4 Environmental limits
[#1] Both the translation and execution environments
constrain the implementation of language translators and
libraries. The following summarizes the language-related |
environmental limits on a conforming implementation; the |
library-related limits are discussed in clause 7.
5.2.2 Environment 5.2.4
22 Committee Draft -- August 3, 1998 WG14/N843
5.2.4.1 Translation limits
[#1] The implementation shall be able to translate and
execute at least one program that contains at least one
instance of every one of the following limits:12)
-- 127 nesting levels of compound statements, iteration
statements, and selection statements
-- 63 nesting levels of conditional inclusion
-- 12 pointer, array, and function declarators (in any
combinations) modifying an arithmetic, structure,
union, or incomplete type in a declaration
-- 63 nesting levels of parenthesized declarators within a
full declarator
-- 63 nesting levels of parenthesized expressions within a
full expression
-- 63 significant initial characters in an internal
identifier or a macro name (each universal character |
name or extended source character is considered a |
single character)
-- 31 significant initial characters in an external
identifier (each universal character name specifying a |
character short identifier of 0000FFFF or less is |
considered 6 characters, each universal character name |
specifying a character short identifier of 00010000 or |
more is considered 10 characters, and each extended |
source character is considered the same number of |
characters as the corresponding universal character |
name, if any)
-- 4095 external identifiers in one translation unit
-- 511 identifiers with block scope declared in one block
-- 4095 macro identifiers simultaneously defined in one
preprocessing translation unit
-- 127 parameters in one function definition
-- 127 arguments in one function call
-- 127 parameters in one macro definition
____________________
12)Implementations should avoid imposing fixed translation
limits whenever possible.
5.2.4.1 Environment 5.2.4.1
WG14/N843 Committee Draft -- August 3, 1998 23
-- 127 arguments in one macro invocation
-- 4095 characters in a logical source line
-- 4095 characters in a character string literal or wide
string literal (after concatenation)
-- 65535 bytes in an object (in a hosted environment only)
-- 15 nesting levels for #included files
-- 1023 case labels for a switch statement (excluding
those for any nested switch statements)
-- 1023 members in a single structure or union
-- 1023 enumeration constants in a single enumeration
-- 63 levels of nested structure or union definitions in a
single struct-declaration-list
5.2.4.2 Numerical limits
[#1] A conforming implementation shall document all the
limits specified in this subclause, which are specified in |
the headers <limits.h> and <float.h>. Additional limits are |
specified in <stdint.h>.
5.2.4.2.1 Sizes of integer types <limits.h>
[#1] The values given below shall be replaced by constant
expressions suitable for use in #if preprocessing
directives. Moreover, except for CHAR_BIT and MB_LEN_MAX,
the following shall be replaced by expressions that have the
same type as would an expression that is an object of the
corresponding type converted according to the integer
promotions. Their implementation-defined values shall be
equal or greater in magnitude (absolute value) to those
shown, with the same sign.
-- number of bits for smallest object that is not a bit-
field (byte)
CHAR_BIT 8
-- minimum value for an object of type signed char
SCHAR_MIN -127 // -(27-1)
-- maximum value for an object of type signed char
SCHAR_MAX +127 // 27-1
-- maximum value for an object of type unsigned char
UCHAR_MAX 255 // 28-1
5.2.4.1 Environment 5.2.4.2.1
24 Committee Draft -- August 3, 1998 WG14/N843
-- minimum value for an object of type char
CHAR_MIN see below
-- maximum value for an object of type char
CHAR_MAX see below
-- maximum number of bytes in a multibyte character, for
any supported locale
MB_LEN_MAX 1
-- minimum value for an object of type short int
SHRT_MIN -32767 // -(215-1)
-- maximum value for an object of type short int
SHRT_MAX +32767 // 215-1
-- maximum value for an object of type unsigned short int
USHRT_MAX 65535 // 216-1
-- minimum value for an object of type int
INT_MIN -32767 // -(215-1)
-- maximum value for an object of type int
INT_MAX +32767 // 215-1
-- maximum value for an object of type unsigned int
UINT_MAX 65535 // 216-1
-- minimum value for an object of type long int
LONG_MIN -2147483647 // -(231-1)
-- maximum value for an object of type long int
LONG_MAX +2147483647 // 231-1
-- maximum value for an object of type unsigned long int
ULONG_MAX 4294967295 // 232-1
-- minimum value for an object of type long long int
LLONG_MIN -9223372036854775807 // -(263-1)
-- maximum value for an object of type long long int
LLONG_MAX +9223372036854775807 // 263-1
-- maximum value for an object of type unsigned long long
int
ULLONG_MAX 18446744073709551615 // 264-1
[#2] If the value of an object of type char is treated as a
signed integer when used in an expression, the value of
CHAR_MIN shall be the same as that of SCHAR_MIN and the
value of CHAR_MAX shall be the same as that of SCHAR_MAX.
Otherwise, the value of CHAR_MIN shall be 0 and the value of
CHAR_MAX shall be the same as that of UCHAR_MAX.13) The
value UCHAR_MAX+1 shall equal 2 raised to the power
5.2.4.2.1 Environment 5.2.4.2.1
WG14/N843 Committee Draft -- August 3, 1998 25
CHAR_BIT.
5.2.4.2.2 Characteristics of floating types <float.h>
[#1] The characteristics of floating types are defined in
terms of a model that describes a representation of
floating-point numbers and values that provide information
about an implementation's floating-point arithmetic.14) The
following parameters are used to define the model for each
floating-point type:
s sign (±1)
b base or radix of exponent representation (an integer > 1)
e exponent (an integer between a minimum emin and a maximum emax)
p precision (the number of base-b digits in the significand)
fk nonnegative integers less than b (the significand digits)
[#2] A normalized floating-point number x (f1 > 0 if x != 0)
is defined by the following model:
x=s×be×k=1fk×b-k,emin<=e<=emax
[#3] Floating types may include values that are not
normalized floating-point numbers, for example subnormal |
floating-point numbers (x!=0,e=emin,f1=0), infinities, and |
NaNs.15) A NaN is an encoding signifying Not-a-Number. A
quiet NaN propagates through almost every arithmetic
operation without raising an exception; a signaling NaN
generally raises an exception when occurring as an
arithmetic operand.16)
[#4] The accuracy of the floating-point operations (+, -, *,
/) and of the library functions in <math.h> and <complex.h> |
that return floating-point results is implementation |
defined. The implementation may state that the accuracy is |
unknown.
[#5] All integer values in the <float.h> header, except
____________________
13)See 6.2.5.
14)The floating-point model is intended to clarify the
description of each floating-point characteristic and
does not require the floating-point arithmetic of the
implementation to be identical.
15)Although they are stored in floating types, infinities
and NaNs are not floating-point numbers.
16)IEC 60559:1989 specifies quiet and signaling NaNs. For
implementations that do not support IEC 60559:1989, the
terms quiet NaN and signaling NaN are intended to apply
to encodings with similar behavior.
5.2.4.2.1 Environment 5.2.4.2.2
26 Committee Draft -- August 3, 1998 WG14/N843
FLT_ROUNDS, shall be constant expressions suitable for use
in #if preprocessing directives; all floating values shall
be constant expressions. All except DECIMAL_DIG, |
FLT_EVAL_METHOD, FLT_RADIX, and FLT_ROUNDS have separate
names for all three floating-point types. The floating-
point model representation is provided for all values except
FLT_EVAL_METHOD and FLT_ROUNDS.
[#6] The rounding mode for floating-point addition is
characterized by the value of FLT_ROUNDS:17)
-1 indeterminable
0 toward zero
1 to nearest
2 toward positive infinity
3 toward negative infinity
All other values for FLT_ROUNDS characterize implementation-
defined rounding behavior.
[#7] The values of operations with floating operands and
values subject to the usual arithmetic conversions and of
floating constants are evaluated to a format whose range and
precision may be greater than required by the type. The use
of evaluation formats is characterized by the value of
FLT_EVAL_METHOD:18)
-1 indeterminable;
0 evaluate all operations and constants just
to the range and precision of the type;
1 evaluate operations and constants of type
float and double to the range and precision
of the double type, evaluate long double
operations and constants to the range and
precision of the long double type;
2 evaluate all operations and constants to the
range and precision of the long double type.
All other negative values for FLT_EVAL_METHOD characterize
implementation-defined behavior.
____________________
17)Evaluation of FLT_ROUNDS correctly reflects any
execution-time change of rounding mode through the
function fesetround in <fenv.h>.
18)The evaluation method determines evaluation formats of
expressions involving all floating types, not just real
types. For example, if FLT_EVAL_METHOD is 1, then the
product of two float _Complex operands is represented in
the double _Complex format, and its parts are evaluated
to double.
5.2.4.2.2 Environment 5.2.4.2.2
WG14/N843 Committee Draft -- August 3, 1998 27
[#8] The values given in the following list shall be
replaced by implementation-defined constant expressions with
values that are greater or equal in magnitude (absolute
value) to those shown, with the same sign:
-- radix of exponent representation, b
FLT_RADIX 2
-- number of base-FLT_RADIX digits in the floating-point
significand, p
FLT_MANT_DIG
DBL_MANT_DIG
LDBL_MANT_DIG
-- number of decimal digits, n, such that any floating- |
point number in the widest supported floating type with |
pmax radix b digits can be rounded to a floating-point |
number with n decimal digits and back again without |
changpmax×log10blueif b is a power of 10 |
|1+pmax×log10b|otherwise
DECIMAL_DIG 10
-- number of decimal digits, q, such that any floating-
point number with q decimal digits can be rounded into
a floating-point number with p radix b digits and back
again without change to the q decimal digits, |
5.2.4.2.2 Environment 5.2.4.2.2
28 Committee Draft -- August 3, 1998 WG14/N843
p×log10b if b is a power of 10
|(p-1)×log10b|otherwise
FLT_DIG 6
DBL_DIG 10
LDBL_DIG 10
-- minimum negative integer such that FLT_RADIX raised to
one less than that power is a normalized floating-point
number, emin
FLT_MIN_EXP
DBL_MIN_EXP
LDBL_MIN_EXP
-- minimum negative integer such that 10 raised to that
power is in the range of normalized floating-point
numbers, |log10bemin-1|
FLT_MIN_10_EXP -37
DBL_MIN_10_EXP -37
LDBL_MIN_10_EXP -37
-- maximum integer such that FLT_RADIX raised to one less
than that power is a representable finite floating-
point number, emax
FLT_MAX_EXP
DBL_MAX_EXP
LDBL_MAX_EXP
-- maximum integer such that 10 raised to that power is in
the range of representable finite floating-point
numbers, |log10((1-b-p)×bemax)|
5.2.4.2.2 Environment 5.2.4.2.2
WG14/N843 Committee Draft -- August 3, 1998 29
FLT_MAX_10_EXP +37
DBL_MAX_10_EXP +37
LDBL_MAX_10_EXP +37
[#9] The values given in the following list shall be
replaced by implementation-defined constant expressions with
values that are greater than or equal to those shown:
-- maximum representable finite floating-point number,
(1-b-p)×bemax
FLT_MAX 1E+37
DBL_MAX 1E+37
LDBL_MAX 1E+37
[#10] The values given in the following list shall be
replaced by implementation-defined constant expressions with
(positive) values that are less than or equal to those
shown:
-- the difference between 1 and the least value greater
than 1 that is representable in the given floating
point type, b1-p
FLT_EPSILON 1E-5
DBL_EPSILON 1E-9
LDBL_EPSILON 1E-9
-- minimum normalized positive floating-point number,
bemin-1
FLT_MIN 1E-37
DBL_MIN 1E-37
LDBL_MIN 1E-37
[#11] EXAMPLE 1 The following describes an artificial
floating-point representation that meets the minimum
requirements of this International Standard, and the
appropriate values in a <float.h> header for type float:
x=s×16e×k=1fk×16-k,-31<=e<=+32
FLT_RADIX 16
FLT_MANT_DIG 6
FLT_EPSILON 9.53674316E-07F
FLT_DIG 6
FLT_MIN_EXP -31
FLT_MIN 2.93873588E-39F
FLT_MIN_10_EXP -38
FLT_MAX_EXP +32
FLT_MAX 3.40282347E+38F
FLT_MAX_10_EXP +38
5.2.4.2.2 Environment 5.2.4.2.2
30 Committee Draft -- August 3, 1998 WG14/N843
[#12] EXAMPLE 2 The following describes floating-point
representations that also meet the requirements for single-
precision and double-precision normalized numbers in IEC
60559,19) and the appropriate values in a <float.h> header
for types float and double:
xf=s×2e×k=1fk×2-k,-125<=e<=+128
xd=s×2e×k=1fk×2-k,-1021<=e<=+1024
FLT_RADIX 2
DECIMAL_DIG 17 |
FLT_MANT_DIG 24
FLT_EPSILON 1.19209290E-07F // decimal constant
FLT_EPSILON 0X1P-23F // hex constant
FLT_DIG 6
FLT_MIN_EXP -125
FLT_MIN 1.17549435E-38F // decimal constant
FLT_MIN 0X1P-126F // hex constant
FLT_MIN_10_EXP -37
FLT_MAX_EXP +128
FLT_MAX 3.40282347E+38F // decimal constant
FLT_MAX 0X1.fffffeP127F // hex constant
FLT_MAX_10_EXP +38
DBL_MANT_DIG 53
DBL_EPSILON 2.2204460492503131E-16 // decimal constant
DBL_EPSILON 0X1P-52 // hex constant
DBL_DIG 15
DBL_MIN_EXP -1021
DBL_MIN 2.2250738585072014E-308 // decimal constant
DBL_MIN 0X1P-1022 // hex constant
DBL_MIN_10_EXP -307
DBL_MAX_EXP +1024
DBL_MAX 1.7976931348623157E+308 // decimal constant
DBL_MAX 0X1.ffffffffffffeP1023 // hex constant
DBL_MAX_10_EXP +308
If a type wider than double were supported, then DECIMAL_DIG |
would be greater than 17. For example, if the widest type |
were to use the minimal-width IEC 60559 double-extended |
format (64 bits of precision), then DECIMAL_DIG would be 21.
Forward references: conditional inclusion (6.10.1), complex |
arithmetic <complex.h> (7.3), mathematics <math.h> (7.12), |
integer types <stdint.h> (7.18).
____________________
19)The floating-point model in that standard sums powers of
b from zero, so the values of the exponent limits are one
less than shown here.
5.2.4.2.2 Environment 5.2.4.2.2
WG14/N843 Committee Draft -- August 3, 1998 31
6. Language
6.1 Notation
[#1] In the syntax notation used in this clause, syntactic
categories (nonterminals) are indicated by italic type, and
literal words and character set members (terminals) by bold
type. A colon (:) following a nonterminal introduces its
definition. Alternative definitions are listed on separate
lines, except when prefaced by the words ``one of''. An
optional symbol is indicated by the suffix ``-opt'', so that
{ expression-opt }
indicates an optional expression enclosed in braces.
[#2] A summary of the language syntax is given in annex A.
6.2 Concepts
6.2.1 Scopes of identifiers
[#1] An identifier can denote an object; a function; a tag |
or a member of a structure, union, or enumeration; a typedef |
name; a label name; a macro name; or a macro parameter. The |
same identifier can denote different entities at different |
points in the program. A member of an enumeration is called |
an enumeration constant. Macro names and macro parameters |
are not considered further here, because prior to the |
semantic phase of program translation any occurrences of |
macro names in the source file are replaced by the |
preprocessing token sequences that constitute their macro |
definitions. |
[#2] For each different entity that an identifier
designates, the identifier is visible (i.e., can be used)
only within a region of program text called its scope.
Different entities designated by the same identifier either |
have different scopes, or are in different name spaces.
There are four kinds of scopes: function, file, block, and
function prototype. (A function prototype is a declaration
of a function that declares the types of its parameters.)
[#3] A label name is the only kind of identifier that has
function scope. It can be used (in a goto statement)
anywhere in the function in which it appears, and is
declared implicitly by its syntactic appearance (followed by
a : and a statement). Label names shall be unique within a
function.
[#4] Every other identifier has scope determined by the
placement of its declaration (in a declarator or type
specifier). If the declarator or type specifier that
declares the identifier appears outside of any block or list
6 Language 6.2.1
32 Committee Draft -- August 3, 1998 WG14/N843
of parameters, the identifier has file scope, which
terminates at the end of the translation unit. If the
declarator or type specifier that declares the identifier
appears inside a block or within the list of parameter
declarations in a function definition, the identifier has
block scope, which terminates at the } that closes the
associated block. If the declarator or type specifier that
declares the identifier appears within the list of parameter
declarations in a function prototype (not part of a function
definition), the identifier has function prototype scope,
which terminates at the end of the function declarator. If
an identifier designates two different entities in the same
name space, the scopes might overlap. If so, the scope of
one entity (the inner scope) will be a strict subset of the
scope of the other entity (the outer scope). Within the
inner scope, the identifier designates the entity declared
in the inner scope; the entity declared in the outer scope
is hidden (and not visible) within the inner scope.
[#5] Unless explicitly stated otherwise, where this
International Standard uses the term identifier to refer to
some entity (as opposed to the syntactic construct), it
refers to the entity in the relevant name space whose
declaration is visible at the point the identifier occurs.
[#6] Two identifiers have the same scope if and only if
their scopes terminate at the same point.
[#7] Structure, union, and enumeration tags have scope that
begins just after the appearance of the tag in a type
specifier that declares the tag. Each enumeration constant
has scope that begins just after the appearance of its
defining enumerator in an enumerator list. Any other
identifier has scope that begins just after the completion
of its declarator.
Forward references: compound statement, or block (6.8.2),
declarations (6.7), enumeration specifiers (6.7.2.2),
function calls (6.5.2.2), function declarators (including
prototypes) (6.7.5.3), function definitions (6.9.1), the
goto statement (6.8.6.1), labeled statements (6.8.1), name
spaces of identifiers (6.2.3), scope of macro definitions
(6.10.3.5), source file inclusion (6.10.2), tags (6.7.2.3),
type specifiers (6.7.2).
6.2.2 Linkages of identifiers
[#1] An identifier declared in different scopes or in the
same scope more than once can be made to refer to the same
object or function by a process called linkage. There are
three kinds of linkage: external, internal, and none.
[#2] In the set of translation units and libraries that
constitutes an entire program, each declaration of a |
6.2.1 Language 6.2.2
WG14/N843 Committee Draft -- August 3, 1998 33
particular identifier with external linkage denotes the same
object or function. Within one translation unit, each |
declaration of an identifier with internal linkage denotes
the same object or function. Each declaration of an |
identifier with no linkage denotes a unique entity. |
[#3] If the declaration of a file scope identifier for an
object or a function contains the storage-class specifier
static, the identifier has internal linkage.20)
[#4] For an identifier declared with the storage-class
specifier extern in a scope in which a prior declaration of
that identifier is visible,21) if the prior declaration
specifies internal or external linkage, the linkage of the
identifier at the later declaration is the same as the |
linkage specified at the prior declaration. If no prior
declaration is visible, or if the prior declaration
specifies no linkage, then the identifier has external
linkage.
[#5] If the declaration of an identifier for a function has
no storage-class specifier, its linkage is determined
exactly as if it were declared with the storage-class
specifier extern. If the declaration of an identifier for
an object has file scope and no storage-class specifier, its
linkage is external.
[#6] The following identifiers have no linkage: an
identifier declared to be anything other than an object or a
function; an identifier declared to be a function parameter;
a block scope identifier for an object declared without the
storage-class specifier extern.
[#7] If, within a translation unit, the same identifier
appears with both internal and external linkage, the
behavior is undefined.
Forward references: compound statement, or block (6.8.2),
declarations (6.7), expressions (6.5), external definitions
(6.9).
____________________
20)A function declaration can contain the storage-class
specifier static only if it is at file scope; see 6.7.1.
21)As specified in 6.2.1, the later declaration might hide
the prior declaration.
6.2.2 Language 6.2.2
34 Committee Draft -- August 3, 1998 WG14/N843
6.2.3 Name spaces of identifiers
[#1] If more than one declaration of a particular identifier
is visible at any point in a translation unit, the syntactic
context disambiguates uses that refer to different entities.
Thus, there are separate name spaces for various categories
of identifiers, as follows:
-- label names (disambiguated by the syntax of the label
declaration and use);
-- the tags of structures, unions, and enumerations
(disambiguated by following any22) of the keywords
struct, union, or enum);
-- the members of structures or unions; each structure or
union has a separate name space for its members
(disambiguated by the type of the expression used to
access the member via the . or -> operator);
-- all other identifiers, called ordinary identifiers
(declared in ordinary declarators or as enumeration
constants).
Forward references: enumeration specifiers (6.7.2.2),
labeled statements (6.8.1), structure and union specifiers
(6.7.2.1), structure and union members (6.5.2.3), tags
(6.7.2.3).
6.2.4 Storage durations of objects
[#1] An object has a storage duration that determines its
lifetime. There are three storage durations: static,
automatic, and allocated. Allocated storage is described in
7.20.3.
[#2] An object whose identifier is declared with external or
internal linkage, or with the storage-class specifier static
has static storage duration. For such an object, storage is
reserved and its stored value is initialized only once,
prior to program startup. The object exists, has a constant
address, and retains its last-stored value throughout the
execution of the entire program.23)
[#3] An object whose identifier is declared with no linkage
and without the storage-class specifier static has automatic
storage duration. For objects that do not have a variable |
length array type, storage is guaranteed to be reserved for |
a new instance of the object on each entry into the block |
with which it is associated; the initial value of the object |
is indeterminate. If an initialization is specified for the |
object, it is performed each time the declaration is reached |
in the execution of the block; otherwise, the value becomes |
indeterminate each time the declaration is reached. Storage
for the object is no longer guaranteed to be reserved when
execution of the block ends in any way. (Entering an |
enclosed block or calling a function suspends, but does not |
end, execution of the current block.) |
WG14/N843 Committee Draft -- August 3, 1998 35
[#4] For objects that do have a variable length array type, |
storage is guaranteed to be reserved for a new instance of |
the object each time the declaration is reached in the |
execution of the program. The initial value of the object |
is indeterminate. Storage for the object is no longer |
guaranteed to be reserved when the execution of the program |
leaves the scope of the declaration.24) |
[#5] If an object is referred to when storage is not |
reserved for it, the behavior is undefined. The value of a
pointer that referred to an object whose storage is no |
longer reserved is indeterminate. During the time that its |
storage is reserved, an object has a constant address.
Forward references: compound statement, or block (6.8.2),
function calls (6.5.2.2), declarators (6.7.5), array
declarators (6.7.5.2), initialization (6.7.8).
6.2.5 Types
[#1] The meaning of a value stored in an object or returned
by a function is determined by the type of the expression
used to access it. (An identifier declared to be an object
is the simplest such expression; the type is specified in
the declaration of the identifier.) Types are partitioned
into object types (types that describe objects), function
types (types that describe functions), and incomplete types
(types that describe objects but lack information needed to
determine their sizes).
[#2] An object declared as type _Bool is large enough to |
store the values 0 and 1. |
[#3] An object declared as type char is large enough to
store any member of the basic execution character set. If a
member of the required source character set enumerated in
5.2.1 is stored in a char object, its value is guaranteed to
____________________
22)There is only one name space for tags even though three
are possible.
23)The term constant address means that two pointers to the
object constructed at possibly different times will
compare equal. The address may be different during two
different executions of the same program.
In the case of a volatile object, the last store need not
be explicit in the program.
24)Leaving the innermost block containing the declaration,
or jumping to a point in that block or an embedded block
prior to the declaration, leaves the scope of the
declaration.
6.2.4 Language 6.2.5
36 Committee Draft -- August 3, 1998 WG14/N843
be positive. If any other character is stored in a char
object, the resulting value is implementation-defined but
shall be within the range of values that can be represented
in that type.
[#4] There are five standard signed integer types,
designated as signed char, short int, int, long int, and
long long int. (These and other types may be designated in
several additional ways, as described in 6.7.2.) There may
also be implementation-defined extended signed integer
types.25) The standard and extended signed integer types
are collectively called signed integer types.26) |
[#5] An object declared as type signed char occupies the
same amount of storage as a ``plain'' char object. A
``plain'' int object has the natural size suggested by the
architecture of the execution environment (large enough to
contain any value in the range INT_MIN to INT_MAX as defined
in the header <limits.h>).
[#6] For each of the signed integer types, there is a
corresponding (but different) unsigned integer type
(designated with the keyword unsigned) that uses the same
amount of storage (including sign information) and has the
same alignment requirements. The type _Bool and the |
unsigned integer types that correspond to the standard |
signed integer types are the standard unsigned integer
types. The unsigned integer types that correspond to the
extended signed integer types are the extended unsigned
integer types.
[#7] The standard signed integer types and standard unsigned |
integer types are collectively called the standard integer |
types, the extended signed integer types and extended |
unsigned integer types are collectively called the extended
integer types.
[#8] For any two types with the same signedness and
different integer conversion rank (see 6.3.1.1), the range |
of values of the type with smaller integer conversion rank
is a subrange of the values of the other type.
[#9] The range of nonnegative values of a signed integer
type is a subrange of the corresponding unsigned integer
type, and the representation of the same value in each type
is the same.27) A computation involving unsigned operands
____________________
25)Implementation-defined keywords shall have the form of an
identifier reserved for any use as described in 7.1.3.
26)Therefore, any statement in this Standard about signed
integer types also applies to the extended signed integer
types.
6.2.5 Language 6.2.5
WG14/N843 Committee Draft -- August 3, 1998 37
can never overflow, because a result that cannot be
represented by the resulting unsigned integer type is
reduced modulo the number that is one greater than the
largest value that can be represented by the resulting
unsigned integer type.
[#10] There are three real floating types, designated as
float, double, and long double. The set of values of the
type float is a subset of the set of values of the type
double; the set of values of the type double is a subset of
the set of values of the type long double.
[#11] There are three complex types, designated as float
_Complex, double _Complex, and long double _Complex.28) The
real floating and complex types are collectively called the
floating types.
[#12] For each floating type there is a corresponding real
type, which is always a real floating type. For real
floating types, it is the same type. For complex types, it
is the type given by deleting the keyword _Complex from the
type name.
[#13] Each complex type has the same representation and
alignment requirements as an array type containing exactly
two elements of the corresponding real type; the first
element is equal to the real part, and the second element to
the imaginary part, of the complex number.
[#14] The type char, the signed and unsigned integer types,
and the floating types are collectively called the basic
types. Even if the implementation defines two or more basic
types to have the same representation, they are nevertheless
different types.29)
[#15] The three types char, signed char, and unsigned char
are collectively called the character types. The
implementation shall define char to have the same range,
____________________
27)The same representation and alignment requirements are
meant to imply interchangeability as arguments to
functions, return values from functions, and members of
unions.
28)A specification for imaginary types is in informative
annex G.
29)An implementation may define new keywords that provide
alternative ways to designate a basic (or any other)
type; this does not violate the requirement that all
basic types be different. Implementation-defined
keywords shall have the form of an identifier reserved
for any use as described in 7.1.3.
6.2.5 Language 6.2.5
38 Committee Draft -- August 3, 1998 WG14/N843
representation, and behavior as either signed char or
unsigned char.30)
[#16] An enumeration comprises a set of named integer
constant values. Each distinct enumeration constitutes a
different enumerated type.
[#17] The type char, the signed and unsigned integer types, |
and the enumerated types are collectively called integer |
types. The integer and real floating types are collectively |
called real types. |
[#18] The void type comprises an empty set of values; it is
an incomplete type that cannot be completed.
[#19] Any number of derived types can be constructed from
the object, function, and incomplete types, as follows:
-- An array type describes a contiguously allocated
nonempty set of objects with a particular member object
type, called the element type.31) Array types are
characterized by their element type and by the number
of elements in the array. An array type is said to be
derived from its element type, and if its element type
is T, the array type is sometimes called ``array of
T''. The construction of an array type from an element
type is called ``array type derivation''.
-- A structure type describes a sequentially allocated |
nonempty set of member objects (and, in certain |
circumstances, an incomplete array), each of which has
an optionally specified name and possibly distinct
type.
-- A union type describes an overlapping nonempty set of
member objects, each of which has an optionally
specified name and possibly distinct type.
-- A function type describes a function with specified
return type. A function type is characterized by its
return type and the number and types of its parameters.
A function type is said to be derived from its return
type, and if its return type is T, the function type is
sometimes called ``function returning T''. The
____________________
30)CHAR_MIN, defined in <limits.h>, will have one of the
values 0 or SCHAR_MIN, and this can be used to
distinguish the two options. Irrespective of the choice
made, char is a separate type from the other two and is
not compatible with either.
31)Since object types do not include incomplete types, an
array of incomplete type cannot be constructed.
6.2.5 Language 6.2.5
WG14/N843 Committee Draft -- August 3, 1998 39
construction of a function type from a return type is
called ``function type derivation''.
-- A pointer type may be derived from a function type, an
object type, or an incomplete type, called the
referenced type. A pointer type describes an object
whose value provides a reference to an entity of the
referenced type. A pointer type derived from the
referenced type T is sometimes called ``pointer to T''.
The construction of a pointer type from a referenced
type is called ``pointer type derivation''.
[#20] These methods of constructing derived types can be
applied recursively.
[#21] Integer and floating types are collectively called *
arithmetic types. Arithmetic types and pointer types are
collectively called scalar types. Array and structure types
are collectively called aggregate types.32)
[#22] Each arithmetic type belongs to one typedomain. The |
real type domain comprises the real types. The complex type |
domain comprises the complex types.
[#23] An array type of unknown size is an incomplete type.
It is completed, for an identifier of that type, by
specifying the size in a later declaration (with internal or
external linkage). A structure or union type of unknown
content (as described in 6.7.2.3) is an incomplete type. It
is completed, for all declarations of that type, by
declaring the same structure or union tag with its defining
content later in the same scope. A structure type |
containing a flexible array member is an incomplete type |
that cannot be completed.
[#24] Array, function, and pointer types are collectively
called derived declarator types. A declarator type
derivation from a type T is the construction of a derived
declarator type from T by the application of an array-type,
a function-type, or a pointer-type derivation to T.
[#25] A type is characterized by its type category, which is
either the outermost derivation of a derived type (as noted
above in the construction of derived types), or the type
itself if the type consists of no derived types.
[#26] Any type so far mentioned is an unqualified type. Each
unqualified type has several qualified versions of its
type,33) corresponding to the combinations of one, two, or
____________________
32)Note that aggregate type does not include union type
because an object with union type can only contain one
member at a time.
6.2.5 Language 6.2.5
40 Committee Draft -- August 3, 1998 WG14/N843
all three of the const, volatile, and restrict qualifiers.
The qualified or unqualified versions of a type are distinct
types that belong to the same type category and have the
same representation and alignment requirements.27) A
derived type is not qualified by the qualifiers (if any) of
the type from which it is derived.
[#27] A pointer to void shall have the same representation
and alignment requirements as a pointer to a character type.
Similarly, pointers to qualified or unqualified versions of
compatible types shall have the same representation and
alignment requirements.27) All pointers to structure types
shall have the same representation and alignment
requirements as each other. All pointers to union types
shall have the same representation and alignment
requirements as each other. Pointers to other types need
not have the same representation or alignment requirements.
[#28] EXAMPLE 1 The type designated as ``float *'' has type
``pointer to float''. Its type category is pointer, not a
floating type. The const-qualified version of this type is
designated as ``float * const'' whereas the type designated
as ``const float *'' is not a qualified type -- its type is
``pointer to const-qualified float'' and is a pointer to a
qualified type.
[#29] EXAMPLE 2 The type designated as ``struct tag
(*[5])(float)'' has type ``array of pointer to function
returning struct tag''. The array has length five and the
function has a single parameter of type float. Its type
category is array.
Forward references: character constants (6.4.4.4),
compatible type and composite type (6.2.7), declarations *
(6.7), tags (6.7.2.3), type qualifiers (6.7.3).
6.2.6 Representations of types
[#1] The representations of all types are unspecified except
as stated in this subclause.
6.2.6.1 General
[#1] Except for bit-fields, objects are composed of |
contiguous sequences of one or more bytes, the number, |
order, and encoding of which are either explicitly specified |
or implementation-defined. |
[#2] Values stored in objects of type unsigned char shall be
____________________
33)See 6.7.3 regarding qualified array and function types.
6.2.5 Language 6.2.6.1
WG14/N843 Committee Draft -- August 3, 1998 41
represented using a pure binary notation.34)
[#3] Values stored in objects of any other object type |
consist of n×CHAR_BIT bits, where n is the size of an object |
of that type, in bytes. The value may be copied into an
object of type unsigned char [n] (e.g., by memcpy); the
resulting set of bytes is called the object representation
of the value. Two values (other than NaNs) with the same |
object representation compare equal, but values that compare |
equal may have different object representations.
[#4] Certain object representations need not represent a |
value of the object type. If the stored value of an object
has such a representation and is accessed by an lvalue
expression that does not have character type, the behavior
is undefined. If such a representation is produced by a
side effect that modifies all or any part of the object by
an lvalue expression that does not have character type, the
behavior is undefined.35) Such a representation is called a
trap representation.
[#5] When a value is stored in an object of structure or
union type, including in a member object, the bytes of the
object representation that correspond to any padding bytes
take unspecified values.36) The values of padding bytes
shall not affect whether the value of such an object is a
trap representation. Those bits of a structure or union
object that are in the same byte as a bit-field member, but
are not part of that member, shall similarly not affect
whether the value of such an object is a trap
representation.
[#6] When a value is stored in a member of an object of
union type, the bytes of the object representation that do
not correspond to that member but do correspond to other
____________________
34)A positional representation for integers that uses the
binary digits 0 and 1, in which the values represented by
successive bits are additive, begin with 1, and are
multiplied by successive integral powers of 2, except
perhaps the bit with the highest position. (Adapted from
the American National Dictionary for Information
Processing Systems.) A byte contains CHAR_BIT bits, and
the values of type unsigned char range from 0 to
2CHAR_BIT-1.
35)Thus an automatic variable can be initialized to a trap
representation without causing undefined behavior, but
the value of the variable cannot be used until a proper
value is stored in it.
36)Thus, for example, structure assignment may be
implemented element-at-a-time or via memcpy.
6.2.6.1 Language 6.2.6.1
42 Committee Draft -- August 3, 1998 WG14/N843
members take unspecified values, but the value of the union
object shall not thereby become a trap representation.
[#7] Where an operator is applied to a value which has more
than one object representation, which object representation
is used shall not affect the value of the result. Where a
value is stored in an object using a type that has more than
one object representation for that value, it is unspecified
which representation is used, but a trap representation
shall not be generated.
6.2.6.2 Integer types
[#1] For unsigned integer types other than unsigned char,
the bits of the object representation shall be divided into
two groups: value bits and padding bits (there need not be
any of the latter). If there are N value bits, each bit
shall represent a different power of 2 between 1 and 2N-1,
so that objects of that type shall be capable of
representing values from 0 to 2N-1 using a pure binary
representation; this shall be known as the value
representation. The values of any padding bits are
unspecified.37)
[#2] For signed integer types, the bits of the object
representation shall be divided into three groups: value
bits, padding bits, and the sign bit. There need not be any
padding bits; there shall be exactly one sign bit. Each bit
that is a value bit shall have the same value as the same
bit in the object representation of the corresponding
unsigned type (if there are M value bits in the signed type
and N in the unsigned type, then M<=N). If the sign bit is
zero, it shall not affect the resulting value. If the sign
bit is one, then the value shall be modified in one of the
following ways:
-- the corresponding value with sign bit 0 is negated;
-- the sign bit has the value -2N;
-- the sign bit has the value 1-2N.
[#3] The values of any padding bits are unspecified.37) A
valid (non-trap) object representation of a signed integer
____________________
37)Some combinations of padding bits might generate trap
representations, for example, if one padding bit is a
parity bit. Regardless, no arithmetic operation on valid
values can generate a trap representation other than as
part of an exception such as an overflow, and this cannot
occur with unsigned types. All other combinations of
padding bits are alternative object representations of
the value specified by the value bits.
6.2.6.1 Language 6.2.6.2
WG14/N843 Committee Draft -- August 3, 1998 43
type where the sign bit is zero is a valid object
representation of the corresponding unsigned type, and shall
represent the same value.
[#4] The precision of an integer type is the number of bits
it uses to represent values, excluding any sign and padding
bits. The width of an integer type is the same but
including any sign bit; thus for unsigned integer types the
two values are the same, while for signed integer types the
width is one greater than the precision.
6.2.7 Compatible type and composite type
[#1] Two types have compatible type if their types are the
same. Additional rules for determining whether two types
are compatible are described in 6.7.2 for type specifiers,
in 6.7.3 for type qualifiers, and in 6.7.5 for
declarators.38) Moreover, two structure, union, or
enumerated types declared in separate translation units are
compatible if their tags and members satisfy the following
requirements: If one is declared with a tag, the other
shall be declared with the same tag. If both are completed
types, then the following additional requirements apply:
there shall be a one-to-one correspondence between their
members such that each pair of corresponding members are
declared with compatible types, and such that if one member
of a corresponding pair is declared with a name, the other
member is declared with the same name. For two structures,
corresponding members shall be declared in the same order.
For two structures or unions, corresponding bit-fields shall
have the same widths. For two enumerations, corresponding
members shall have the same values.
[#2] All declarations that refer to the same object or
function shall have compatible type; otherwise, the behavior
is undefined.
[#3] A composite type can be constructed from two types that
are compatible; it is a type that is compatible with both of
the two types and satisfies the following conditions:
-- If one type is an array of known constant size, the
composite type is an array of that size; otherwise, if
one type is a variable length array, the composite type
is that type.
-- If only one type is a function type with a parameter
type list (a function prototype), the composite type is
a function prototype with the parameter type list.
____________________
38)Two types need not be identical to be compatible.
6.2.6.2 Language 6.2.7
44 Committee Draft -- August 3, 1998 WG14/N843
-- If both types are function types with parameter type
lists, the type of each parameter in the composite
parameter type list is the composite type of the
corresponding parameters.
These rules apply recursively to the types from which the
two types are derived.
[#4] For an identifier with internal or external linkage
declared in a scope in which a prior declaration of that
identifier is visible,39) if the prior declaration specifies
internal or external linkage, the type of the identifier at
the later declaration becomes the composite type.
[#5] EXAMPLE Given the following two file scope
declarations:
int f(int (*)(), double (*)[3]);
int f(int (*)(char *), double (*)[]);
The resulting composite type for the function is:
int f(int (*)(char *), double (*)[3]);
Forward references: declarators (6.7.5), enumeration
specifiers (6.7.2.2), structure and union specifiers
(6.7.2.1), type definitions (6.7.7), type qualifiers
(6.7.3), type specifiers (6.7.2).
____________________
39)As specified in 6.2.1, the later declaration might hide
the prior declaration.
6.2.7 Language 6.2.7
WG14/N843 Committee Draft -- August 3, 1998 45
6.3 Conversions
[#1] Several operators convert operand values from one type
to another automatically. This subclause specifies the
result required from such an implicit conversion, as well as
those that result from a cast operation (an explicit
conversion). The list in 6.3.1.8 summarizes the conversions
performed by most ordinary operators; it is supplemented as
required by the discussion of each operator in 6.5.
[#2] Conversion of an operand value to a compatible type
causes no change to the value or the representation.
Forward references: cast operators (6.5.4).
6.3.1 Arithmetic operands
6.3.1.1 Boolean, characters, and integers |
[#1] Every integer type has an integer conversion rank
defined as follows:
-- No two signed integer types shall have the same rank,
even if they have the same representation.
-- The rank of a signed integer type shall be greater than
the rank of any signed integer type with less
precision.
-- The rank of long long int shall be greater than the *
rank of long int, which shall be greater than the rank
of int, which shall be greater than the rank of short
int, which shall be greater than the rank of signed
char.
-- The rank of any unsigned integer type shall equal the
rank of the corresponding signed integer type, if any. |
-- The rank of any standard integer type shall be greater |
than the rank of any extended integer type with the |
same width.
-- The rank of char shall equal the rank of signed char
and unsigned char.
-- The rank of _Bool shall be less than the rank of all |
other standard integer types. |
-- The rank of any enumerated type shall equal the rank of
the compatible integer type.
-- The rank of any extended signed integer type relative
to another extended signed integer type with the same
precision is implementation-defined, but still subject
6.3 Language 6.3.1.1
46 Committee Draft -- August 3, 1998 WG14/N843
to the other rules for determining the integer
conversion rank.
-- For all integer types T1, T2, and T3, if T1 has greater
rank than T2 and T2 has greater rank than T3, then T1
has greater rank than T3.
[#2] The following may be used in an expression wherever an
int or unsigned int may be used:
-- An object or expression with an integer type whose
integer conversion rank is less than the rank of int
and unsigned int.
-- A bit-field of type _Bool, int, signed int, or unsigned |
int.
If an int can represent all values of the original type, the |
value is converted to an int; otherwise, it is converted to
an unsigned int. These are called the integer
promotions.40) All other types are unchanged by the integer
promotions.
[#3] The integer promotions preserve value including sign.
As discussed earlier, whether a ``plain'' char is treated as
signed is implementation-defined.
Forward references: enumeration specifiers (6.7.2.2),
structure and union specifiers (6.7.2.1). |
6.3.1.2 Boolean type |
[#1] When any scalar value is converted to _Bool, the result |
is 0 if the value compares equal to 0; otherwise, the result |
is 1.
6.3.1.3 Signed and unsigned integers
[#1] When a value with integer type is converted to another |
integer type other than _Bool, if the value can be
represented by the new type, it is unchanged.
[#2] Otherwise, if the new type is unsigned, the value is
converted by repeatedly adding or subtracting one more than
the maximum value that can be represented in the new type
until the value is in the range of the new type.
____________________
40)The integer promotions are applied only: as part of the |
usual arithmetic conversions, to certain argument
expressions, to the operands of the unary +, -, and ~
operators, and to both operands of the shift operators,
as specified by their respective subclauses.
6.3.1.1 Language 6.3.1.3
WG14/N843 Committee Draft -- August 3, 1998 47
[#3] Otherwise, the new type is signed and the value cannot
be represented in it; the result is implementation-defined.
6.3.1.4 Real floating and integer
[#1] When a finite value of real floating type is converted |
to integer type other than _Bool, the fractional part is |
discarded (i.e., the value is truncated toward zero). If
the value of the integral part cannot be represented by the
integer type, the behavior is undefined.41)
[#2] When a value of integer type is converted to real
floating type, if the value being converted is in the range
of values that can be represented but cannot be represented
exactly, the result is either the nearest higher or nearest
lower value, chosen in an implementation-defined manner. If
the value being converted is outside the range of values
that can be represented, the behavior is undefined.
6.3.1.5 Real floating types
[#1] When a float is promoted to double or long double, or a
double is promoted to long double, its value is unchanged.
[#2] When a double is demoted to float or a long double to
double or float, if the value being converted is outside the
range of values that can be represented, the behavior is
undefined. If the value being converted is in the range of
values that can be represented but cannot be represented
exactly, the result is either the nearest higher or nearest
lower value, chosen in an implementation-defined manner.
6.3.1.6 Complex types
[#1] When a value of complex type is converted to another
complex type, both the real and imaginary parts follow the
conversion rules for the corresponding real types.
6.3.1.7 Real and complex
[#1] When a value of real type is converted to a complex
type, the real part of the complex result value is
determined by the rules of conversion to the corresponding
real type and the imaginary part of the complex result value
is a positive zero or an unsigned zero.
[#2] When a value of complex type is converted to a real
____________________
41)The remaindering operation performed when a value of
integer type is converted to unsigned type need not be
performed when a value of real floating type is converted
to unsigned type. Thus, the range of portable real
floating values is (-1, Utype_MAX+1).
6.3.1.3 Language 6.3.1.7
48 Committee Draft -- August 3, 1998 WG14/N843
type, the imaginary part of the complex value is discarded
and the value of the real part is converted according to the
conversion rules for the corresponding real type.
6.3.1.8 Usual arithmetic conversions
[#1] Many operators that expect operands of arithmetic type
cause conversions and yield result types in a similar way.
The purpose is to determine a common real type for the
operands and result. For the specified operands, each
operand is converted, without change of type domain, to a |
type whose corresponding real type is the common real type.
Unless explicitly stated otherwise, the common real type is
also the corresponding real type of the result, whose type |
domain is determined by the operator. This pattern is
called the usual arithmetic conversions:
First, if the corresponding real type of either operand
is long double, the other operand is converted, without
change of type domain, to a type whose corresponding |
real type is long double.
Otherwise, if the corresponding real type of either
operand is double, the other operand is converted,
without change of type domain, to a type whose |
corresponding real type is double.
Otherwise, if the corresponding real type of either
operand is float, the other operand is converted, |
without change of type domain, to a type whose
corresponding real type is float.42)
Otherwise, the integer promotions are performed on both
operands. Then the following rules are applied to the
promoted operands:
If both operands have the same type, then no
further conversion is needed.
Otherwise, if both operands have signed integer
types or both have unsigned integer types, the
operand with the type of lesser integer conversion
rank is converted to the type of the operand with
greater rank.
Otherwise, if the operand that has unsigned
integer type has rank greater or equal to the rank
of the type of the other operand, then the operand
with signed integer type is converted to the type
____________________
42)For example, addition of a double _Complex and a float
entails just the conversion of the float operand to
double (and yields a double _Complex result).
6.3.1.7 Language 6.3.1.8
WG14/N843 Committee Draft -- August 3, 1998 49
of the operand with unsigned integer type.
Otherwise, if the type of the operand with signed
integer type can represent all of the values of
the type of the operand with unsigned integer
type, then the operand with unsigned integer type
is converted to the type of the operand with
signed integer type.
Otherwise, both operands are converted to the
unsigned integer type corresponding to the type of
the operand with signed integer type.
[#2] The values of floating operands and of the results of
floating expressions may be represented in greater precision
and range than that required by the type; the types are not
changed thereby.43)
6.3.2 Other operands
6.3.2.1 Lvalues and function designators
[#1] An lvalue is an expression with an object type or an |
incomplete type other than void;44) if an lvalue does not |
designate an object when it is evaluated, the behavior is |
undefined. When an object is said to have a particular
type, the type is specified by the lvalue used to designate
the object. A modifiable lvalue is an lvalue that does not
have array type, does not have an incomplete type, does not
have a const-qualified type, and if it is a structure or
union, does not have any member (including, recursively, any
member or element of all contained aggregates or unions)
with a const-qualified type.
[#2] Except when it is the operand of the sizeof operator,
the unary & operator, the ++ operator, the -- operator, or
____________________
43)The cast and assignment operators are still required to
perform their specified conversions as described in
6.3.1.4 and 6.3.1.5.
44)The name ``lvalue'' comes originally from the assignment
expression E1 = E2, in which the left operand E1 is
required to be a (modifiable) lvalue. It is perhaps
better considered as representing an object ``locator
value''. What is sometimes called ``rvalue'' is in this
International Standard described as the ``value of an
expression''.
An obvious example of an lvalue is an identifier of an
object. As a further example, if E is a unary expression
that is a pointer to an object, *E is an lvalue that
designates the object to which E points.
6.3.1.8 Language 6.3.2.1
50 Committee Draft -- August 3, 1998 WG14/N843
the left operand of the . operator or an assignment
operator, an lvalue that does not have array type is
converted to the value stored in the designated object (and
is no longer an lvalue). If the lvalue has qualified type,
the value has the unqualified version of the type of the
lvalue; otherwise, the value has the type of the lvalue. If
the lvalue has an incomplete type and does not have array
type, the behavior is undefined.
[#3] Except when it is the operand of the sizeof operator or
the unary & operator, or is a string literal used to |
initialize an array, an expression that has type ``array of |
type'' is converted to an expression with type ``pointer to |
type'' that points to the initial element of the array
object and is not an lvalue. If the array object has
register storage class, the behavior is undefined.
[#4] A function designator is an expression that has
function type. Except when it is the operand of the sizeof
operator45) or the unary & operator, a function designator
with type ``function returning type'' is converted to an
expression that has type ``pointer to function returning
type''.
Forward references: address and indirection operators
(6.5.3.2), assignment operators (6.5.16), common definitions
<stddef.h> (7.17), initialization (6.7.8), postfix increment
and decrement operators (6.5.2.4), prefix increment and
decrement operators (6.5.3.1), the sizeof operator
(6.5.3.4), structure and union members (6.5.2.3).
6.3.2.2 void
[#1] The (nonexistent) value of a void expression (an
expression that has type void) shall not be used in any way,
and implicit or explicit conversions (except to void) shall
not be applied to such an expression. If an expression of
any other type is evaluated as a void expression, its value |
or designator is discarded. (A void expression is evaluated
for its side effects.)
6.3.2.3 Pointers
[#1] A pointer to void may be converted to or from a pointer
to any incomplete or object type. A pointer to any
incomplete or object type may be converted to a pointer to
void and back again; the result shall compare equal to the
original pointer.
____________________
45)Because this conversion does not occur, the operand of
the sizeof operator remains a function designator and
violates the constraint in 6.5.3.4.
6.3.2.1 Language 6.3.2.3
WG14/N843 Committee Draft -- August 3, 1998 51
[#2] For any qualifier q, a pointer to a non-q-qualified
type may be converted to a pointer to the q-qualified
version of the type; the values stored in the original and
converted pointers shall compare equal.
[#3] An integer constant expression with the value 0, or
such an expression cast to type void *, is called a null
pointer constant.46) If a null pointer constant is assigned
to or compared for equality to a pointer, the constant is
converted to a pointer of that type. Such a pointer, called
a null pointer, is guaranteed to compare unequal to a
pointer to any object or function.
[#4] Conversion of a null pointer to another pointer type
yields a null pointer of that type. Any two null pointers
shall compare equal.
[#5] An integer may be converted to any pointer type. The
result is implementation-defined, might not be properly
aligned, and might not point to an entity of the referenced
type.47)
[#6] Any pointer type may be converted to an integer type;
the result is implementation-defined. If the result cannot
be represented in the integer type, the behavior is
undefined. The result need not be in the range of values of |
any integer type.
[#7] A pointer to an object or incomplete type may be
converted to a pointer to a different object or incomplete
type. If the resulting pointer is not correctly aligned48)
for the pointed-to type, the behavior is undefined. |
Otherwise, when converted back again, the result shall
compare equal to the original pointer. When a pointer to an |
object is converted to a pointer to a character type, the |
result points to the lowest addressed byte of the object. |
Successive increments of the result, up to the size of the |
object, yield pointers to the remaining bytes of the object.
____________________
46)The macro NULL is defined in <stddef.h> as a null pointer
constant; see 7.17.
47)The mapping functions for converting a pointer to an
integer or an integer to a pointer are intended to be
consistent with the addressing structure of the execution
environment.
48)In general, the concept ``correctly aligned'' is |
transitive: if a pointer to type A is correctly aligned
for a pointer to type B, which in turn is correctly
aligned for a pointer to type C, then a pointer to type A
is correctly aligned for a pointer to type C.
6.3.2.3 Language 6.3.2.3
52 Committee Draft -- August 3, 1998 WG14/N843
[#8] A pointer to a function of one type may be converted to
a pointer to a function of another type and back again; the
result shall compare equal to the original pointer. If a |
converted pointer is used to call a function whose type is |
not compatible with the pointed-to type, the behavior is |
undefined.
Forward references: cast operators (6.5.4), equality
operators (6.5.9), simple assignment (6.5.16.1).
6.3.2.3 Language 6.3.2.3
WG14/N843 Committee Draft -- August 3, 1998 53
6.4 Lexical elements
Syntax
[#1]
token:
keyword
identifier
constant
string-literal
punctuator
preprocessing-token:
header-name
identifier
pp-number
character-constant
string-literal
punctuator
each universal-character-name that cannot be one of the above|
each non-white-space character that cannot be one of the above
Constraints
[#2] Each preprocessing token that is converted to a token
shall have the lexical form of a keyword, an identifier, a
constant, a string literal, or a punctuator.
Semantics
[#3] A token is the minimal lexical element of the language
in translation phases 7 and 8. The categories of tokens
are: keywords, identifiers, constants, string literals, and
punctuators. A preprocessing token is the minimal lexical
element of the language in translation phases 3 through 6.
The categories of preprocessing token are: header names,
identifiers, preprocessing numbers, character constants,
string literals, punctuators, and single non-white-space
characters that do not lexically match the other
preprocessing token categories.49) If a ' or a " character
matches the last category, the behavior is undefined.
Preprocessing tokens can be separated by white space; this
consists of comments (described later), or white-space
characters (space, horizontal tab, new-line, vertical tab,
and form-feed), or both. As described in 6.10, in certain
circumstances during translation phase 4, white space (or
the absence thereof) serves as more than preprocessing token
separation. White space may appear within a preprocessing
____________________
49)An additional category, placemarkers, is used internally
in translation phase 4 (see 6.10.3.3); it cannot occur in
source files.
6.4 Language 6.4
54 Committee Draft -- August 3, 1998 WG14/N843
token only as part of a header name or between the quotation
characters in a character constant or string literal.
[#4] If the input stream has been parsed into preprocessing
tokens up to a given character, the next preprocessing token
is the longest sequence of characters that could constitute
a preprocessing token. There is one exception to this rule:
a header name preprocessing token is only recognized within
a #include preprocessing directive, and within such a
directive, a sequence of characters that could be either a
header name or a string literal is recognized as the former.
[#5] EXAMPLE 1 The program fragment 1Ex is parsed as a
preprocessing number token (one that is not a valid floating
or integer constant token), even though a parse as the pair
of preprocessing tokens 1 and Ex might produce a valid
expression (for example, if Ex were a macro defined as +1).
Similarly, the program fragment 1E1 is parsed as a
preprocessing number (one that is a valid floating constant
token), whether or not E is a macro name.
[#6] EXAMPLE 2 The program fragment x+++++y is parsed as
x+++++y, which violates a constraint on increment operators,
even though the parse x+++++y might yield a correct
expression.
Forward references: character constants (6.4.4.4), comments
(6.4.9), expressions (6.5), floating constants (6.4.4.2),
header names (6.4.7), macro replacement (6.10.3), postfix
increment and decrement operators (6.5.2.4), prefix
increment and decrement operators (6.5.3.1), preprocessing
directives (6.10), preprocessing numbers (6.4.8), string
literals (6.4.5).
6.4.1 Keywords
Syntax
[#1]
keyword: one of
auto enum restrict unsigned
break extern return void
case float short volatile
char for signed while
const goto sizeof _Bool |
continue if static _Complex
default inline struct _Imaginary
do int switch
double long typedef
else register union
6.4 Language 6.4.1
WG14/N843 Committee Draft -- August 3, 1998 55
Semantics
[#2] The above tokens (case sensitive) are reserved (in |
translation phases 7 and 8) for use as keywords, and shall
not be used otherwise.
6.4.2 Identifiers
6.4.2.1 General
Syntax
[#1]
identifier: |
identifier-nondigit ||
identifier identifier-nondigit ||
identifier digit ||
identifier-nondigit: |
nondigit
universal-character-name |
other implementation-defined characters |
nondigit: one of
_ a b c d e f g h i j k l m *
n o p q r s t u v w x y z
A B C D E F G H I J K L M
N O P Q R S T U V W X Y Z
digit: one of
0 1 2 3 4 5 6 7 8 9
Semantics |
[#2] An identifier is a sequence of nondigit characters
(including the underscore _, the lowercase and uppercase
Latin letters, and other characters) and digits, which |
designates one or more entities as described in 6.2.1. |
Lower-case and upper-case letters are distinct. There is no |
specific limit on the maximum length of an identifier. |
[#3] Each universal character name in an identifier shall
designate a character whose encoding in ISO/IEC 10646 falls |
into one of the ranges specified in annex I.50) The initial
____________________
50)On systems in which linkers cannot accept extended
characters, an encoding of the universal character name
may be used in forming valid external identifiers. For
example, some otherwise unused character or sequence of
characters may be used to encode the \u in a universal
character name. Extended characters may produce a long
external identifier.
6.4.1 Language 6.4.2.1
56 Committee Draft -- August 3, 1998 WG14/N843
nondigit character shall not be a universal character name
designating a digit. An implementation may allow multibyte |
characters that are not part of the required source |
character set to appear in identifiers; which characters and |
their correspondence to universal character names is |
implementation defined.
[#4] When preprocessing tokens are converted to tokens
during translation phase 7, if a preprocessing token could
be converted to either a keyword or an identifier, it is
converted to a keyword.
Implementation limits
[#5] As discussed in 5.2.4.1, an implementation may limit |
the number of significant initial characters in an |
identifier; the limit for an external name (an identifier
that has external linkage) may be more restrictive than that |
for an internal name (a macro name or an identifier that |
does not have external linkage). The number of significant
characters in an identifier is implementation-defined.
[#6] Any identifiers that differ in a significant character
are different identifiers. If two identifiers differ only
in nonsignificant characters, the behavior is undefined.
Forward references: universal character names (6.4.3), *
macro replacement (6.10.3).
6.4.2.2 Predefined identifiers
Semantics
[#1] The identifier __func__ shall be implicitly declared by
the translator as if, immediately following the opening
brace of each function definition, the declaration
static const char __func__[] = "function-name";
appeared, where function-name is the name of the lexically-
enclosing function.51) *
[#2] This name is encoded as if the implicit declaration had
been written in the source character set and then translated
into the execution character set as indicated in translation
phase 5.
[#3] EXAMPLE Consider the code fragment:
____________________
51)Note that since the name __func__ is reserved for any use
by the implementation (7.1.3), if any other identifier is
explicitly declared using the name __func__, the behavior
is undefined.
6.4.2.1 Language 6.4.2.2
WG14/N843 Committee Draft -- August 3, 1998 57
#include <stdio.h>
void myfunc(void)
{
printf("%s\n", __func__);
/* ... */
}
Each time the function is called, it will print to the
standard output stream:
myfunc
|
Forward references: function definitions (6.9.1).
6.4.3 Universal character names
Syntax
[#1]
universal-character-name:
\u hex-quad
\U hex-quad hex-quad
hex-quad:
hexadecimal-digit hexadecimal-digit
hexadecimal-digit hexadecimal-digit
Constraints
[#2] A universal character name shall not specify a
character short identifier in the range 00000000 through |
00000020, 0000007F through 0000009F, or 0000D800 through |
0000DFFF inclusive. A universal character name shall not
designate a character in the required character set. |
Description
[#3] Universal character names may be used in identifiers,
character constants, and string literals to designate
characters that are not in the required character set. |
Semantics
[#4] The universal character name \Unnnnnnnn designates the
character whose character short identifier (as specified by |
ISO/IEC 10646) is nnnnnnnn. Similarly, the universal
character name \unnnn designates the character whose
character short identifier is 0000nnnn.
6.4.2.2 Language 6.4.3
58 Committee Draft -- August 3, 1998 WG14/N843
6.4.4 Constants
Syntax
[#1]
constant:
integer-constant
floating-constant
enumeration-constant
character-constant
Constraints
[#2] The value of a constant shall be in the range of
representable values for its type.
Semantics
[#3] Each constant has a type, determined by its form and
value, as detailed later.
6.4.4.1 Integer constants
Syntax
[#1]
integer-constant:
decimal-constant integer-suffix-opt
octal-constant integer-suffix-opt
hexadecimal-constant integer-suffix-opt
decimal-constant:
nonzero-digit
decimal-constant digit
octal-constant:
0
octal-constant octal-digit
hexadecimal-constant:
hexadecimal-prefix hexadecimal-digit
hexadecimal-constant hexadecimal-digit
hexadecimal-prefix: one of
0x 0X
nonzero-digit: one of
1 2 3 4 5 6 7 8 9
octal-digit: one of
0 1 2 3 4 5 6 7
6.4.4 Language 6.4.4.1
WG14/N843 Committee Draft -- August 3, 1998 59
hexadecimal-digit: one of
0 1 2 3 4 5 6 7 8 9
a b c d e f
A B C D E F
integer-suffix:
unsigned-suffix long-suffix-opt |
unsigned-suffix long-long-suffix |
long-suffix unsigned-suffix-opt |
long-long-suffix unsigned-suffix-opt |
unsigned-suffix: one of
u U
long-suffix: one of
l L
long-long-suffix: one of
ll LL
Description
[#2] An integer constant begins with a digit, but has no
period or exponent part. It may have a prefix that
specifies its base and a suffix that specifies its type.
[#3] A decimal constant begins with a nonzero digit and
consists of a sequence of decimal digits. An octal constant
consists of the prefix 0 optionally followed by a sequence
of the digits 0 through 7 only. A hexadecimal constant
consists of the prefix 0x or 0X followed by a sequence of
the decimal digits and the letters a (or A) through f (or F)
with values 10 through 15 respectively.
Semantics
[#4] The value of a decimal constant is computed base 10;
that of an octal constant, base 8; that of a hexadecimal
constant, base 16. The lexically first digit is the most
significant.
[#5] The type of an integer constant is the first of the
corresponding list in which its value can be represented.
6.4.4.1 Language 6.4.4.1
60 Committee Draft -- August 3, 1998 WG14/N843
|| |
|| | Octal or Hexadecimal
Suffix || Decimal Constant | Constant
-------------++-----------------------+------------------------
none ||int | int
||long int | unsigned int
||long long int | long int
|| | unsigned long int
|| | long long int
|| | unsigned long long int
-------------++-----------------------+------------------------
u or U ||unsigned int | unsigned int
||unsigned long int | unsigned long int
||unsigned long long int | unsigned long long int
-------------++-----------------------+------------------------
l or L ||long int | long int
||long long int | unsigned long int
|| | long long int
|| | unsigned long long int
-------------++-----------------------+------------------------
Both u or U ||unsigned long int | unsigned long int
and l or L ||unsigned long long int | unsigned long long int
-------------++-----------------------+------------------------
ll or LL ||long long int | long long int
|| | unsigned long long int
-------------++-----------------------+------------------------
Both u or U ||unsigned long long int | unsigned long long int
and ll or LL || |
If an integer constant cannot be represented by any type in
its list, it may have an extended integer type, if the
extended integer type can represent its value. If all of
the types in the list for the constant are signed, the
extended integer type shall be signed. If all of the types
in the list for the constant are unsigned, the extended
integer type shall be unsigned. If the list contains both
signed and unsigned types, the extended integer type may be
signed or unsigned.
6.4.4.2 Floating constants
Syntax
[#1]
floating-constant:
decimal-floating-constant
hexadecimal-floating-constant
decimal-floating-constant:
fractional-constant exponent-part-opt floating-suffix-opt
digit-sequence exponent-part floating-suffix-opt
6.4.4.1 Language 6.4.4.2
WG14/N843 Committee Draft -- August 3, 1998 61
hexadecimal-floating-constant:
hexadecimal-prefix hexadecimal-fractional-constant
binary-exponent-part floating-suffix-opt
hexadecimal-prefix hexadecimal-digit-sequence
binary-exponent-part floating-suffix-opt
fractional-constant:
digit-sequence-opt . digit-sequence
digit-sequence .
exponent-part:
e sign-opt digit-sequence
E sign-opt digit-sequence
sign: one of
+ -
digit-sequence:
digit
digit-sequence digit
hexadecimal-fractional-constant:
hexadecimal-digit-sequence-opt .
hexadecimal-digit-sequence
hexadecimal-digit-sequence .
binary-exponent-part:
p sign-opt digit-sequence
P sign-opt digit-sequence
hexadecimal-digit-sequence:
hexadecimal-digit
hexadecimal-digit-sequence hexadecimal-digit
floating-suffix: one of
f l F L
Description
[#2] A floating constant has a significand part that may be
followed by an exponent part and a suffix that specifies its
type. The components of the significand part may include a
digit sequence representing the whole-number part, followed
by a period (.), followed by a digit sequence representing
the fraction part. The components of the exponent part are
an e, E, p, or P followed by an exponent consisting of an
optionally signed digit sequence. Either the whole-number |
part or the fraction part has to be present; for decimal
floating constants, either the period or the exponent part |
has to be present.
Semantics
[#3] The significand part is interpreted as a (decimal or
6.4.4.2 Language 6.4.4.2
62 Committee Draft -- August 3, 1998 WG14/N843
hexadecimal) rational number; the digit sequence in the
exponent part is interpreted as a decimal integer. For
decimal floating constants, the exponent indicates the power
of 10 by which the significand part is to be scaled. For
hexadecimal floating constants, the exponent indicates the
power of 2 by which the significand part is to be scaled.
For decimal floating constants, and also for hexadecimal
floating constants when FLT_RADIX is not a power of 2, the |
result is either the nearest representable value, or the
larger or smaller representable value immediately adjacent
to the nearest representable value, chosen in an
implementation-defined manner. For hexadecimal floating |
constants when FLT_RADIX is a power of 2, the result is |
correctly rounded.
[#4] An unsuffixed floating constant has type double. If
suffixed by the letter f or F, it has type float. If
suffixed by the letter l or L, it has type long double.
Recommended practice
[#5] The implementation should produce a diagnostic message
if a hexadecimal constant cannot be represented exactly in
its evaluation format; the implementation should then |
proceed with the translation of the program.
[#6] The translation-time conversion of floating constants
should match the execution-time conversion of character
strings by library functions, such as strtod, given matching
inputs suitable for both conversions, the same result
format, and default execution-time rounding.52)
6.4.4.3 Enumeration constants
Syntax
[#1]
enumeration-constant:
identifier
Semantics
[#2] An identifier declared as an enumeration constant has
type int.
Forward references: enumeration specifiers (6.7.2.2).
____________________
52)The specification for the library functions recommends
more accurate conversion than required for floating
constants (see 7.20.1.3).
6.4.4.2 Language 6.4.4.3
WG14/N843 Committee Draft -- August 3, 1998 63
6.4.4.4 Character constants
Syntax
[#1]
character-constant:
'c-char-sequence'
L'c-char-sequence'
c-char-sequence:
c-char
c-char-sequence c-char
c-char: *
any member of the source character set except
the single-quote ', backslash \, or new-line character
escape-sequence
escape-sequence:
simple-escape-sequence
octal-escape-sequence
hexadecimal-escape-sequence
universal-character-name |
simple-escape-sequence: one of
\' \" \? \\
\a \b \f \n \r \t \v
octal-escape-sequence:
\ octal-digit
\ octal-digit octal-digit
\ octal-digit octal-digit octal-digit
hexadecimal-escape-sequence:
\x hexadecimal-digit
hexadecimal-escape-sequence hexadecimal-digit
Description
[#2] An integer character constant is a sequence of one or
more multibyte characters enclosed in single-quotes, as in
'x' or 'ab'. A wide character constant is the same, except
prefixed by the letter L. With a few exceptions detailed
later, the elements of the sequence are any members of the
source character set; they are mapped in an implementation-
defined manner to members of the execution character set.
[#3] The single-quote ', the double-quote ", the question-
mark ?, the backslash \, and arbitrary integer values, are
representable according to the following table of escape
sequences:
6.4.4.4 Language 6.4.4.4
64 Committee Draft -- August 3, 1998 WG14/N843
single quote ' \'
double quote " \"
question mark ? \?
backslash \ \\
octal character \octal digits
hexadecimal character \xhexadecimal digits
[#4] The double-quote " and question-mark ? are
representable either by themselves or by the escape
sequences \" and \?, respectively, but the single-quote '
and the backslash \ shall be represented, respectively, by
the escape sequences \' and \\.
[#5] The octal digits that follow the backslash in an octal
escape sequence are taken to be part of the construction of
a single character for an integer character constant or of a
single wide character for a wide character constant. The
numerical value of the octal integer so formed specifies the
value of the desired character or wide character.
[#6] The hexadecimal digits that follow the backslash and
the letter x in a hexadecimal escape sequence are taken to
be part of the construction of a single character for an
integer character constant or of a single wide character for
a wide character constant. The numerical value of the
hexadecimal integer so formed specifies the value of the
desired character or wide character.
[#7] Each octal or hexadecimal escape sequence is the
longest sequence of characters that can constitute the
escape sequence.
[#8] In addition, graphic characters not in the required |
character set are representable by universal character names |
and certain nongraphic characters are representable by
escape sequences consisting of the backslash \ followed by a
lowercase letter: \a, \b, \f, \n, \r, \t, and \v.53)
Constraints
[#9] The value of an octal or hexadecimal escape sequence
shall be in the range of representable values for the type
unsigned char for an integer character constant, or the
unsigned type corresponding to wchar_t for a wide character
constant.
____________________
53)The semantics of these characters were discussed in
5.2.2. If any other character follows a backslash, the
result is not a token and a diagnostic is required. See
``future language directions'' (6.11.1).
6.4.4.4 Language 6.4.4.4
WG14/N843 Committee Draft -- August 3, 1998 65
Semantics
[#10] An integer character constant has type int. The value
of an integer character constant containing a single
character that maps to a member of the basic execution
character set is the numerical value of the representation
of the mapped character interpreted as an integer. The
value of an integer character constant containing more than
one character, or containing a character or escape sequence
not represented in the basic execution character set, is
implementation-defined. If an integer character constant
contains a single character or escape sequence, its value is
the one that results when an object with type char whose
value is that of the single character or escape sequence is
converted to type int.
[#11] A wide character constant has type wchar_t, an integer
type defined in the <stddef.h> header. The value of a wide
character constant containing a single multibyte character
that maps to a member of the extended execution character
set is the wide character (code) corresponding to that
multibyte character, as defined by the mbtowc function, with
an implementation-defined current locale. The value of a
wide character constant containing more than one multibyte
character, or containing a multibyte character or escape
sequence not represented in the extended execution character
set, is implementation-defined.
[#12] EXAMPLE 1 The construction '\0' is commonly used to
represent the null character.
[#13] EXAMPLE 2 Consider implementations that use two's-
complement representation for integers and eight bits for
objects that have type char. In an implementation in which
type char has the same range of values as signed char, the
integer character constant '\xFF' has the value -1; if type
char has the same range of values as unsigned char, the
character constant '\xFF' has the value +255 .
[#14] EXAMPLE 3 Even if eight bits are used for objects that
have type char, the construction '\x123' specifies an
integer character constant containing only one character,
since a hexadecimal escape sequence is terminated only by a
non-hexadecimal character. To specify an integer character
constant containing the two characters whose values are
'\x12' and '3', the construction '\0223' may be used, since
an octal escape sequence is terminated after three octal
digits. (The value of this two-character integer character
constant is implementation-defined.)
[#15] EXAMPLE 4 Even if 12 or more bits are used for objects
6.4.4.4 Language 6.4.4.4
66 Committee Draft -- August 3, 1998 WG14/N843
that have type wchar_t, the construction L'\1234' specifies
the implementation-defined value that results from the
combination of the values 0123 and '4'.
Forward references: common definitions <stddef.h> (7.17), *
the mbtowc function (7.20.7.2).
6.4.5 String literals
Syntax
[#1]
string-literal:
"s-char-sequence-opt"
L"s-char-sequence-opt"
s-char-sequence:
s-char
s-char-sequence s-char
s-char: *
any member of the source character set except
the double-quote ", backslash \, or new-line character
escape-sequence
Description
[#2] A character string literal is a sequence of zero or
more multibyte characters enclosed in double-quotes, as in
"xyz". A wide string literal is the same, except prefixed
by the letter L.
[#3] The same considerations apply to each element of the
sequence in a character string literal or a wide string
literal as if it were in an integer character constant or a
wide character constant, except that the single-quote ' is
representable either by itself or by the escape sequence \',
but the double-quote " shall be represented by the escape
sequence \".
Semantics
[#4] In translation phase 6, the multibyte character
sequences specified by any sequence of adjacent character
and wide string literal tokens are concatenated into a
single multibyte character sequence. If any of the tokens
are wide string literal tokens, the resulting multibyte
character sequence is treated as a wide string literal;
otherwise, it is treated as a character string literal.
[#5] In translation phase 7, a byte or code of value zero is
appended to each multibyte character sequence that results
6.4.4.4 Language 6.4.5
WG14/N843 Committee Draft -- August 3, 1998 67
from a string literal or literals.54) The multibyte
character sequence is then used to initialize an array of
static storage duration and length just sufficient to
contain the sequence. For character string literals, the
array elements have type char, and are initialized with the
individual bytes of the multibyte character sequence; for
wide string literals, the array elements have type wchar_t,
and are initialized with the sequence of wide characters |
corresponding to the multibyte character sequence, as |
defined by the mbstowcs function with an implementation- |
defined current locale. The value of a string literal |
containing a multibyte character or escape sequence not |
represented in the execution character set is |
implementation-defined.
[#6] It is unspecified whether these arrays are distinct |
provided their elements have the appropriate values. If the
program attempts to modify such an array, the behavior is
undefined.
[#7] EXAMPLE This pair of adjacent character string
literals
"\x12" "3"
produces a single character string literal containing the
two characters whose values are '\x12' and '3', because
escape sequences are converted into single members of the
execution character set just prior to adjacent string
literal concatenation.
Forward references: common definitions <stddef.h> (7.17).
6.4.6 Punctuators
Syntax
[#1]
punctuator: one of
[ ] ( ) { } . ->
++ -- & * + - ~ !
/ % << >> < > <= >= == != ^ | && ||
? : ; ...
= *= /= %= += -= <<= >>= &= ^= |=
, # ## |
<: :> <% %> %: %:%: |
____________________
54)A character string literal need not be a string (see
7.1.1), because a null character may be embedded in it by
a \0 escape sequence.
6.4.5 Language 6.4.6
68 Committee Draft -- August 3, 1998 WG14/N843
Semantics
[#2] A punctuator is a symbol that has independent syntactic
and semantic significance. Depending on context, it may
specify an operation to be performed (which in turn may |
yield a value or a function designator, produce a side
effect, or some combination thereof) in which case it is
known as an operator (other forms of operator also exist in
some contexts). An operand is an entity on which an
operator acts.
[#3] In all aspects of the language, these six tokens
<: :> <% %> %: %:%:
behave, respectively, the same as these six tokens
[ ] { } # ##
except for their spelling.55)
Forward references: expressions (6.5), declarations (6.7),
preprocessing directives (6.10), statements (6.8).
6.4.7 Header names
Syntax
[#1]
header-name:
<h-char-sequence>
"q-char-sequence"
h-char-sequence:
h-char
h-char-sequence h-char
h-char:
any member of the source character set except
the new-line character and >
q-char-sequence:
q-char
q-char-sequence q-char
q-char:
any member of the source character set except
the new-line character and "
____________________
55)Thus [ and <: behave differently when ``stringized'' (see
6.10.3.2), but can otherwise be freely interchanged.
6.4.6 Language 6.4.7
WG14/N843 Committee Draft -- August 3, 1998 69
Semantics
[#2] The sequences in both forms of header names are mapped
in an implementation-defined manner to headers or external
source file names as specified in 6.10.2.
[#3] If the characters ', \, ", //, or /* occur in the
sequence between the < and > delimiters, the behavior is
undefined. Similarly, if the characters ', \, //, or /*
occur in the sequence between the " delimiters, the behavior
is undefined.56) A header name preprocessing token is
recognized only within a #include preprocessing directive.
[#4] EXAMPLE The following sequence of characters:
0x3<1/a.h>1e2
#include <1/a.h>
#define const.member@$
forms the following sequence of preprocessing tokens (with
each individual preprocessing token delimited by a { on the
left and a } on the right).
{0x3}{<}{1}{/}{a}{.}{h}{>}{1e2}
{#}{include} {<1/a.h>}
{#}{define} {const}{.}{member}{@}{$}
Forward references: source file inclusion (6.10.2).
6.4.8 Preprocessing numbers
Syntax
[#1]
pp-number:
digit
. digit
pp-number digit
pp-number identifier-nondigit |
pp-number e sign
pp-number E sign
pp-number p sign
pp-number P sign
pp-number .
Description
____________________
56)Thus, sequences of characters that resemble escape
sequences cause undefined behavior.
6.4.7 Language 6.4.8
70 Committee Draft -- August 3, 1998 WG14/N843
[#2] A preprocessing number begins with a digit optionally
preceded by a period (.) and may be followed by letters,
underscores, digits, periods, and e+, e-, E+, E-, p+, p-,
P+, or P- character sequences.
[#3] Preprocessing number tokens lexically include all
floating and integer constant tokens.
Semantics
[#4] A preprocessing number does not have type or a value;
it acquires both after a successful conversion (as part of
translation phase 7) to a floating constant token or an
integer constant token.
6.4.9 Comments
[#1] Except within a character constant, a string literal,
or a comment, the characters /* introduce a comment. The
contents of a comment are examined only to identify
multibyte characters and to find the characters */ that
terminate it.57)
[#2] Except within a character constant, a string literal,
or a comment, the characters // introduce a comment that
includes all multibyte characters up to, but not including,
the next new-line character. The contents of such a comment
are examined only to identify multibyte characters and to
find the terminating new-line character.
[#3] EXAMPLE 1
"a//b" // four-character string literal|
#include "//e" // undefined behavior |
// */ // comment, not syntax error|
f = g/**//h; // equivalent to f = g / h; |
//\ |
i(); // part of a two-line comment|
/\ |
/ j(); // part of a two-line comment|
#define glue(x,y) x##y
glue(/,/) k(); // syntax error, not comment|
/*//*/ l(); // equivalent to l(); |
m = n//**/o
+ p; // equivalent to m = n + p; |
____________________
57)Thus, /* ... */ comments do not nest.
6.4.8 Language 6.4.9
WG14/N843 Committee Draft -- August 3, 1998 71
6.5 Expressions
[#1] An expression is a sequence of operators and operands
that specifies computation of a value, or that designates an
object or a function, or that generates side effects, or
that performs a combination thereof.
[#2] Between the previous and next sequence point an object
shall have its stored value modified at most once by the
evaluation of an expression. Furthermore, the prior value
shall be accessed only to determine the value to be |
stored.58) Informative annex D presents an algorithm for |
determining whether an expression or set of expressions |
meets these requirements.
[#3] The grouping of operators and operands is indicated by |
the syntax.59) Except as specified later (for the function- |
call (), &&, ||, ?:, and comma operators), the order of
evaluation of subexpressions and the order in which side
effects take place are both unspecified.
[#4] Some operators (the unary operator ~, and the binary
operators <<, >>, &, ^, and |, collectively described as
bitwise operators) are required to have operands that have |
integer type. These operators return values that depend on
the internal representations of integers, and have
implementation-defined and undefined aspects for signed
____________________
58)This paragraph renders undefined statement expressions
such as
i = ++i + 1;
a[i++] = i; |
while allowing
i = i + 1;
a[i] = i; |
59)The syntax specifies the precedence of operators in the
evaluation of an expression, which is the same as the
order of the major subclauses of this subclause, highest
precedence first. Thus, for example, the expressions
allowed as the operands of the binary + operator (6.5.6) |
are those expressions defined in 6.5.1 through 6.5.6.
The exceptions are cast expressions (6.5.4) as operands
of unary operators (6.5.3), and an operand contained
between any of the following pairs of operators: grouping
parentheses () (6.5.1), subscripting brackets []
(6.5.2.1), function-call parentheses () (6.5.2.2), and
the conditional operator ?: (6.5.15).
Within each major subclause, the operators have the same
precedence. Left- or right-associativity is indicated in
each subclause by the syntax for the expressions
discussed therein.
6.5 Language 6.5
72 Committee Draft -- August 3, 1998 WG14/N843
types.
[#5] If an exception occurs during the evaluation of an
expression (that is, if the result is not mathematically
defined or not in the range of representable values for its
type), the behavior is undefined.
[#6] The effective type of an object for an access to its
stored value is the declared type of the object, if any.60) |
If a value is stored into an object having no declared type
through an lvalue having a type that is not a character
type, then the type of the lvalue becomes the effective type
of the object for that access and for subsequent accesses
that do not modify the stored value. If a value is copied
into an object having no declared type using memcpy or
memmove, or is copied as an array of character type, then
the effective type of the modified object for that access
and for subsequent accesses that do not modify the value is
the effective type of the object from which the value is
copied, if it has one. For all other accesses to an object
having no declared type, the effective type of the object is
simply the type of the lvalue used for the access.
[#7] An object shall have its stored value accessed only by
an lvalue expression that has one of the following types:61)
-- a type compatible with the effective type of the
object,
-- a qualified version of a type compatible with the
effective type of the object,
-- a type that is the signed or unsigned type
corresponding to the effective type of the object,
-- a type that is the signed or unsigned type
corresponding to a qualified version of the effective
type of the object,
-- an aggregate or union type that includes one of the
aforementioned types among its members (including,
recursively, a member of a subaggregate or contained
union), or
-- a character type.
[#8] A floating expression may be contracted, that is,
evaluated as though it were an atomic operation, thereby
____________________
60)Allocated objects have no declared type.
61)The intent of this list is to specify those circumstances
in which an object may or may not be aliased.
6.5 Language 6.5
WG14/N843 Committee Draft -- August 3, 1998 73
omitting rounding errors implied by the source code and the
expression evaluation method.62) The FP_CONTRACT pragma in
<math.h> provides a way to disallow contracted expressions.
Otherwise, whether and how expressions are contracted is
implementation-defined.63)
6.5.1 Primary expressions
Syntax
[#1]
primary-expr:
identifier
constant
string-literal
( expression )
Semantics
[#2] An identifier is a primary expression, provided it has
been declared as designating an object (in which case it is
an lvalue) or a function (in which case it is a function
designator).64)
[#3] A constant is a primary expression. Its type depends
on its form and value, as detailed in 6.4.4.
[#4] A string literal is a primary expression. It is an
lvalue with type as detailed in 6.4.5.
[#5] A parenthesized expression is a primary expression.
Its type and value are identical to those of the
unparenthesized expression. It is an lvalue, a function
designator, or a void expression if the unparenthesized
expression is, respectively, an lvalue, a function
designator, or a void expression.
____________________
62)A contracted expression might also omit the raising of
floating-point exception flags.
63)This license is specifically intended to allow
implementations to exploit fast machine instructions that
combine multiple C operators. As contractions
potentially undermine predictability, and can even
decrease accuracy for containing expressions, their use
needs to be well-defined and clearly documented.
64)Thus, an undeclared identifier is a violation of the
syntax.
6.5 Language 6.5.1
74 Committee Draft -- August 3, 1998 WG14/N843
Forward references: declarations (6.7).
6.5.2 Postfix operators
Syntax
[#1]
postfix-expr:
primary-expr
postfix-expr [ expression ]
postfix-expr ( argument-expression-list-opt )
postfix-expr . identifier
postfix-expr -> identifier
postfix-expr ++
postfix-expr --
( type-name ) { initializer-list }
( type-name ) { initializer-list , }
argument-expression-list:
assignment-expr
argument-expression-list , assignment-expr
6.5.2.1 Array subscripting
Constraints
[#1] One of the expressions shall have type ``pointer to
object type'', the other expression shall have integer type,
and the result has type ``type''.
Semantics
[#2] A postfix expression followed by an expression in
square brackets [] is a subscripted designation of an
element of an array object. The definition of the subscript
operator [] is that E1[E2] is identical to (*((E1)+(E2))). |
Because of the conversion rules that apply to the binary +
operator, if E1 is an array object (equivalently, a pointer
to the initial element of an array object) and E2 is an
integer, E1[E2] designates the E2-th element of E1 (counting
from zero).
[#3] Successive subscript operators designate an element of
a multidimensional array object. If E is an n-dimensional
array (n>=2) with dimensions i×j× ... ×k, then E (used as
other than an lvalue) is converted to a pointer to an
(n-1)-dimensional array with dimensions j× ... ×k. If the
unary * operator is applied to this pointer explicitly, or
implicitly as a result of subscripting, the result is the
pointed-to (n-1)-dimensional array, which itself is
converted into a pointer if used as other than an lvalue.
It follows from this that arrays are stored in row-major
order (last subscript varies fastest).
6.5.1 Language 6.5.2.1
WG14/N843 Committee Draft -- August 3, 1998 75
[#4] EXAMPLE Consider the array object defined by the
declaration
int x[3][5];
Here x is a 3×5 array of ints; more precisely, x is an array
of three element objects, each of which is an array of five
ints. In the expression x[i], which is equivalent to |
(*((x)+(i))), x is first converted to a pointer to the
initial array of five ints. Then i is adjusted according to
the type of x, which conceptually entails multiplying i by
the size of the object to which the pointer points, namely
an array of five int objects. The results are added and
indirection is applied to yield an array of five ints. When
used in the expression x[i][j], that array is in turn |
converted to a pointer to the first of the ints, so x[i][j]
yields an int.
Forward references: additive operators (6.5.6), address and
indirection operators (6.5.3.2), array declarators
(6.7.5.2).
6.5.2.2 Function calls
Constraints
[#1] The expression that denotes the called function65)
shall have type pointer to function returning void or
returning an object type other than an array type.
[#2] If the expression that denotes the called function has
a type that includes a prototype, the number of arguments
shall agree with the number of parameters. Each argument
shall have a type such that its value may be assigned to an
object with the unqualified version of the type of its
corresponding parameter.
Semantics
[#3] A postfix expression followed by parentheses ()
containing a possibly empty, comma-separated list of
expressions is a function call. The postfix expression
denotes the called function. The list of expressions
specifies the arguments to the function.
[#4] An argument may be an expression of any object type.
In preparing for the call to a function, the arguments are
evaluated, and each parameter is assigned the value of the |
corresponding argument.66) |
____________________
65)Most often, this is the result of converting an
identifier that is a function designator.
6.5.2.1 Language 6.5.2.2
76 Committee Draft -- August 3, 1998 WG14/N843
[#5] If the expression that denotes the called function has
type pointer to function returning an object type, the
function call expression has the same type as that object
type, and has the value determined as specified in 6.8.6.4.
Otherwise, the function call has type void. If an attempt |
is made to modify the result of a function call or to access |
it after the next sequence point, the behavior is undefined.
[#6] If the expression that denotes the called function has
a type that does not include a prototype, the integer
promotions are performed on each argument, and arguments
that have type float are promoted to double. These are
called the default argument promotions. If the number of
arguments does not agree with the number of parameters, the
behavior is undefined. If the function is defined with a
type that includes a prototype, and either the prototype
ends with an ellipsis (, ...) or the types of the arguments
after promotion are not compatible with the types of the
parameters, the behavior is undefined. If the function is
defined with a type that does not include a prototype, and
the types of the arguments after promotion are not
compatible with those of the parameters after promotion, the
behavior is undefined, except for the following cases:
-- one promoted type is a signed integer type, the other
promoted type is the corresponding unsigned integer
type, and the value is representable in both types;
-- one type is pointer to void and the other is a pointer
to a character type.
[#7] If the expression that denotes the called function has |
a type that does include a prototype, the arguments are
implicitly converted, as if by assignment, to the types of
the corresponding parameters, taking the type of each
parameter to be the unqualified version of its declared
type. The ellipsis notation in a function prototype
declarator causes argument type conversion to stop after the
last declared parameter. The default argument promotions
are performed on trailing arguments.
[#8] No other conversions are performed implicitly; in
particular, the number and types of arguments are not
compared with those of the parameters in a function
definition that does not include a function prototype
____________________
66)A function may change the values of its parameters, but
these changes cannot affect the values of the arguments.
On the other hand, it is possible to pass a pointer to an
object, and the function may change the value of the
object pointed to. A parameter declared to have array or
function type is converted to a parameter with a pointer
type as described in 6.9.1.
6.5.2.2 Language 6.5.2.2
WG14/N843 Committee Draft -- August 3, 1998 77
declarator.
[#9] If the function is defined with a type that is not
compatible with the type (of the expression) pointed to by
the expression that denotes the called function, the
behavior is undefined.
[#10] The order of evaluation of the function designator, |
the actual arguments, and subexpressions within the actual |
arguments is unspecified, but there is a sequence point
before the actual call.
[#11] Recursive function calls shall be permitted, both
directly and indirectly through any chain of other
functions.
[#12] EXAMPLE In the function call
(*pf[f1()]) (f2(), f3() + f4())
the functions f1, f2, f3, and f4 may be called in any order. |
All side effects have to be completed before the function
pointed to by pf[f1()] is called. |
Forward references: function declarators (including
prototypes) (6.7.5.3), function definitions (6.9.1), the
return statement (6.8.6.4), simple assignment (6.5.16.1).
6.5.2.3 Structure and union members
Constraints
[#1] The first operand of the . operator shall have a
qualified or unqualified structure or union type, and the
second operand shall name a member of that type.
[#2] The first operand of the -> operator shall have type
``pointer to qualified or unqualified structure'' or
``pointer to qualified or unqualified union'', and the
second operand shall name a member of the type pointed to.
Semantics
[#3] A postfix expression followed by the . operator and an
identifier designates a member of a structure or union
object. The value is that of the named member, and is an
lvalue if the first expression is an lvalue. If the first
expression has qualified type, the result has the so-
qualified version of the type of the designated member.
[#4] A postfix expression followed by the -> operator and an
identifier designates a member of a structure or union
object. The value is that of the named member of the object
6.5.2.2 Language 6.5.2.3
78 Committee Draft -- August 3, 1998 WG14/N843
to which the first expression points, and is an lvalue.67)
If the first expression is a pointer to a qualified type,
the result has the so-qualified version of the type of the
designated member.
[#5] With one exception, if the value of a member of a union
object is used when the most recent store to the object was
to a different member, the behavior is
implementation-defined.68) One special guarantee is made in
order to simplify the use of unions: If a union contains
several structures that share a common initial sequence (see
below), and if the union object currently contains one of
these structures, it is permitted to inspect the common
initial part of any of them anywhere that a declaration of
the completed type of the union is visible. Two structures
share a common initial sequence if corresponding members
have compatible types (and, for bit-fields, the same widths)
for a sequence of one or more initial members.
[#6] EXAMPLE 1 If f is a function returning a structure or
union, and x is a member of that structure or union, f().x
is a valid postfix expression but is not an lvalue.
[#7] EXAMPLE 2 In: |
struct s { int i; const int ci; };
struct s s;
const struct s cs;
volatile struct s vs;
the various members have the types: |
s.i int
s.ci const int
cs.i const int
cs.ci const int
vs.i volatile int
vs.ci volatile const int
|
____________________
67)If &E is a valid pointer expression (where & is the
``address-of'' operator, which generates a pointer to its
operand), the expression (&E)->MOS is the same as E.MOS.
68)The ``byte orders'' for scalar types are invisible to
isolated programs that do not indulge in type punning
(for example, by assigning to one member of a union and
inspecting the storage by accessing another member that
is an appropriately sized array of character type), but
have to be accounted for when conforming to externally
imposed storage layouts.
6.5.2.3 Language 6.5.2.3
WG14/N843 Committee Draft -- August 3, 1998 79
[#8] EXAMPLE 3 The following is a valid fragment:
union {
struct {
int alltypes;
} n;
struct {
int type;
int intnode;
} ni;
struct {
int type;
double doublenode;
} nf;
} u;
u.nf.type = 1;
u.nf.doublenode = 3.14;
/* ... */
if (u.n.alltypes == 1)
if (sin(u.nf.doublenode) == 0.0)
/* ... */
The following is not a valid fragment (because the union *
type is not visible within function f):
struct t1 { int m; };
struct t2 { int m; };
int f(struct t1 * p1, struct t2 * p2)
{
if (p1->m < 0)
p2->m = -p2->m;
return p1->m;
}
int g()
{
union {
struct t1 s1;
struct t2 s2;
} u;
/* ... */
return f(&u.s1, &u.s2);
}
Forward references: address and indirection operators
(6.5.3.2), structure and union specifiers (6.7.2.1).
6.5.2.3 Language 6.5.2.3
80 Committee Draft -- August 3, 1998 WG14/N843
6.5.2.4 Postfix increment and decrement operators
Constraints
[#1] The operand of the postfix increment or decrement
operator shall have qualified or unqualified real or pointer
type and shall be a modifiable lvalue.
Semantics
[#2] The result of the postfix ++ operator is the value of
the operand. After the result is obtained, the value of the
operand is incremented. (That is, the value 1 of the
appropriate type is added to it.) See the discussions of
additive operators and compound assignment for information
on constraints, types, and conversions and the effects of
operations on pointers. The side effect of updating the
stored value of the operand shall occur between the previous
and the next sequence point.
[#3] The postfix -- operator is analogous to the postfix ++
operator, except that the value of the operand is
decremented (that is, the value 1 of the appropriate type is
subtracted from it).
Forward references: additive operators (6.5.6), compound
assignment (6.5.16.2).
6.5.2.5 Compound literals
Constraints
[#1] The type name shall specify an object type or an array
of unknown size.
[#2] No initializer shall attempt to provide a value for an
object not contained within the entire unnamed object
specified by the compound literal.
[#3] If the compound literal occurs outside the body of a
function, the initializer list shall consist of constant
expressions.
Semantics
[#4] A postfix expression that consists of a parenthesized
type name followed by a brace-enclosed list of initializers
is a compound literal. It provides an unnamed object whose
value is given by the initializer list.69)
[#5] If the type name specifies an array of unknown size,
the size is determined by the initializer list as specified
in 6.7.7, and the type of the compound literal is that of
the completed array type. Otherwise (when the type name
specifies an object type), the type of the compound literal
is that specified by the type name. In either case, the
result is an lvalue.
[#6] The value of the compound literal is that of an unnamed
WG14/N843 Committee Draft -- August 3, 1998 81
object initialized by the initializer list. The object has
static storage duration if and only if the compound literal
occurs outside the body of a function; otherwise, it has
automatic storage duration associated with the enclosing
block.
[#7] All the semantic rules and constraints for initializer
lists in 6.7.8 are applicable to compound literals.70)
[#8] String literals, and compound literals with const-
qualified types, need not designate distinct objects.71)
[#9] EXAMPLE 1 The file scope definition
int *p = (int []){2, 4};
initializes p to point to the first element of an array of
two ints, the first having the value two and the second,
four. The expressions in this compound literal are required
to be constant. The unnamed object has static storage
duration.
[#10] EXAMPLE 2 In contrast, in
void f(void)
{
int *p;
/*...*/
p = (int [2]){*p};
/*...*/
}
p is assigned the address of the first element of an array
of two ints, the first having the value previously pointed
to by p and the second, zero. The expressions in this
compound literal need not be constant. The unnamed object
has automatic storage duration.
[#11] EXAMPLE 3 Initializers with designations can be
____________________
69)Note that this differs from a cast expression. For
example, a cast specifies a conversion to scalar types or
void only, and the result of a cast expression is not an
lvalue.
70)For example, subobjects without explicit initializers are
initialized to zero.
71)This allows implementations to share storage for string
literals and constant compound literals with the same or
overlapping representations.
6.5.2.5 Language 6.5.2.5
82 Committee Draft -- August 3, 1998 WG14/N843
combined with compound literals. Structure objects created
using compound literals can be passed to functions without
depending on member order:
drawline((struct point){.x=1, .y=1},
(struct point){.x=3, .y=4});
Or, if drawline instead expected pointers to struct point:
drawline(&(struct point){.x=1, .y=1},
&(struct point){.x=3, .y=4});
[#12] EXAMPLE 4 A read-only compound literal can be
specified through constructions like:
(const float []){1e0, 1e1, 1e2, 1e3, 1e4, 1e5, 1e6}
[#13] EXAMPLE 5 The following three expressions have
different meanings:
"/tmp/fileXXXXXX"
(char []){"/tmp/fileXXXXXX"}
(const char []){"/tmp/fileXXXXXX"}
The first always has static storage duration and has type
array of char, but need not be modifiable; the last two have
automatic storage duration when they occur within the body
of a function, and the first of these two is modifiable.
[#14] EXAMPLE 6 Like string literals, const-qualified
compound literals can be placed into read-only memory and
can even be shared. For example,
(const char []){"abc"} == "abc"
might yield 1 if the literals' storage is shared.
[#15] EXAMPLE 7 Since compound literals are unnamed, a
single compound literal cannot specify a circularly linked
object. For example, there is no way to write a self-
referential compound literal that could be used as the
function argument in place of the named object endless_zeros
below:
struct int_list { int car; struct int_list *cdr; };
struct int_list endless_zeros = {0, &endless_zeros};
eval(endless_zeros);
6.5.2.5 Language 6.5.2.5
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[#16] EXAMPLE 8 Each compound literal creates only a single *
object in a given scope:
struct s { int i; };
int f (void)
{
struct s *p = 0, *q;
int j = 0; |
while (j < 2) |
q = p, p = &((struct s){ j++ }); |
return p == q && q->i == 1; |
}
The function f() always returns the value 1.
[#17] Note that if a for loop were used instead of a while |
loop, the lifetime of the unnamed object would be the body
of the loop only, and on entry next time around p would be
pointing to an object which is no longer guaranteed to |
exist, which would result in undefined behavior.
6.5.3 Unary operators
Syntax
[#1]
unary-expr:
postfix-expr
++ unary-expr
-- unary-expr
unary-operator cast-expr
sizeof unary-expr
sizeof ( type-name )
unary-operator: one of
& * + - ~ !
6.5.3.1 Prefix increment and decrement operators
Constraints
[#1] The operand of the prefix increment or decrement
operator shall have qualified or unqualified real or pointer
type and shall be a modifiable lvalue.
Semantics
[#2] The value of the operand of the prefix ++ operator is
6.5.2.5 Language 6.5.3.1
84 Committee Draft -- August 3, 1998 WG14/N843
incremented. The result is the new value of the operand
after incrementation. The expression ++E is equivalent to
(E+=1). See the discussions of additive operators and
compound assignment for information on constraints, types,
side effects, and conversions and the effects of operations
on pointers.
[#3] The prefix -- operator is analogous to the prefix ++
operator, except that the value of the operand is
decremented.
Forward references: additive operators (6.5.6), compound
assignment (6.5.16.2).
6.5.3.2 Address and indirection operators
Constraints
[#1] The operand of the unary & operator shall be either a
function designator, the result of a [] or unary * operator,
or an lvalue that designates an object that is not a bit-
field and is not declared with the register storage-class
specifier.
[#2] The operand of the unary * operator shall have pointer
type.
Semantics
[#3] The result of the unary & (address-of) operator is a
pointer to the object or function designated by its operand.
If the operand has type ``type'', the result has type
``pointer to type''. If the operand is the result of a
unary * operator, neither that operator nor the & operator |
is evaluated, and the result is as if both were omitted, |
except that the constraints on the operators still apply and |
the result is not an lvalue. Similarly, if the operand is
the result of a [] operator, neither the & operator nor the
unary * that is implied by the [] is evaluated, and the |
result is as if the & operator were removed and the [] |
operator were changed to a + operator.
[#4] The unary * operator denotes indirection. If the
operand points to a function, the result is a function
designator; if it points to an object, the result is an
lvalue designating the object. If the operand has type
``pointer to type'', the result has type ``type''. If an
invalid value has been assigned to the pointer, the behavior
of the unary * operator is undefined.72)
Forward references: storage-class specifiers (6.7.1),
structure and union specifiers (6.7.2.1).
6.5.3.1 Language 6.5.3.2
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6.5.3.3 Unary arithmetic operators
Constraints
[#1] The operand of the unary + or - operator shall have
arithmetic type; of the ~ operator, integer type; of the !
operator, scalar type.
Semantics
[#2] The result of the unary + operator is the value of its |
(promoted) operand. The integer promotions are performed on
the operand, and the result has the promoted type.
[#3] The result of the unary - operator is the negative of |
its (promoted) operand. The integer promotions are
performed on the operand, and the result has the promoted
type.
[#4] The result of the ~ operator is the bitwise complement |
of its (promoted) operand (that is, each bit in the result
is set if and only if the corresponding bit in the converted
operand is not set). The integer promotions are performed
on the operand, and the result has the promoted type. If |
the promoted type is an unsigned type, the expression ~E is |
equivalent to the maximum value representable in that type |
minus E.
[#5] The result of the logical negation operator ! is 0 if
the value of its operand compares unequal to 0, 1 if the
value of its operand compares equal to 0. The result has
type int. The expression !E is equivalent to (0==E).
Forward references: characteristics of floating types
<float.h> (7.7), sizes of integer types <limits.h> (7.10).
____________________
72)Thus, &*E is equivalent to E (even if E is a null |
pointer), and &(E1[E2]) to ((E1)+(E2)). It is always |
true that if E is a function designator or an lvalue that
is a valid operand of the unary & operator, *&E is a
function designator or an lvalue equal to E. If *P is an
lvalue and T is the name of an object pointer type, *(T)P
is an lvalue that has a type compatible with that to
which T points.
Among the invalid values for dereferencing a pointer by
the unary * operator are a null pointer, an address
inappropriately aligned for the type of object pointed
to, and the address of an automatic storage duration
object when execution of the block with which the object
is associated has terminated.
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6.5.3.4 The sizeof operator
Constraints
[#1] The sizeof operator shall not be applied to an
expression that has function type or an incomplete type, to
the parenthesized name of such a type, or to an lvalue that
designates a bit-field object.
Semantics
[#2] The sizeof operator yields the size (in bytes) of its
operand, which may be an expression or the parenthesized
name of a type. The size is determined from the type of the
operand. The result is an integer. If the type of the
operand is a variable length array type, the operand is
evaluated; otherwise, the operand is not evaluated and the
result is an integer constant.
[#3] When applied to an operand that has type char, unsigned
char, or signed char, (or a qualified version thereof) the
result is 1. When applied to an operand that has array
type, the result is the total number of bytes in the
array.73) When applied to an operand that has structure or
union type, the result is the total number of bytes in such
an object, including internal and trailing padding.
[#4] The value of the result is implementation-defined, and
its type (an unsigned integer type) is size_t, defined in |
the <stddef.h> header.
[#5] EXAMPLE 1 A principal use of the sizeof operator is in
communication with routines such as storage allocators and
I/O systems. A storage-allocation function might accept a
size (in bytes) of an object to allocate and return a
pointer to void. For example:
extern void *alloc(size_t);
double *dp = alloc(sizeof *dp);
The implementation of the alloc function should ensure that
its return value is aligned suitably for conversion to a
pointer to double.
[#6] EXAMPLE 2 Another use of the sizeof operator is to
compute the number of elements in an array:
____________________
73)When applied to a parameter declared to have array or
function type, the sizeof operator yields the size of the
pointer obtained by converting as in 6.3.2.1; see 6.9.1.
6.5.3.3 Language 6.5.3.4
WG14/N843 Committee Draft -- August 3, 1998 87
sizeof array / sizeof array[0]
[#7] EXAMPLE 3 In this example, the size of a variable-
length array is computed and returned from a function:
size_t fsize3 (int n)
{
char b[n+3]; // Variable length array.
return sizeof b; // Execution time sizeof.
}
int main()
{
size_t size;
size = fsize3(10); // fsize3 returns 13.
return 0;
}
Forward references: common definitions <stddef.h> (7.17),
declarations (6.7), structure and union specifiers
(6.7.2.1), type names (6.7.6), array declarators (6.7.5.2).
6.5.4 Cast operators
Syntax
[#1]
cast-expr:
unary-expr
( type-name ) cast-expr
Constraints
[#2] Unless the type name specifies a void type, the type
name shall specify qualified or unqualified scalar type and
the operand shall have scalar type.
[#3] Conversions that involve pointers, other than where
permitted by the constraints of 6.5.16.1, shall be specified
by means of an explicit cast.
Semantics
[#4] Preceding an expression by a parenthesized type name
converts the value of the expression to the named type.
This construction is called a cast.74) A cast that
specifies no conversion has no effect on the type or value
of an expression.75)
Forward references: equality operators (6.5.9), function
declarators (including prototypes) (6.7.5.3), simple
assignment (6.5.16.1), type names (6.7.6).
88 Committee Draft -- August 3, 1998 WG14/N843
6.5.5 Multiplicative operators
Syntax
[#1]
multiplicative-expr:
cast-expr
multiplicative-expr * cast-expr
multiplicative-expr / cast-expr
multiplicative-expr % cast-expr
Constraints
[#2] Each of the operands shall have arithmetic type. The
operands of the % operator shall have integer type.
Semantics
[#3] The usual arithmetic conversions are performed on the
operands. If either operand has complex type, the result
has complex type.
[#4] The result of the binary * operator is the product of
the operands.
[#5] The result of the / operator is the quotient from the
division of the first operand by the second; the result of
the % operator is the remainder. In both operations, if the
value of the second operand is zero, the behavior is
undefined.
[#6] When integers are divided, the result of the / operator
is the algebraic quotient with any fractional part
discarded.76) If the quotient a/b is representable, the
expression (a/b)*b + a%b shall equal a.
____________________
75)If the value of the expression is represented with
greater precision or range than required by the type
named by the cast (6.3.1.8), then the cast specifies a
conversion even if the type of the expression is the same
as the named type.
76)This is often called ``truncation toward zero''.
6.5.5 Language 6.5.5
WG14/N843 Committee Draft -- August 3, 1998 89
6.5.6 Additive operators
Syntax
[#1]
additive-expr:
multiplicative-expr
additive-expr + multiplicative-expr
additive-expr - multiplicative-expr
Constraints
[#2] For addition, either both operands shall have
arithmetic type, or one operand shall be a pointer to an
object type and the other shall have integer type.
(Incrementing is equivalent to adding 1.)
[#3] For subtraction, one of the following shall hold:
-- both operands have arithmetic type;
-- both operands are pointers to qualified or unqualified
versions of compatible object types; or
-- the left operand is a pointer to an object type and the
right operand has integer type. *
(Decrementing is equivalent to subtracting 1.) |
Semantics
[#4] If both operands have arithmetic type, the usual
arithmetic conversions are performed on them. If either
operand has complex type, the result has complex type.
[#5] The result of the binary + operator is the sum of the
operands.
[#6] The result of the binary - operator is the difference
resulting from the subtraction of the second operand from
the first.
[#7] For the purposes of these operators, a pointer to a
nonarray object behaves the same as a pointer to the first
element of an array of length one with the type of the
object as its element type.
[#8] When an expression that has integer type is added to or
subtracted from a pointer, the result has the type of the
pointer operand. If the pointer operand points to an
element of an array object, and the array is large enough,
the result points to an element offset from the original
element such that the difference of the subscripts of the
6.5.6 Language 6.5.6
90 Committee Draft -- August 3, 1998 WG14/N843
resulting and original array elements equals the integer
expression. In other words, if the expression P points to
the i-th element of an array object, the expressions (P)+N
(equivalently, N+(P)) and (P)-N (where N has the value n)
point to, respectively, the i+n-th and i-n-th elements of
the array object, provided they exist. Moreover, if the
expression P points to the last element of an array object,
the expression (P)+1 points one past the last element of the
array object, and if the expression Q points one past the
last element of an array object, the expression (Q)-1 points
to the last element of the array object. If both the
pointer operand and the result point to elements of the same
array object, or one past the last element of the array
object, the evaluation shall not produce an overflow;
otherwise, the behavior is undefined. If the result points |
one past the last element of the array object, it shall not |
be used as the operand of a unary * operator that is |
evaluated.
[#9] When two pointers are subtracted, both shall point to |
elements of the same array object, or one past the last |
element of the array object; the result is the difference of
the subscripts of the two array elements. The size of the
result is implementation-defined, and its type (a signed
integer type) is ptrdiff_t defined in the <stddef.h> header.
If the result is not representable in an object of that
type, the behavior is undefined. In other words, if the
expressions P and Q point to, respectively, the i-th and j-
th elements of an array object, the expression (P)-(Q) has
the value i-j provided the value fits in an object of type
ptrdiff_t. Moreover, if the expression P points either to
an element of an array object or one past the last element
of an array object, and the expression Q points to the last
element of the same array object, the expression ((Q)+1)-(P)
has the same value as ((Q)-(P))+1 and as -((P)-((Q)+1)), and
has the value zero if the expression P points one past the
last element of the array object, even though the expression
(Q)+1 does not point to an element of the array object.77) |
____________________
77)Another way to approach pointer arithmetic is first to
convert the pointer(s) to character pointer(s): In this
scheme the integer expression added to or subtracted from
the converted pointer is first multiplied by the size of
the object originally pointed to, and the resulting
pointer is converted back to the original type. For
pointer subtraction, the result of the difference between
the character pointers is similarly divided by the size
of the object originally pointed to.
When viewed in this way, an implementation need only
provide one extra byte (which may overlap another object
in the program) just after the end of the object in order
to satisfy the ``one past the last element''
requirements.
6.5.6 Language 6.5.6
WG14/N843 Committee Draft -- August 3, 1998 91
[#10] EXAMPLE Pointer arithmetic is well defined with
pointers to variable length array types.
{
int n = 4, m = 3;
int a[n][m];
int (*p)[m] = a; // p == &a[0]
p += 1; // p == &a[1]
(*p)[2] = 99; // a[1][2] == 99
n = p - a; // n == 1
}
[#11] If array a in the above example were declared to be an |
array of known constant size, and pointer p were declared to |
be a pointer to an array of the same known constant size |
(pointing to a), the results would be the same.
Forward references: array declarators (6.7.5.2), common
definitions <stddef.h> (7.17).
6.5.7 Bitwise shift operators
Syntax
[#1]
shift-expr:
additive-expr
shift-expr << additive-expr
shift-expr >> additive-expr
Constraints
[#2] Each of the operands shall have integer type.
Semantics
[#3] The integer promotions are performed on each of the
operands. The type of the result is that of the promoted
left operand. If the value of the right operand is negative |
or is greater than or equal to the width of the promoted
left operand, the behavior is undefined.
[#4] The result of E1 << E2 is E1 left-shifted E2 bit
positions; vacated bits are filled with zeros. If E1 has an
unsigned type, the value of the result is E1×2E2, reduced |
modulo one more than the maximum value representable in the |
result type. If E1 has a signed type and nonnegative value,
and E1×2E2 is representable in the result type, then that is |
the resulting value; otherwise, the behavior is undefined.
[#5] The result of E1 >> E2 is E1 right-shifted E2 bit
positions. If E1 has an unsigned type or if E1 has a signed
6.5.6 Language 6.5.7
92 Committee Draft -- August 3, 1998 WG14/N843
type and a nonnegative value, the value of the result is the
integral part of the quotient of E1 divided by the quantity,
2 raised to the power E2. If E1 has a signed type and a
negative value, the resulting value is implementation-
defined.
6.5.8 Relational operators
Syntax
[#1]
relational-expr:
shift-expr
relational-expr < shift-expr
relational-expr > shift-expr
relational-expr <= shift-expr
relational-expr >= shift-expr
Constraints
[#2] One of the following shall hold:
-- both operands have real type;
-- both operands are pointers to qualified or unqualified
versions of compatible object types; or
-- both operands are pointers to qualified or unqualified
versions of compatible incomplete types.
Semantics
[#3] If both of the operands have arithmetic type, the usual
arithmetic conversions are performed.
[#4] For the purposes of these operators, a pointer to a
nonarray object behaves the same as a pointer to the first
element of an array of length one with the type of the
object as its element type.
[#5] When two pointers are compared, the result depends on
the relative locations in the address space of the objects
pointed to. If two pointers to object or incomplete types
both point to the same object, or both point one past the
last element of the same array object, they compare equal.
If the objects pointed to are members of the same aggregate
object, pointers to structure members declared later compare
greater than pointers to members declared earlier in the
structure, and pointers to array elements with larger
subscript values compare greater than pointers to elements
of the same array with lower subscript values. All pointers
to members of the same union object compare equal. If the
expression P points to an element of an array object and the
6.5.7 Language 6.5.8
WG14/N843 Committee Draft -- August 3, 1998 93
expression Q points to the last element of the same array
object, the pointer expression Q+1 compares greater than P.
In all other cases, the behavior is undefined.
[#6] Each of the operators < (less than), > (greater than),
<= (less than or equal to), and >= (greater than or equal
to) shall yield 1 if the specified relation is true and 0 if
it is false.78) The result has type int.
6.5.9 Equality operators
Syntax
[#1]
equality-expr:
relational-expr
equality-expr == relational-expr
equality-expr != relational-expr
Constraints
[#2] One of the following shall hold:
-- both operands have arithmetic type;
-- both operands are pointers to qualified or unqualified
versions of compatible types;
-- one operand is a pointer to an object or incomplete
type and the other is a pointer to a qualified or
unqualified version of void; or
-- one operand is a pointer and the other is a null
pointer constant.
Semantics
[#3] The == (equal to) and != (not equal to) operators are |
analogous to the relational operators except for their lower
precedence.79) Each of the operators yields 1 if the |
specified relation is true and 0 if it is false. The result |
has type int. For any pair of operands, exactly one of the |
relations is true.
____________________
78)The expression a<b<c is not interpreted as in ordinary
mathematics. As the syntax indicates, it means (a<b)<c;
in other words, ``if a is less than b, compare 1 to c; |
otherwise, compare 0 to c''.
79)Because of the precedences, a<b == c<d is 1 whenever a<b
and c<d have the same truth-value.
6.5.8 Language 6.5.9
94 Committee Draft -- August 3, 1998 WG14/N843
[#4] If both of the operands have arithmetic type, the usual |
arithmetic conversions are performed. Values of complex
types are equal if and only if both their real parts are
equal and also their imaginary parts are equal. Any two |
values of arithmetic types from different type domains are
equal if and only if the results of their conversion to the
complex type corresponding to the common real type
determined by the usual arithmetic conversions are equal. |
[#5] Otherwise, at least one operand is a pointer. If one |
operand is a null pointer constant, it is converted to the |
type of the other operand. If one operand is a pointer to |
an object or incomplete type and the other is a pointer to a |
qualified or unqualified version of void, the former is |
converted to the type of the latter. |
[#6] Two pointers compare equal if both are null pointers, |
both are pointers to the same object (including a pointer to |
an object and a subobject at its beginning) or function, |
both are pointers to one past the last element of the same |
array object, or one is a pointer to one past the end of one |
array object and the other is a pointer to the start of a |
different array object that happens to immediately follow |
the first array object in the address space.80)
6.5.10 Bitwise AND operator
Syntax
[#1]
AND-expr:
equality-expr
AND-expr & equality-expr
Constraints
[#2] Each of the operands shall have integer type.
Semantics
[#3] The usual arithmetic conversions are performed on the
operands.
____________________
80)Two objects may be adjacent in memory because they are
adjacent elements of a larger array or adjacent members
of a structure with no padding between them, or because
the implementation chose to place them so, even though
they are unrelated. If prior invalid pointer operations,
such as accesses outside array bounds, produced undefined
behavior, the effect of subsequent comparisons is also
undefined.
6.5.9 Language 6.5.10
WG14/N843 Committee Draft -- August 3, 1998 95
[#4] The result of the binary & operator is the bitwise AND
of the operands (that is, each bit in the result is set if
and only if each of the corresponding bits in the converted
operands is set).
6.5.11 Bitwise exclusive OR operator
Syntax
[#1]
exclusive-OR-expr:
AND-expr
exclusive-OR-expr ^ AND-expr
Constraints
[#2] Each of the operands shall have integer type.
Semantics
[#3] The usual arithmetic conversions are performed on the
operands.
[#4] The result of the ^ operator is the bitwise exclusive
OR of the operands (that is, each bit in the result is set
if and only if exactly one of the corresponding bits in the
converted operands is set).
6.5.12 Bitwise inclusive OR operator
Syntax
[#1]
inclusive-OR-expr:
exclusive-OR-expr
inclusive-OR-expr | exclusive-OR-expr
Constraints
[#2] Each of the operands shall have integer type.
Semantics
[#3] The usual arithmetic conversions are performed on the
operands.
[#4] The result of the | operator is the bitwise inclusive
OR of the operands (that is, each bit in the result is set
if and only if at least one of the corresponding bits in the
converted operands is set).
6.5.10 Language 6.5.12
96 Committee Draft -- August 3, 1998 WG14/N843
6.5.13 Logical AND operator
Syntax
[#1]
logical-AND-expr:
inclusive-OR-expr
logical-AND-expr && inclusive-OR-expr
Constraints
[#2] Each of the operands shall have scalar type.
Semantics
[#3] The && operator shall yield 1 if both of its operands
compare unequal to 0; otherwise, it yields 0. The result
has type int.
[#4] Unlike the bitwise binary & operator, the && operator
guarantees left-to-right evaluation; there is a sequence
point after the evaluation of the first operand. If the
first operand compares equal to 0, the second operand is not
evaluated.
6.5.14 Logical OR operator
Syntax
[#1]
logical-OR-expr:
logical-AND-expr
logical-OR-expr || logical-AND-expr
Constraints
[#2] Each of the operands shall have scalar type.
Semantics
[#3] The || operator shall yield 1 if either of its operands
compare unequal to 0; otherwise, it yields 0. The result
has type int.
[#4] Unlike the bitwise | operator, the || operator
guarantees left-to-right evaluation; there is a sequence
point after the evaluation of the first operand. If the
first operand compares unequal to 0, the second operand is
not evaluated.
6.5.13 Language 6.5.14
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6.5.15 Conditional operator
Syntax
[#1]
conditional-expr:
logical-OR-expr
logical-OR-expr ? expr : conditional-expr
Constraints
[#2] The first operand shall have scalar type.
[#3] One of the following shall hold for the second and
third operands:
-- both operands have arithmetic type;
-- both operands have compatible structure or union types;
-- both operands have void type;
-- both operands are pointers to qualified or unqualified
versions of compatible types;
-- one operand is a pointer and the other is a null
pointer constant; or
-- one operand is a pointer to an object or incomplete
type and the other is a pointer to a qualified or
unqualified version of void.
Semantics
[#4] The first operand is evaluated; there is a sequence
point after its evaluation. The second operand is evaluated
only if the first compares unequal to 0; the third operand
is evaluated only if the first compares equal to 0; the
result is the value of the second or third operand
(whichever is evaluated), converted to the type described
below.81) If an attempt is made to modify the result of a |
conditional operator or to access it after the next sequence |
point, the behavior is undefined.
[#5] If both the second and third operands have arithmetic
type, the type that the usual arithmetic conversions would
yield if applied to those two operands is the type of the
result. If both the operands have structure or union type,
the result has that type. If both operands have void type,
the result has void type.
____________________
81)A conditional expression does not yield an lvalue.
6.5.15 Language 6.5.15
98 Committee Draft -- August 3, 1998 WG14/N843
[#6] If both the second and third operands are pointers or
one is a null pointer constant and the other is a pointer,
the result type is a pointer to a type qualified with all
the type qualifiers of the types pointed-to by both
operands. Furthermore, if both operands are pointers to
compatible types or to differently qualified versions of
compatible types, the result type is a pointer to an
appropriately qualified version of the composite type; if
one operand is a null pointer constant, the result has the
type of the other operand; otherwise, one operand is a
pointer to void or a qualified version of void, in which
case the result type is a pointer to an appropriately
qualified version of void.
[#7] EXAMPLE The common type that results when the second
and third operands are pointers is determined in two
independent stages. The appropriate qualifiers, for
example, do not depend on whether the two pointers have
compatible types.
[#8] Given the declarations
const void *c_vp;
void *vp;
const int *c_ip;
volatile int *v_ip;
int *ip;
const char *c_cp;
the third column in the following table is the common type
that is the result of a conditional expression in which the
first two columns are the second and third operands (in
either order):
c_vp c_ip const void *
v_ip 0 volatile int *
c_ip v_ip const volatile int *
vp c_cp const void *
ip c_ip const int *
vp ip void *
6.5.16 Assignment operators
Syntax
[#1]
assignment-expr:
conditional-expr
unary-expr assignment-operator assignment-expr
6.5.15 Language 6.5.16
WG14/N843 Committee Draft -- August 3, 1998 99
assignment-operator: one of
= *= /= %= += -= <<= >>= &= ^= |=
Constraints
[#2] An assignment operator shall have a modifiable lvalue
as its left operand.
Semantics
[#3] An assignment operator stores a value in the object
designated by the left operand. An assignment expression
has the value of the left operand after the assignment, but
is not an lvalue. The type of an assignment expression is
the type of the left operand unless the left operand has
qualified type, in which case it is the unqualified version
of the type of the left operand. The side effect of
updating the stored value of the left operand shall occur
between the previous and the next sequence point.
[#4] The order of evaluation of the operands is unspecified. |
If an attempt is made to modify the result of an assignment |
operator or to access it after the next sequence point, the |
behavior is undefined.
6.5.16.1 Simple assignment
Constraints
[#1] One of the following shall hold:82)
-- the left operand has qualified or unqualified
arithmetic type and the right has arithmetic type;
-- the left operand has a qualified or unqualified version
of a structure or union type compatible with the type
of the right;
-- both operands are pointers to qualified or unqualified
versions of compatible types, and the type pointed to
by the left has all the qualifiers of the type pointed
to by the right;
-- one operand is a pointer to an object or incomplete
type and the other is a pointer to a qualified or
unqualified version of void, and the type pointed to by
the left has all the qualifiers of the type pointed to
____________________
82)The asymmetric appearance of these constraints with
respect to type qualifiers is due to the conversion
(specified in 6.3.2.1) that changes lvalues to ``the
value of the expression'' which removes any type
qualifiers from the type category of the expression.
6.5.16 Language 6.5.16.1
100 Committee Draft -- August 3, 1998 WG14/N843
by the right; or
-- the left operand is a pointer and the right is a null
pointer constant.
Semantics
[#2] In simple assignment (=), the value of the right
operand is converted to the type of the assignment
expression and replaces the value stored in the object
designated by the left operand.
[#3] If the value being stored in an object is accessed from
another object that overlaps in any way the storage of the
first object, then the overlap shall be exact and the two
objects shall have qualified or unqualified versions of a
compatible type; otherwise, the behavior is undefined.
[#4] EXAMPLE 1 In the program fragment
int f(void);
char c;
/* ... */
if ((c = f()) == -1)
/* ... */
the int value returned by the function may be truncated when
stored in the char, and then converted back to int width
prior to the comparison. In an implementation in which
``plain'' char has the same range of values as unsigned char
(and char is narrower than int), the result of the
conversion cannot be negative, so the operands of the
comparison can never compare equal. Therefore, for full
portability, the variable c should be declared as int.
[#5] EXAMPLE 2 In the fragment:
char c;
int i;
long l;
l = (c = i);
the value of i is converted to the type of the assignment-
expression c = i, that is, char type. The value of the
expression enclosed in parentheses is then converted to the
type of the outer assignment-expression, that is, long int
type.
|
[#6] EXAMPLE 3 Consider the fragment: |
6.5.16.1 Language 6.5.16.1
WG14/N843 Committee Draft -- August 3, 1998 101
const char **cpp;
char *p;
const char c = 'A';
cpp = &p; // constraint violation
*cpp = &c; // valid
*p = 0; // valid
The first assignment is unsafe because it would allow the |
following valid code to attempt to change the value of the |
const object c. |
6.5.16.2 Compound assignment
Constraints
[#1] For the operators += and -= only, either the left
operand shall be a pointer to an object type and the right
shall have integer type, or the left operand shall have
qualified or unqualified arithmetic type and the right shall
have arithmetic type.
[#2] For the other operators, each operand shall have
arithmetic type consistent with those allowed by the
corresponding binary operator.
Semantics
[#3] A compound assignment of the form E1 op= E2 differs
from the simple assignment expression E1 = E1 op (E2) only
in that the lvalue E1 is evaluated only once.
6.5.17 Comma operator
Syntax
[#1]
expression:
assignment-expr
expression , assignment-expr
Semantics
[#2] The left operand of a comma operator is evaluated as a
void expression; there is a sequence point after its
evaluation. Then the right operand is evaluated; the result
has its type and value.83) If an attempt is made to modify |
the result of a comma operator or to access it after the |
next sequence point, the behavior is undefined.
____________________
83)A comma operator does not yield an lvalue.
6.5.16.1 Language 6.5.17
102 Committee Draft -- August 3, 1998 WG14/N843
[#3] EXAMPLE As indicated by the syntax, the comma operator
(as described in this subclause) cannot appear in contexts
where a comma is used to separate items in a list (such as
arguments to functions or lists of initializers). On the
other hand, it can be used within a parenthesized expression
or within the second expression of a conditional operator in
such contexts. In the function call
f(a, (t=3, t+2), c)
the function has three arguments, the second of which has
the value 5.
Forward references: initialization (6.7.8).
6.5.17 Language 6.5.17
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6.6 Constant expressions
Syntax
[#1]
constant-expression:
conditional-expression
Description
[#2] A constant expression can be evaluated during
translation rather than runtime, and accordingly may be used
in any place that a constant may be.
Constraints
[#3] Constant expressions shall not contain assignment,
increment, decrement, function-call, or comma operators, |
except when they are contained within a subexpression that |
is not evaluated.84)
[#4] Each constant expression shall evaluate to a constant
that is in the range of representable values for its type.
Semantics
[#5] An expression that evaluates to a constant is required
in several contexts. If a floating expression is evaluated
in the translation environment, the arithmetic precision and
range shall be at least as great as if the expression were
being evaluated in the execution environment.
[#6] An integer constant expression85) shall have integer
type and shall only have operands that are integer
constants, enumeration constants, character constants,
sizeof expressions whose results are integer constants, and |
floating constants that are the immediate operands of casts.
Cast operators in an integer constant expression shall only
convert arithmetic types to integer types, except as part of
an operand to the sizeof operator.
____________________
84)The operand of a sizeof operator is usually not evaluated |
(6.5.3.4).
85)An integer constant expression is used to specify the
size of a bit-field member of a structure, the value of
an enumeration constant, the size of an array, or the
value of a case constant. Further constraints that apply
to the integer constant expressions used in conditional-
inclusion preprocessing directives are discussed in
6.10.1.
6.6 Language 6.6
104 Committee Draft -- August 3, 1998 WG14/N843
[#7] More latitude is permitted for constant expressions in
initializers. Such a constant expression shall be, or
evaluate to, one of the following:
-- an arithmetic constant expression,
-- a null pointer constant,
-- an address constant, or
-- an address constant for an object type plus or minus an
integer constant expression.
[#8] An arithmetic constant expression shall have arithmetic
type and shall only have operands that are integer
constants, floating constants, enumeration constants,
character constants, and sizeof expressions. Cast operators
in an arithmetic constant expression shall only convert
arithmetic types to arithmetic types, except as part of an
operand to the sizeof operator.
[#9] An address constant is a null pointer, a pointer to an
lvalue designating an object of static storage duration, or
to a function designator; it shall be created explicitly
using the unary & operator or an integer constant cast to
pointer type, or implicitly by the use of an expression of
array or function type. The array-subscript [] and member-
access . and -> operators, the address & and indirection *
unary operators, and pointer casts may be used in the
creation of an address constant, but the value of an object
shall not be accessed by use of these operators.
[#10] An implementation may accept other forms of constant
expressions.
[#11] The semantic rules for the evaluation of a constant
expression are the same as for nonconstant expressions.86)
Forward references: array declarators (6.7.5.2),
initialization (6.7.8).
____________________
86)Thus, in the following initialization,
static int i = 2 || 1 / 0;
the expression is a valid integer constant expression
with value one.
6.6 Language 6.6
WG14/N843 Committee Draft -- August 3, 1998 105
6.7 Declarations
Syntax
[#1]
declaration:
declaration-specifiers init-declarator-list-opt ;
declaration-specifiers:
storage-class-specifier declaration-specifiers-opt
type-specifier declaration-specifiers-opt
type-qualifier declaration-specifiers-opt
function-specifier declaration-specifiers-opt
init-declarator-list:
init-declarator
init-declarator-list , init-declarator
init-declarator:
declarator
declarator = initializer
Constraints
[#2] A declaration shall declare at least a declarator |
(other than the parameters of a function or the members of a
structure or union), a tag, or the members of an
enumeration.
[#3] If an identifier has no linkage, there shall be no more
than one declaration of the identifier (in a declarator or
type specifier) with the same scope and in the same name
space, except for tags as specified in 6.7.2.3.
[#4] All declarations in the same scope that refer to the
same object or function shall specify compatible types.
Semantics
[#5] A declaration specifies the interpretation and
attributes of a set of identifiers. A definition of an
identifier is a declaration for that identifier that:
-- for an object, causes storage to be reserved for that
object;
-- for a function, includes the function body;87)
____________________
87)Function definitions have a different syntax, described
in 6.9.1.
6.7 Language 6.7
106 Committee Draft -- August 3, 1998 WG14/N843
-- for an enumeration constant or typedef name, is the
(only) declaration of the identifier.
[#6] The declaration specifiers consist of a sequence of
specifiers that indicate the linkage, storage duration, and
part of the type of the entities that the declarators
denote. The init-declarator-list is a comma-separated
sequence of declarators, each of which may have additional
type information, or an initializer, or both. The
declarators contain the identifiers (if any) being declared.
[#7] If an identifier for an object is declared with no
linkage, the type for the object shall be complete by the
end of its declarator, or by the end of its init-declarator
if it has an initializer.
Forward references: declarators (6.7.5), enumeration
specifiers (6.7.2.2), initialization (6.7.8), tags
(6.7.2.3).
6.7.1 Storage-class specifiers
Syntax
[#1]
storage-class-specifier:
typedef
extern
static
auto
register
Constraints
[#2] At most, one storage-class specifier may be given in
the declaration specifiers in a declaration.88)
Semantics
[#3] The typedef specifier is called a ``storage-class
specifier'' for syntactic convenience only; it is discussed
in 6.7.7. The meanings of the various linkages and storage
durations were discussed in 6.2.2 and 6.2.4.
[#4] A declaration of an identifier for an object with
storage-class specifier register suggests that access to the
object be as fast as possible. The extent to which such
suggestions are effective is implementation-defined.89)
[#5] The declaration of an identifier for a function that
____________________
88)See ``future language directions'' (6.11.2).
6.7 Language 6.7.1
WG14/N843 Committee Draft -- August 3, 1998 107
has block scope shall have no explicit storage-class
specifier other than extern.
[#6] If an aggregate or union object is declared with a
storage-class specifier other than typedef, the properties
resulting from the storage-class specifier, except with
respect to linkage, also apply to the members of the object,
and so on recursively for any aggregate or union member
objects.
Forward references: type definitions (6.7.7).
6.7.2 Type specifiers
Syntax
[#1]
type-specifier:
void
char
short
int
long
float
double
signed
unsigned
_Bool |
_Complex
_Imaginary
struct-or-union-specifier
enum-specifier
typedef-name
Constraints
[#2] At least one type specifier shall be given in the
declaration specifiers in each declaration, and in the |
specifier-qualifier list in each struct declaration and type |
name. Each list of type specifiers shall be one of the
following sets (delimited by commas, when there is more than
____________________
89)The implementation may treat any register declaration
simply as an auto declaration. However, whether or not
addressable storage is actually used, the address of any
part of an object declared with storage-class specifier
register cannot be computed, either explicitly (by use of
the unary & operator as discussed in 6.5.3.2) or
implicitly (by converting an array name to a pointer as
discussed in 6.3.2.1). Thus the only operator that can
be applied to an array declared with storage-class
specifier register is sizeof.
6.7.1 Language 6.7.2
108 Committee Draft -- August 3, 1998 WG14/N843
one set on a line); the type specifiers may occur in any
order, possibly intermixed with the other declaration
specifiers.
-- void
-- char
-- signed char
-- unsigned char
-- short, signed short, short int, or signed short int
-- unsigned short, or unsigned short int
-- int, signed, or signed int
-- unsigned, or unsigned int
-- long, signed long, long int, or signed long int
-- unsigned long, or unsigned long int
-- long long, signed long long, long long int, or signed
long long int
-- unsigned long long, or unsigned long long int
-- float
-- double
-- long double
-- _Bool |
-- float _Complex
-- double _Complex
-- long double _Complex
-- float _Imaginary
-- double _Imaginary
-- long double _Imaginary
-- struct or union specifier |
-- enum specifier |
6.7.2 Language 6.7.2
WG14/N843 Committee Draft -- August 3, 1998 109
-- typedef name |
[#3] The type specifiers _Complex and _Imaginary shall not
be used if the implementation does not provide those
types.90)
Semantics
[#4] Specifiers for structures, unions, and enumerations are
discussed in 6.7.2.1 through 6.7.2.3. Declarations of
typedef names are discussed in 6.7.7. The characteristics
of the other types are discussed in 6.2.5.
[#5] Each of the comma-separated sets designates the same
type, except that for bit-fields, it is implementation-
defined whether the specifier int designates the same type
as signed int or the same type as unsigned int.
Forward references: enumeration specifiers (6.7.2.2),
structure and union specifiers (6.7.2.1), tags (6.7.2.3),
type definitions (6.7.7).
6.7.2.1 Structure and union specifiers
Syntax
[#1]
struct-or-union-specifier:
struct-or-union identifier-opt { struct-declaration-list }
struct-or-union identifier
struct-or-union:
struct
union
struct-declaration-list:
struct-declaration
struct-declaration-list struct-declaration
struct-declaration:
specifier-qualifier-list struct-declarator-list ;
specifier-qualifier-list:
type-specifier specifier-qualifier-list-opt
type-qualifier specifier-qualifier-list-opt
____________________
90)Implementations are not required to provide imaginary
types. Freestanding implementations are not required to
provide complex types.
6.7.2 Language 6.7.2.1
110 Committee Draft -- August 3, 1998 WG14/N843
struct-declarator-list:
struct-declarator
struct-declarator-list , struct-declarator
struct-declarator:
declarator
declarator-opt : constant-expression
Constraints
[#2] A structure or union shall not contain a member with |
incomplete or function type (hence, a structure shall not |
contain an instance of itself, but may contain a pointer to |
an instance of itself), except that the last member of a |
structure with more than one named member may have |
incomplete array type; such a structure (and any union |
containing, possibly recursively, a member that is such a |
structure) shall not be a member of a structure or an |
element of an array.
[#3] The expression that specifies the width of a bit-field
shall be an integer constant expression that has nonnegative
value that shall not exceed the number of bits in an object
of the type that is specified if the colon and expression
are omitted. If the value is zero, the declaration shall
have no declarator.
Semantics
[#4] As discussed in 6.2.5, a structure is a type consisting
of a sequence of members, whose storage is allocated in an
ordered sequence, and a union is a type consisting of a
sequence of members whose storage overlap.
[#5] Structure and union specifiers have the same form.
[#6] The presence of a struct-declaration-list in a struct-
or-union-specifier declares a new type, within a translation
unit. The struct-declaration-list is a sequence of
declarations for the members of the structure or union. If
the struct-declaration-list contains no named members, the
behavior is undefined. The type is incomplete until after
the } that terminates the list.
[#7] A member of a structure or union may have any object
type other than a variably modified type.91) In addition, a
member may be declared to consist of a specified number of
bits (including a sign bit, if any). Such a member is
called a bit-field;92) its width is preceded by a colon.
____________________
91)A structure or union can not contain a member with a
variably modified type because member names are not
ordinary identifiers as defined in 6.2.3.
6.7.2.1 Language 6.7.2.1
WG14/N843 Committee Draft -- August 3, 1998 111
[#8] A bit-field shall have a type that is a qualified or
unqualified version of _Bool, signed int, or unsigned int. |
A bit-field is interpreted as a signed or unsigned integer
type consisting of the specified number of bits.93) If the |
value 0 or 1 is stored into a nonzero-width bit-field of |
type _Bool, the value of the bit-field shall compare equal |
to the value stored.
[#9] An implementation may allocate any addressable storage
unit large enough to hold a bit-field. If enough space
remains, a bit-field that immediately follows another bit-
field in a structure shall be packed into adjacent bits of
the same unit. If insufficient space remains, whether a
bit-field that does not fit is put into the next unit or
overlaps adjacent units is implementation-defined. The
order of allocation of bit-fields within a unit (high-order
to low-order or low-order to high-order) is implementation-
defined. The alignment of the addressable storage unit is
unspecified.
[#10] A bit-field declaration with no declarator, but only a
colon and a width, indicates an unnamed bit-field.94) As a |
special case, a bit-field structure member with a width of 0
indicates that no further bit-field is to be packed into the
unit in which the previous bit-field, if any, was placed.
[#11] Each non-bit-field member of a structure or union
object is aligned in an implementation-defined manner
appropriate to its type.
[#12] Within a structure object, the non-bit-field members
and the units in which bit-fields reside have addresses that
increase in the order in which they are declared. A pointer
to a structure object, suitably converted, points to its
initial member (or if that member is a bit-field, then to
the unit in which it resides), and vice versa. There may be
unnamed padding within a structure object, but not at its
beginning.
[#13] The size of a union is sufficient to contain the
largest of its members. The value of at most one of the
____________________
92)The unary & (address-of) operator cannot be applied to a
bit-field object; thus, there are no pointers to or
arrays of bit-field objects.
93)As specified in 6.7.2 above, if the actual type specifier
used is int or a typedef-name defined as int, then it is
implementation-defined whether the bit-field is signed or
unsigned.
94)An unnamed bit-field structure member is useful for
padding to conform to externally imposed layouts.
6.7.2.1 Language 6.7.2.1
112 Committee Draft -- August 3, 1998 WG14/N843
members can be stored in a union object at any time. A
pointer to a union object, suitably converted, points to
each of its members (or if a member is a bit-field, then to
the unit in which it resides), and vice versa.
[#14] There may be unnamed padding at the end of a structure |
or union.
[#15] As a special case, the last element of a structure |
with more than one named member may have an incomplete array |
type. This is called a flexible array member, and the size
of the structure shall be equal to the offset of the last
element of an otherwise identical structure that replaces
the flexible array member with an array of unspecified |
length.95) When an lvalue whose type is a structure with a
flexible array member is used to access an object, it
behaves as if that member were replaced with the longest |
array, with the same element type, that would not make the |
structure larger than the object being accessed; the offset |
of the array shall remain that of the flexible array member, |
even if this would differ from that of the replacement |
array. If this array would have no elements, then it |
behaves as if it had one element, but the behavior is
undefined if any attempt is made to access that element or |
to generate a pointer one past it.
[#16] EXAMPLE Assuming that all array members are aligned |
the same, after the declarations:
struct s { int n; double d[]; };
struct ss { int n; double d[1]; };
the three expressions:
sizeof (struct s)
offsetof(struct s, d)
offsetof(struct ss, d)
have the same value. The structure struct s has a flexible
array member d.
[#17] If sizeof (double) is 8, then after the following code
is executed:
struct s *s1;
struct s *s2;
s1 = malloc(sizeof (struct s) + 64);
s2 = malloc(sizeof (struct s) + 46);
____________________
95)The length is unspecified to allow for the fact that
implementations may give array members different
alignments according to their lengths.
6.7.2.1 Language 6.7.2.1
WG14/N843 Committee Draft -- August 3, 1998 113
and assuming that the calls to malloc succeed, the objects |
pointed to by s1 and s2 behave as if the identifiers had |
been declared as:
struct { int n; double d[8]; } *s1;
struct { int n; double d[5]; } *s2;
[#18] Following the further successful assignments:
s1 = malloc(sizeof (struct s) + 10);
s2 = malloc(sizeof (struct s) + 6);
they then behave as if the declarations were: |
struct { int n; double d[1]; } *s1, *s2;
and:
double *dp;
dp = &(s1->d[0]); // Permitted
*dp = 42; // Permitted
dp = &(s2->d[0]); // Permitted
*dp = 42; // Undefined behavior
Forward references: tags (6.7.2.3).
6.7.2.2 Enumeration specifiers
Syntax
[#1]
enum-specifier:
enum identifier-opt { enumerator-list }
enum identifier-opt { enumerator-list , }
enum identifier
enumerator-list:
enumerator
enumerator-list , enumerator
enumerator:
enumeration-constant
enumeration-constant = constant-expression
Constraints
[#2] The expression that defines the value of an enumeration
constant shall be an integer constant expression that has a
value representable as an int.
6.7.2.1 Language 6.7.2.2
114 Committee Draft -- August 3, 1998 WG14/N843
Semantics
[#3] The identifiers in an enumerator list are declared as
constants that have type int and may appear wherever such
are permitted.96) An enumerator with = defines its
enumeration constant as the value of the constant
expression. If the first enumerator has no =, the value of
its enumeration constant is 0. Each subsequent enumerator
with no = defines its enumeration constant as the value of
the constant expression obtained by adding 1 to the value of
the previous enumeration constant. (The use of enumerators
with = may produce enumeration constants with values that
duplicate other values in the same enumeration.) The
enumerators of an enumeration are also known as its members.
[#4] Each enumerated type shall be compatible with an
integer type. The choice of type is |
implementation-defined,97) but shall be capable of
representing the values of all the members of the
enumeration. The enumerated type is incomplete until after *
the } that terminates the list of enumerator declarations.
[#5] EXAMPLE The following fragment: |
enum hue { chartreuse, burgundy, claret=20, winedark };
enum hue col, *cp;
col = claret;
cp = &col;
if (*cp != burgundy)
/* ... */
makes hue the tag of an enumeration, and then declares col
as an object that has that type and cp as a pointer to an
object that has that type. The enumerated values are in the
set {0, 1, 20, 21}.
Forward references: tags (6.7.2.3).
6.7.2.3 Tags
Constraints
[#1] A specific type shall have its content defined at most
once.
____________________
96)Thus, the identifiers of enumeration constants declared
in the same scope shall all be distinct from each other
and from other identifiers declared in ordinary
declarators.
97)An implementation may delay the choice of which integer
type until all enumeration constants have been seen.
6.7.2.2 Language 6.7.2.3
WG14/N843 Committee Draft -- August 3, 1998 115
[#2] A type specifier of the form
enum identifier
without an enumerator list shall only appear after the type
it specifies is completed.
Semantics
[#3] All declarations of structure, union, or enumerated
types that have the same scope and use the same tag declare
the same type. The type is incomplete98) until the closing
brace of the list defining the content, and complete
thereafter.
[#4] Two declarations of structure, union, or enumerated
types which are in different scopes or use different tags
declare distinct types. Each declaration of a structure,
union, or enumerated type which does not include a tag
declares a distinct type.
[#5] A type specifier of the form
struct-or-union identifier-opt { struct-declaration-list }
or
enum identifier { enumerator-list }
or
enum identifier { enumerator-list , }
declares a structure, union, or enumerated type. The list
defines the structure content, union content, or enumeration
content. If an identifier is provided,99) the type
specifier also declares the identifier to be the tag of that
type.
[#6] A declaration of the form
____________________
98)An incomplete type may only by used when the size of an
object of that type is not needed. It is not needed, for
example, when a typedef name is declared to be a
specifier for a structure or union, or when a pointer to
or a function returning a structure or union is being
declared. (See incomplete types in 6.2.5.) The |
specification has to be complete before such a function
is called or defined.
99)If there is no identifier, the type can, within the
translation unit, only be referred to by the declaration
of which it is a part. Of course, when the declaration
is of a typedef name, subsequent declarations can make
use of that typedef name to declare objects having the
specified structure, union, or enumerated type.
6.7.2.3 Language 6.7.2.3
116 Committee Draft -- August 3, 1998 WG14/N843
struct-or-union identifier ;
specifies a structure or union type and declares the
identifier as a tag of that type.100)
[#7] If a type specifier of the form
struct-or-union identifier
occurs other than as part of one of the above forms, and no
other declaration of the identifier as a tag is visible,
then it declares an incomplete structure or union type, and
declares the identifier as the tag of that type.100)
[#8] If a type specifier of the form
struct-or-union identifier
or
enum identifier
occurs other than as part of one of the above forms, and a
declaration of the identifier as a tag is visible, then it
specifies the same type as that other declaration, and does
not redeclare the tag.
[#9] EXAMPLE 1 This mechanism allows declaration of a self-
referential structure.
struct tnode {
int count;
struct tnode *left, *right;
};
specifies a structure that contains an integer and two
pointers to objects of the same type. Once this declaration
has been given, the declaration
struct tnode s, *sp;
declares s to be an object of the given type and sp to be a
pointer to an object of the given type. With these
declarations, the expression sp->left refers to the left
struct tnode pointer of the object to which sp points; the
expression s.right->count designates the count member of the
right struct tnode pointed to from s.
[#10] The following alternative formulation uses the typedef
mechanism:
____________________
100A similar construction with enum does not exist.
6.7.2.3 Language 6.7.2.3
WG14/N843 Committee Draft -- August 3, 1998 117
typedef struct tnode TNODE;
struct tnode {
int count;
TNODE *left, *right;
};
TNODE s, *sp;
[#11] EXAMPLE 2 To illustrate the use of prior declaration
of a tag to specify a pair of mutually referential
structures, the declarations
struct s1 { struct s2 *s2p; /* ... */ }; // D1
struct s2 { struct s1 *s1p; /* ... */ }; // D2
specify a pair of structures that contain pointers to each
other. Note, however, that if s2 were already declared as a
tag in an enclosing scope, the declaration D1 would refer to
it, not to the tag s2 declared in D2. To eliminate this
context sensitivity, the declaration
struct s2;
may be inserted ahead of D1. This declares a new tag s2 in
the inner scope; the declaration D2 then completes the
specification of the new type.
*
Forward references: declarators (6.7.5), array declarators
(6.7.5.2), type definitions (6.7.7).
6.7.3 Type qualifiers
Syntax
[#1]
type-qualifier:
const
restrict
volatile
Constraints
[#2] Types other than pointer types derived from object or
incomplete types shall not be restrict-qualified.
Semantics
[#3] The properties associated with qualified types are
meaningful only for expressions that are lvalues.101)
[#4] If the same qualifier appears more than once in the
6.7.2.3 Language 6.7.3
118 Committee Draft -- August 3, 1998 WG14/N843
same specifier-qualifier-list, either directly or via one or
more typedefs, the behavior is the same as if it appeared
only once.
[#5] If an attempt is made to modify an object defined with
a const-qualified type through use of an lvalue with non-
const-qualified type, the behavior is undefined. If an
attempt is made to refer to an object defined with a
volatile-qualified type through use of an lvalue with non-
volatile-qualified type, the behavior is undefined.102)
[#6] An object that has volatile-qualified type may be
modified in ways unknown to the implementation or have other
unknown side effects. Therefore any expression referring to
such an object shall be evaluated strictly according to the
rules of the abstract machine, as described in 5.1.2.3.
Furthermore, at every sequence point the value last stored
in the object shall agree with that prescribed by the
abstract machine, except as modified by the unknown factors
mentioned previously.103) What constitutes an access to an
object that has volatile-qualified type is implementation-
defined.
[#7] An object that is accessed through a restrict-qualified |
pointer has a special association with that pointer. This
association, defined in 6.7.3.1 below, requires that all |
accesses to that object use, directly or indirectly, the |
value of that particular pointer.104) The intended use of
the restrict qualifier (like the register storage class) is
to promote optimization, and deleting all instances of the
qualifier from a conforming program does not change its
____________________
101The implementation may place a const object that is not
volatile in a read-only region of storage. Moreover, the
implementation need not allocate storage for such an
object if its address is never used.
102This applies to those objects that behave as if they were
defined with qualified types, even if they are never
actually defined as objects in the program (such as an
object at a memory-mapped input/output address).
103A volatile declaration may be used to describe an object
corresponding to a memory-mapped input/output port or an
object accessed by an asynchronously interrupting
function. Actions on objects so declared shall not be
``optimized out'' by an implementation or reordered
except as permitted by the rules for evaluating
expressions.
104For example, a statement that assigns a value returned by |
malloc to a single pointer establishes this association |
between the allocated object and the pointer. ||
6.7.3 Language 6.7.3
WG14/N843 Committee Draft -- August 3, 1998 119
meaning (i.e., observable behavior).
[#8] If the specification of an array type includes any type
qualifiers, the element type is so-qualified, not the array
type. If the specification of a function type includes any
type qualifiers, the behavior is undefined.105)
[#9] For two qualified types to be compatible, both shall
have the identically qualified version of a compatible type;
the order of type qualifiers within a list of specifiers or
qualifiers does not affect the specified type.
[#10] EXAMPLE 1 An object declared
extern const volatile int real_time_clock;
may be modifiable by hardware, but cannot be assigned to,
incremented, or decremented.
[#11] EXAMPLE 2 The following declarations and expressions
illustrate the behavior when type qualifiers modify an
aggregate type:
const struct s { int mem; } cs = { 1 };
struct s ncs; // the object ncs is modifiable
typedef int A[2][3];
const A a = {{4, 5, 6}, {7, 8, 9}}; // array of array of
// const int
int *pi;
const int *pci;
ncs = cs; // valid
cs = ncs; // violates modifiable lvalue constraint for =
pi = &ncs.mem; // valid
pi = &cs.mem; // violates type constraints for =
pci = &cs.mem; // valid
pi = a[0]; // invalid: a[0] has type ``const int *''
6.7.3.1 Formal definition of restrict
[#1] Let D be a declaration of an ordinary identifier that
provides a means of designating an object P as a restrict-
qualified pointer.
[#2] If D appears inside a block and does not have storage
class extern, let B denote the block. If D appears in the
list of parameter declarations of a function definition, let
B denote the associated block. Otherwise, let B denote the
____________________ |
105Both of these can occur through the use of typedefs. ||
6.7.3 Language 6.7.3.1
120 Committee Draft -- August 3, 1998 WG14/N843
block of main (or the block of whatever function is called
at program startup in a freestanding environment).
[#3] In what follows, a pointer expression E is said to be
based on object P if (at some sequence point in the
execution of B prior to the evaluation of E) modifying P to
point to a copy of the array object into which it formerly
pointed would change the value of E.106) Note that |
``based'' is defined only for expressions with pointer
types.
[#4] During each execution of B, let A be the array object
that is determined dynamically by all accesses through |
pointer expressions based on P. Then all accesses to values |
of A shall be through pointer expressions based on P.
Furthermore, if P is assigned the value of a pointer
expression E that is based on another restricted pointer
object P2, associated with block B2, then either the
execution of B2 shall begin before the execution of B, or
the execution of B2 shall end prior to the assignment. If
these requirements are not met, then the behavior is
undefined.
[#5] Here an execution of B means that portion of the
execution of the program during which storage is guaranteed
to be reserved for an instance of an object that is
associated with B and that has automatic storage duration. |
An access to a value means either fetching it or modifying |
it; expressions that are not evaluated do not access values.
[#6] A translator is free to ignore any or all aliasing
implications of uses of restrict.
[#7] EXAMPLE 1 The file scope declarations
int * restrict a;
int * restrict b;
extern int c[];
assert that if an object is accessed using the value of one |
of a, b, or c, then it is never accessed using the value of |
either of the other two.
[#8] EXAMPLE 2 The function parameter declarations in the
following example
____________________ |
106In other words, E depends on the value of P itself rather ||
than on the value of an object referenced indirectly ||
through P. For example, if identifier p has type (int ||
**restrict), then the pointer expressions p and p+1 are ||
based on the restricted pointer object designated by p, ||
but the pointer expressions *p and p[1] are not. ||
6.7.3.1 Language 6.7.3.1
WG14/N843 Committee Draft -- August 3, 1998 121
void f(int n, int * restrict p, int * restrict q)
{
while (n-- > 0)
*p++ = *q++;
}
assert that, during each execution of the function, if an |
object is accessed through one of the pointer parameters, |
then it is not also accessed through the other.
[#9] The benefit of the restrict qualifiers is that they
enable a translator to make an effective dependence analysis
of function f without examining any of the calls of f in the
program. The cost is that the programmer has to examine all
of those calls to ensure that none give undefined behavior.
For example, the second call of f in g has undefined
behavior because each of d[1] through d[49] is accessed |
through both p and q.
void g(void)
{
extern int d[100]; |
f(50, d + 50, d); // ok
f(50, d + 1, d); // undefined behavior
}
[#10] EXAMPLE 3 The function parameter declarations
void h(int n, int * const restrict p,
int * const q, int * const r)
{
int i;
for (i = 0; i < n; i++)
p[i] = q[i] + r[i];
}
show how const can be used in conjunction with restrict.
The const qualifiers imply, without the need to examine the
body of h, that q and r cannot become based on p. The fact
that p is restrict-qualified therefore implies that an |
object accessed through p is never accessed through either |
of q or r. This is the precise assertion required to
optimize the loop. Note that a call of the form h(100, a,
b, b) has defined behavior, which would not be true if all
three of p, q, and r were restrict-qualified.
[#11] EXAMPLE 4 The rule limiting assignments between
restricted pointers does not distinguish between a function
call and an equivalent nested block. With one exception,
only ``outer-to-inner'' assignments between restricted
pointers declared in nested blocks have defined behavior.
6.7.3.1 Language 6.7.3.1
122 Committee Draft -- August 3, 1998 WG14/N843
{
int * restrict p1;
int * restrict q1;
p1 = q1; // undefined behavior
{
int * restrict p2 = p1; // ok
int * restrict q2 = q1; // ok
p1 = q2; // undefined behavior
p2 = q2; // undefined behavior
}
}
The exception allows the value of a restricted pointer to be
carried out of the block in which it (or, more precisely,
the ordinary identifier used to designate it) is declared
when that block finishes execution. For example, this
permits new_vector to return a vector.
typedef struct { int n; float * restrict v; } vector;
vector new_vector(int n)
{
vector t;
t.n = n;
t.v = malloc(n * sizeof (float));
return t;
}
6.7.4 Function specifiers
Syntax
[#1]
function-specifier:
inline
Constraints
[#2] Function specifiers shall be used only in the
declaration of an identifier for a function.
[#3] An inline definition of a function with external |
linkage shall not contain a definition of a modifiable |
object with static storage duration, and shall not contain a |
reference to an identifier with internal linkage.
[#4] The inline function specifier shall not appear in a
declaration of main.
Semantics
[#5] A function declared with an inline function specifier
6.7.3.1 Language 6.7.4
WG14/N843 Committee Draft -- August 3, 1998 123
is an inline function. (The function specifier may appear |
more than once; the behavior is the same as if it appeared |
only once.) Making a function an inline function suggests
that calls to the function be as fast as possible.107) The |
extent to which such suggestions are effective is
implementation-defined.108)
[#6] Any function with internal linkage can be an inline
function. For a function with external linkage, the
following restrictions apply: If a function is declared with |
an inline function specifier, then it shall also be defined
in the same translation unit. If all of the file scope
declarations for a function in a translation unit include
the inline function specifier without extern, then the
definition in that translation unit is an inline definition.
An inline definition does not provide an external definition
for the function, and does not forbid an external definition
in another translation unit. An inline definition provides
an alternative to an external definition, which a translator
may use to implement any call to the function in the same
translation unit. It is unspecified whether a call to the
function uses the inline definition or the external |
definition.109)
[#7] EXAMPLE The declaration of an inline function with
external linkage can result in either an external
definition, or a definition available for use only within
the translation unit. A file scope declaration with extern
creates an external definition. The following example shows
an entire translation unit.
____________________
107By using, for example, an alternative to the usual |
function call mechanism, such as ``inline substitution''. |
Inline substitution is not textual substitution, nor does
it create a new function. Therefore, for example, the
expansion of a macro used within the body of the function
uses the definition it had at the point the function body
appears, and not where the function is called; and
identifiers refer to the declarations in scope where the
body occurs. Similarly, the address of the function is
not affected by the function's being inlined.
108For example, an implementation might never perform inline
substitution, or might only perform inline substitutions
to calls in the scope of an inline declaration.
109Since an inline definition is distinct from the
corresponding external definition, and from any other
corresponding inline definition in another translation
unit, all corresponding objects with static storage
duration are also distinct in each of the definitions.
6.7.4 Language 6.7.4
124 Committee Draft -- August 3, 1998 WG14/N843
inline double fahr(double t)
{
return (9.0 * t) / 5.0 + 32.0;
}
inline double cels(double t)
{
return (5.0 * (t - 32.0)) / 9.0;
}
extern double fahr(double); // creates an external definition|
double convert(int is_fahr, double temp)
{
/* A translator may perform inline substitutions. */
return is_fahr ? cels(temp) : fahr(temp);
}
[#8] Note that the definition of fahr is an external
definition because fahr is also declared with extern, but
the definition of cels is an inline definition. Because |
cels has external linkage and is referenced, an external |
definition has to appear in another translation unit (see |
6.9); the inline definition and the external definition are |
distinct and either may be used for the call.
6.7.5 Declarators
Syntax
[#1]
declarator:
pointer-opt direct-declarator
direct-declarator:
identifier
( declarator )
direct-declarator [ assignment-expression-opt ]
direct-declarator [ * ]
direct-declarator ( parameter-type-list )
direct-declarator ( identifier-list-opt )
pointer:
* type-qualifier-list-opt
* type-qualifier-list-opt pointer
type-qualifier-list:
type-qualifier
type-qualifier-list type-qualifier
6.7.4 Language 6.7.5
WG14/N843 Committee Draft -- August 3, 1998 125
parameter-type-list:
parameter-list
parameter-list , ...
parameter-list:
parameter-declaration
parameter-list , parameter-declaration
parameter-declaration:
declaration-specifiers declarator
declaration-specifiers abstract-declarator-opt
identifier-list:
identifier
identifier-list , identifier
Semantics
[#2] Each declarator declares one identifier, and asserts
that when an operand of the same form as the declarator
appears in an expression, it designates a function or object
with the scope, storage duration, and type indicated by the
declaration specifiers.
[#3] A full declarator is a declarator that is not part of
another declarator. The end of a full declarator is a
sequence point. If the nested sequence of declarators in a
full declarator contains a variable length array type, the
type specified by the full declarator is said to be variably
modified.
[#4] In the following subclauses, consider a declaration
T D1
where T contains the declaration specifiers that specify a
type T (such as int) and D1 is a declarator that contains an
identifier ident. The type specified for the identifier
ident in the various forms of declarator is described
inductively using this notation.
[#5] If, in the declaration ``T D1'', D1 has the form
identifier
then the type specified for ident is T.
[#6] If, in the declaration ``T D1'', D1 has the form
( D )
then ident has the type specified by the declaration ``T
D''. Thus, a declarator in parentheses is identical to the
unparenthesized declarator, but the binding of complicated
6.7.5 Language 6.7.5
126 Committee Draft -- August 3, 1998 WG14/N843
declarators may be altered by parentheses.
Implementation limits
[#7] As discussed in 5.2.4.1, an implementation may limit |
the number of pointer, array, and function declarators that |
modify an arithmetic, structure, union, or incomplete type,
either directly or via one or more typedefs.
Forward references: array declarators (6.7.5.2), type
definitions (6.7.7).
6.7.5.1 Pointer declarators
Semantics
[#1] If, in the declaration ``T D1'', D1 has the form
* type-qualifier-list-opt D
and the type specified for ident in the declaration ``T D''
is ``derived-declarator-type-list T'', then the type
specified for ident is ``derived-declarator-type-list type-
qualifier-list pointer to T''. For each type qualifier in
the list, ident is a so-qualified pointer.
[#2] For two pointer types to be compatible, both shall be
identically qualified and both shall be pointers to
compatible types.
[#3] EXAMPLE The following pair of declarations
demonstrates the difference between a ``variable pointer to
a constant value'' and a ``constant pointer to a variable
value''.
const int *ptr_to_constant;
int *const constant_ptr;
The contents of any object pointed to by ptr_to_constant |
shall not be modified through that pointer, but
ptr_to_constant itself may be changed to point to another
object. Similarly, the contents of the int pointed to by
constant_ptr may be modified, but constant_ptr itself shall
always point to the same location.
[#4] The declaration of the constant pointer constant_ptr
may be clarified by including a definition for the type
``pointer to int''.
typedef int *int_ptr;
const int_ptr constant_ptr;
declares constant_ptr as an object that has type ``const-
qualified pointer to int''.
6.7.5 Language 6.7.5.1
WG14/N843 Committee Draft -- August 3, 1998 127
6.7.5.2 Array declarators
Constraints
[#1] The [ and ] may delimit an expression or *. If [ and ]
delimit an expression (which specifies the size of an
array), it shall have an integer type. If the expression is
a constant expression then it shall have a value greater
than zero. The element type shall not be an incomplete or
function type.
[#2] Only ordinary identifiers (as defined in 6.2.3) with |
both block scope or function prototype scope and no linkage |
shall have a variably modified type. If an identifier is
declared to be an object with static storage duration, it
shall not have a variable length array type.
Semantics
[#3] If, in the declaration ``T D1'', D1 has the form
D[assignment-expr-opt]
or
D[*]
and the type specified for ident in the declaration ``T D''
is ``derived-declarator-type-list T'', then the type
specified for ident is ``derived-declarator-type-list array
of T''.110) If the size is not present, the array type is
an incomplete type. If * is used instead of a size
expression, the array type is a variable length array type
of unspecified size, which can only be used in declarations |
with function prototype scope.111) If the size expression
is an integer constant expression and the element type has a
known constant size, the array type is not a variable length
array type; otherwise, the array type is a variable length |
array type. If the size expression is not a constant
expression, and it is evaluated at program execution time,
it shall evaluate to a value greater than zero. It is
unspecified whether side effects are produced when the size
expression is evaluated. The size of each instance of a
variable length array type does not change during its
lifetime.
____________________
110When several ``array of'' specifications are adjacent, a
multidimensional array is declared.
111Thus, * can be used only in function declarations that
are not definitions (see 6.7.5.3).
6.7.5.1 Language 6.7.5.2
128 Committee Draft -- August 3, 1998 WG14/N843
[#4] For two array types to be compatible, both shall have
compatible element types, and if both size specifiers are
present, and are integer constant expressions, then both
size specifiers shall have the same constant value. If the
two array types are used in a context which requires them to
be compatible, it is undefined behavior if the two size
specifiers evaluate to unequal values.
[#5] EXAMPLE 1
float fa[11], *afp[17];
declares an array of float numbers and an array of pointers
to float numbers.
[#6] EXAMPLE 2 Note the distinction between the declarations
extern int *x;
extern int y[];
The first declares x to be a pointer to int; the second
declares y to be an array of int of unspecified size (an
incomplete type), the storage for which is defined
elsewhere.
[#7] EXAMPLE 3 The following declarations demonstrate the
compatibility rules for variably modified types.
extern int n;
extern int m;
void fcompat(void)
{
int a[n][6][m];
int (*p)[4][n+1];
int c[n][n][6][m];
int (*r)[n][n][n+1];
p = a; // Error - not compatible because 4 != 6.
r = c; // Compatible, but defined behavior
// only if n == 6 and m == n+1.
}
[#8] EXAMPLE 4 All declarations of variably modified (VM)
types have to be at either block scope or function prototype
scope. Array objects declared with the static or extern
storage class specifier cannot have a variable length array
(VLA) type. However, an object declared with the static
storage class specifier can have a VM type (that is, a
pointer to a VLA type). Finally, all identifiers declared |
with a VM type have to be ordinary identifiers and cannot, |
therefore, be members of structures or unions.
6.7.5.2 Language 6.7.5.2
WG14/N843 Committee Draft -- August 3, 1998 129
extern int n;
int A[n]; // Error - file scope VLA.
extern int (*p2)[n]; // Error - file scope VM.
int B[100]; // OK - file scope but not VM.
void fvla(int m, int C[m][m]) // OK - VLA with prototype scope.
{
typedef int VLA[m][m] // OK - block scope typedef VLA.
struct tag {
int (*y)[n]; // Error - y not ordinary identifier.
int z[n]; // Error - z not ordinary identifier.
};
int D[m]; // OK - auto VLA.
static int E[m]; // Error - static block scope VLA.
extern int F[m]; // Error - F has linkage and is VLA.
int (*s)[m]; // OK - auto pointer to VLA.
extern int (*r)[m]; // Error - r had linkage and is
// a pointer to VLA.
static int (*q)[m] = &B; // OK - q is a static block
// pointer to VLA.
}
Forward references: function declarators (6.7.5.3), |
function definitions (6.9.1), initialization (6.7.8).
6.7.5.3 Function declarators (including prototypes)
Constraints
[#1] A function declarator shall not specify a return type
that is a function type or an array type.
[#2] The only storage-class specifier that shall occur in a
parameter declaration is register.
[#3] An identifier list in a function declarator that is not |
part of a definition of that function shall be empty.
[#4] After adjustment, the parameters in a parameter type |
list in a function declarator that is part of a definition |
of that function shall not have incomplete type.112)
Semantics
[#5] If, in the declaration ``T D1'', D1 has the form
____________________
112Arrays and functions are rewritten as pointers.
6.7.5.2 Language 6.7.5.3
130 Committee Draft -- August 3, 1998 WG14/N843
D(parameter-type-list)
or
D(identifier-list-opt)
and the type specified for ident in the declaration ``T D''
is ``derived-declarator-type-list T'', then the type
specified for ident is ``derived-declarator-type-list
function returning T''.
[#6] A parameter type list specifies the types of, and may
declare identifiers for, the parameters of the function. A |
declaration of a parameter as ``array of type'' shall be |
adjusted to ``pointer to type'', and a declaration of a |
parameter as ``function returning type'' shall be adjusted |
to ``pointer to function returning type'', as in 6.3.2.1.
If the list terminates with an ellipsis (, ...), no
information about the number or types of the parameters
after the comma is supplied.113) The special case of an
unnamed parameter of type void as the only item in the list
specifies that the function has no parameters.
[#7] If, in a parameter declaration, an identifier can be
treated as a typedef name or as a parameter name, it shall
be taken as a typedef name.
[#8] If the function declarator is not part of a definition |
of that function, parameters may have incomplete type and |
may use the [*] notation in their sequences of declarator |
specifiers to specify variable length array types.
[#9] The storage-class specifier in the declaration
specifiers for a parameter declaration, if present, is
ignored unless the declared parameter is one of the members
of the parameter type list for a function definition.
[#10] An identifier list declares only the identifiers of
the parameters of the function. An empty list in a function
declarator that is part of a definition of that function |
specifies that the function has no parameters. The empty
list in a function declarator that is not part of a |
definition of that function specifies that no information
about the number or types of the parameters is
supplied.114)
[#11] For two function types to be compatible, both shall
specify compatible return types.115) Moreover, the
parameter type lists, if both are present, shall agree in
the number of parameters and in use of the ellipsis
terminator; corresponding parameters shall have compatible
types. If one type has a parameter type list and the other
type is specified by a function declarator that is not part
of a function definition and that contains an empty
identifier list, the parameter list shall not have an
ellipsis terminator and the type of each parameter shall be
compatible with the type that results from the application
of the default argument promotions. If one type has a
parameter type list and the other type is specified by a
function definition that contains a (possibly empty)
identifier list, both shall agree in the number of
WG14/N843 Committee Draft -- August 3, 1998 131
parameters, and the type of each prototype parameter shall
be compatible with the type that results from the
application of the default argument promotions to the type
of the corresponding identifier. (In the determination of
type compatibility and of a composite type, each parameter
declared with function or array type is taken as having the |
adjusted type and each parameter declared with qualified
type is taken as having the unqualified version of its
declared type.)
[#12] EXAMPLE 1 The declaration
int f(void), *fip(), (*pfi)();
declares a function f with no parameters returning an int, a
function fip with no parameter specification returning a
pointer to an int, and a pointer pfi to a function with no
parameter specification returning an int. It is especially
useful to compare the last two. The binding of *fip() is
*(fip()), so that the declaration suggests, and the same
construction in an expression requires, the calling of a
function fip, and then using indirection through the pointer
result to yield an int. In the declarator (*pfi)(), the
extra parentheses are necessary to indicate that indirection
through a pointer to a function yields a function
designator, which is then used to call the function; it
returns an int.
[#13] If the declaration occurs outside of any function, the
identifiers have file scope and external linkage. If the
declaration occurs inside a function, the identifiers of the
functions f and fip have block scope and either internal or
external linkage (depending on what file scope declarations
for these identifiers are visible), and the identifier of
the pointer pfi has block scope and no linkage.
[#14] EXAMPLE 2 The declaration
int (*apfi[3])(int *x, int *y);
declares an array apfi of three pointers to functions
returning int. Each of these functions has two parameters
that are pointers to int. The identifiers x and y are
declared for descriptive purposes only and go out of scope
____________________
113The macros defined in the <stdarg.h> header (7.15) may be
used to access arguments that correspond to the ellipsis.
114See ``future language directions'' (6.11.3).
115If both function types are ``old style'', parameter types
are not compared.
6.7.5.3 Language 6.7.5.3
132 Committee Draft -- August 3, 1998 WG14/N843
at the end of the declaration of apfi.
[#15] EXAMPLE 3 The declaration
int (*fpfi(int (*)(long), int))(int, ...);
declares a function fpfi that returns a pointer to a
function returning an int. The function fpfi has two
parameters: a pointer to a function returning an int (with
one parameter of type long int), and an int. The pointer
returned by fpfi points to a function that has one int
parameter and accepts zero or more additional arguments of
any type.
[#16] EXAMPLE 4 The following prototype has a variably
modified parameter.
void addscalar(int n, int m,
double a[n][n*m+300], double x);
int main()
{
double b[4][308];
addscalar(4, 2, b, 2.17);
return 0;
}
void addscalar(int n, int m,
double a[n][n*m+300], double x)
{
for (int i = 0; i < n; i++)
for (int j = 0, k = n*m+300; j < k; j++)
// a is a pointer to a VLA
// with n*m+300 elements
a[i][j] += x;
}
[#17] EXAMPLE 5 The following are all compatible function
prototype declarators.
double maximum(int n, int m, double a[n][m]);
double maximum(int n, int m, double a[*][*]);
double maximum(int n, int m, double a[ ][*]);
double maximum(int n, int m, double a[ ][m]);
Forward references: function definitions (6.9.1), type
names (6.7.6).
6.7.5.3 Language 6.7.5.3
WG14/N843 Committee Draft -- August 3, 1998 133
6.7.6 Type names
Syntax
[#1]
type-name:
specifier-qualifier-list abstract-declarator-opt
abstract-declarator:
pointer
pointer-opt direct-abstract-declarator
direct-abstract-declarator:
( abstract-declarator )
direct-abstract-declarator-opt [ assignment-expression-opt ]
direct-abstract-declarator-opt [ * ]
direct-abstract-declarator-opt ( parameter-type-list-opt )
Semantics
[#2] In several contexts, it is necessary to specify a type. |
This is accomplished using a type name, which is
syntactically a declaration for a function or an object of
that type that omits the identifier.116)
[#3] EXAMPLE The constructions
(a) int
(b) int *
(c) int *[3]
(d) int (*)[3]
(e) int (*)[*] |
(f) int *() |
(g) int (*)(void) |
(h) int (*const [])(unsigned int, ...) |
name respectively the types (a) int, (b) pointer to int, (c)
array of three pointers to int, (d) pointer to an array of
three ints, (e) pointer to a variable length array of an |
unspecified number of ints, (f) function with no parameter
specification returning a pointer to int, (g) pointer to |
function with no parameters returning an int, and (h) array |
of an unspecified number of constant pointers to functions,
each with one parameter that has type unsigned int and an
unspecified number of other parameters, returning an int.
____________________
116As indicated by the syntax, empty parentheses in a type
name are interpreted as ``function with no parameter
specification'', rather than redundant parentheses around
the omitted identifier.
6.7.6 Language 6.7.6
134 Committee Draft -- August 3, 1998 WG14/N843
6.7.7 Type definitions
Syntax
[#1]
typedef-name:
identifier
Constraints
[#2] If a typedef name specifies a variably modified type
then it shall have block scope.
Semantics
[#3] In a declaration whose storage-class specifier is
typedef, each declarator defines an identifier to be a |
typedef name that denotes the type specified for the
identifier in the way described in 6.7.5. Any array size
expressions associated with variable length array |
declarators are evaluated each time the declaration of the |
typedef name is reached in the order of execution. A
typedef declaration does not introduce a new type, only a
synonym for the type so specified. That is, in the
following declarations:
typedef T type_ident;
type_ident D;
type_ident is defined as a typedef name with the type
specified by the declaration specifiers in T (known as T),
and the identifier in D has the type ``derived-declarator-
type-list T'' where the derived-declarator-type-list is
specified by the declarators of D. A typedef name shares
the same name space as other identifiers declared in
ordinary declarators. *
[#4] EXAMPLE 1 After
typedef int MILES, KLICKSP();
typedef struct { double hi, lo; } range;
the constructions
MILES distance;
extern KLICKSP *metricp;
range x;
range z, *zp;
are all valid declarations. The type of distance is int,
that of metricp is ``pointer to function with no parameter
specification returning int'', and that of x and z is the
specified structure; zp is a pointer to such a structure.
6.7.7 Language 6.7.7
WG14/N843 Committee Draft -- August 3, 1998 135
The object distance has a type compatible with any other int
object.
[#5] EXAMPLE 2 After the declarations
typedef struct s1 { int x; } t1, *tp1;
typedef struct s2 { int x; } t2, *tp2;
type t1 and the type pointed to by tp1 are compatible. Type
t1 is also compatible with type struct s1, but not
compatible with the types struct s2, t2, the type pointed to
by tp2, or int. |
[#6] EXAMPLE 3 The following obscure constructions
typedef signed int t;
typedef int plain;
struct tag {
unsigned t:4;
const t:5;
plain r:5;
};
declare a typedef name t with type signed int, a typedef
name plain with type int, and a structure with three bit-
field members, one named t that contains values in the range
[0, 15], an unnamed const-qualified bit-field which (if it
could be accessed) would contain values in at least the
range [-15, +15], and one named r that contains values in
the range [0, 31] or values in at least the range [-15,
+15]. (The choice of range is implementation-defined.) The
first two bit-field declarations differ in that unsigned is
a type specifier (which forces t to be the name of a
structure member), while const is a type qualifier (which
modifies t which is still visible as a typedef name). If
these declarations are followed in an inner scope by
t f(t (t));
long t;
then a function f is declared with type ``function returning
signed int with one unnamed parameter with type pointer to
function returning signed int with one unnamed parameter
with type signed int'', and an identifier t with type long
int.
[#7] EXAMPLE 4 On the other hand, typedef names can be used
to improve code readability. All three of the following
declarations of the signal function specify exactly the same
type, the first without making use of any typedef names.
6.7.7 Language 6.7.7
136 Committee Draft -- August 3, 1998 WG14/N843
typedef void fv(int), (*pfv)(int);
void (*signal(int, void (*)(int)))(int);
fv *signal(int, fv *);
pfv signal(int, pfv);
[#8] EXAMPLE 5 If a typedef name denotes a variable length |
array type, the length of the array is fixed at the time the |
typedef name is defined, not each time it is used:
void copyt(int n)
{
typedef int B[n]; // B is n ints, n evaluated now.|
n += 1;
B a; // a is n ints, n without += 1.|
int b[n]; // a and b are different sizes|
for (int i = 1; i < n; i++) |
a[i-1] = b[i]; |
}
Forward references: the signal function (7.14.1.1).
6.7.8 Initialization
Syntax
[#1]
initializer:
assignment-expression
{ initializer-list }
{ initializer-list , }
initializer-list:
designation-opt initializer
initializer-list , designation-opt initializer
designation:
designator-list =
designator-list:
designator
designator-list designator
designator:
[ constant-expression ]
. identifier
Constraints
6.7.7 Language 6.7.8
WG14/N843 Committee Draft -- August 3, 1998 137
[#2] No initializer shall attempt to provide a value for an
object not contained within the entity being initialized.
[#3] The type of the entity to be initialized shall be an
array of unknown size or an object type that is not a
variable length array type.
[#4] All the expressions in an initializer for an object
that has static storage duration shall be constant
expressions or string literals.
[#5] If the declaration of an identifier has block scope,
and the identifier has external or internal linkage, the
declaration shall have no initializer for the identifier.
[#6] If a designator has the form
[ constant-expression ]
then the current object (defined below) shall have array
type and the expression shall be an integer constant
expression. If the array is of unknown size, any
nonnegative value is valid.
[#7] If a designator has the form
. identifier
then the current object (defined below) shall have structure
or union type and the identifier shall be the name of a |
member of that type.
Semantics
[#8] An initializer specifies the initial value stored in an
object.
[#9] Except where explicitly stated otherwise, for the
purposes of this subclause unnamed members of objects of
structure and union type do not participate in
initialization. Unnamed members of structure objects have
indeterminate value even after initialization. *
[#10] If an object that has automatic storage duration is
not initialized explicitly, its value is indeterminate. If
an object that has static storage duration is not
initialized explicitly, then:
-- if it has pointer type, it is initialized to a null
pointer;
-- if it has arithmetic type, it is initialized to |
(positive or unsigned) zero;
6.7.8 Language 6.7.8
138 Committee Draft -- August 3, 1998 WG14/N843
-- if it is an aggregate, every member is initialized
(recursively) according to these rules;
-- if it is a union, the first named member is initialized
(recursively) according to these rules.
[#11] The initializer for a scalar shall be a single
expression, optionally enclosed in braces. The initial
value of the object is that of the expression (after
conversion); the same type constraints and conversions as
for simple assignment apply, taking the type of the scalar
to be the unqualified version of its declared type.
[#12] The rest of this subclause deals with initializers for |
objects that have aggregate or union type. |
[#13] The initializer for a structure or union object that |
has automatic storage duration shall be either an |
initializer list as described below, or a single expression |
that has compatible structure or union type. In the latter |
case, the initial value of the object, including unnamed |
members, is that of the expression. |
[#14] An array of character type may be initialized by a |
character string literal, optionally enclosed in braces. |
Successive characters of the character string literal |
(including the terminating null character if there is room |
or if the array is of unknown size) initialize the elements |
of the array. |
[#15] An array with element type compatible with wchar_t may |
be initialized by a wide string literal, optionally enclosed |
in braces. Successive wide characters of the wide string |
literal (including the terminating null wide character if |
there is room or if the array is of unknown size) initialize |
the elements of the array. |
[#16] Otherwise, the initializer for an object that has |
aggregate or union type shall be a brace-enclosed list of |
initializers for the elements or named members. |
[#17] Each brace-enclosed initializer list has an associated
current object. When no designations are present, subobjects
of the current object are initialized in order according to
the type of the current object: array elements in increasing
subscript order, structure members in declaration order, and
the first named member of a union.117) In contrast, a
designation causes the following initializer to begin
initialization of the subobject described by the designator.
Initialization then continues forward in order, beginning
with the next subobject after that described by the
designator.118)
[#18] Each designator list begins its description with the
current object associated with the closest surrounding brace
pair. Each item in the designator list (in order) specifies
a particular member of its current object and changes the
current object for the next designator (if any) to be that
member.119) The current object that results at the end of
WG14/N843 Committee Draft -- August 3, 1998 139
the designator list is the subobject to be initialized by
the following initializer.
[#19] The initialization shall occur in initializer list
order, each initializer provided for a particular subobject
overriding any previously listed initializer for the same
subobject; all subobjects that are not initialized
explicitly shall be initialized implicitly the same as
objects that have static storage duration.
[#20] If the aggregate contains elements or members that are |
aggregates or unions, or if the first member of a union is
an aggregate or union, these rules apply recursively to the |
subaggregates or contained unions. If the initializer of a
subaggregate or contained union begins with a left brace,
the initializers enclosed by that brace and its matching
right brace initialize the elements or members of the |
subaggregate or the first member of the contained union.
Otherwise, only enough initializers from the list are taken
to account for the elements or members of the subaggregate |
or the first member of the contained union; any remaining
initializers are left to initialize the next element or |
member of the aggregate of which the current subaggregate or
contained union is a part.
[#21] If there are fewer initializers in a brace-enclosed
list than there are elements or members of an aggregate, or |
fewer characters in a string literal used to initialize an
array of known size than there are elements in the array,
the remainder of the aggregate shall be initialized
implicitly the same as objects that have static storage
duration.
[#22] If an array of unknown size is initialized, its size
is determined by the largest indexed element with an
explicit initializer. At the end of its initializer list,
the array no longer has incomplete type.
[#23] The order in which any side effects occur among the
initialization list expressions is unspecified.120)
____________________
118After a union member is initialized, the next object is
not the next member of the union; instead, it is the next
subobject of an object containing the union.
119Thus, a designator can only specify a strict subobject of
the aggregate or union that is associated with the
surrounding brace pair. Note, too, that each separate
designator list is independent.
120In particular, the evaluation order need not be the same
as the order of subobject initialization.
6.7.8 Language 6.7.8
140 Committee Draft -- August 3, 1998 WG14/N843
[#24] EXAMPLE 1 Provided that <complex.h> has been
#included, the declarations
int i = 3.5;
complex c = 5 + 3 * I;
define and initialize i with the value 3 and c with the
value 5.0+3.0i.
[#25] EXAMPLE 2 The declaration
int x[] = { 1, 3, 5 };
defines and initializes x as a one-dimensional array object
that has three elements, as no size was specified and there
are three initializers.
[#26] EXAMPLE 3 The declaration
int y[4][3] = {
{ 1, 3, 5 },
{ 2, 4, 6 },
{ 3, 5, 7 },
};
is a definition with a fully bracketed initialization: 1, 3,
and 5 initialize the first row of y (the array object y[0]),
namely y[0][0], y[0][1], and y[0][2]. Likewise the next two
lines initialize y[1] and y[2]. The initializer ends early,
so y[3] is initialized with zeros. Precisely the same
effect could have been achieved by
int y[4][3] = {
1, 3, 5, 2, 4, 6, 3, 5, 7
};
The initializer for y[0] does not begin with a left brace,
so three items from the list are used. Likewise the next
three are taken successively for y[1] and y[2].
[#27] EXAMPLE 4 The declaration
int z[4][3] = {
{ 1 }, { 2 }, { 3 }, { 4 }
};
initializes the first column of z as specified and
initializes the rest with zeros.
[#28] EXAMPLE 5 The declaration
6.7.8 Language 6.7.8
WG14/N843 Committee Draft -- August 3, 1998 141
struct { int a[3], b; } w[] = { { 1 }, 2 };
is a definition with an inconsistently bracketed
initialization. It defines an array with two element
structures: w[0].a[0] is 1 and w[1].a[0] is 2; all the other
elements are zero.
[#29] EXAMPLE 6 The declaration
short q[4][3][2] = {
{ 1 },
{ 2, 3 },
{ 4, 5, 6 }
};
contains an incompletely but consistently bracketed
initialization. It defines a three-dimensional array
object: q[0][0][0] is 1, q[1][0][0] is 2, q[1][0][1] is 3,
and 4, 5, and 6 initialize q[2][0][0], q[2][0][1], and
q[2][1][0], respectively; all the rest are zero. The
initializer for q[0][0] does not begin with a left brace, so
up to six items from the current list may be used. There is
only one, so the values for the remaining five elements are
initialized with zero. Likewise, the initializers for
q[1][0] and q[2][0] do not begin with a left brace, so each
uses up to six items, initializing their respective two-
dimensional subaggregates. If there had been more than six
items in any of the lists, a diagnostic message would have
been issued. The same initialization result could have been
achieved by:
short q[4][3][2] = {
1, 0, 0, 0, 0, 0,
2, 3, 0, 0, 0, 0,
4, 5, 6
};
or by:
short q[4][3][2] = {
{
{ 1 },
},
{
{ 2, 3 },
},
{
{ 4, 5 },
{ 6 },
}
};
in a fully bracketed form.
6.7.8 Language 6.7.8
142 Committee Draft -- August 3, 1998 WG14/N843
[#30] Note that the fully bracketed and minimally bracketed
forms of initialization are, in general, less likely to
cause confusion.
[#31] EXAMPLE 7 One form of initialization that completes
array types involves typedef names. Given the declaration
typedef int A[]; // OK - declared with block scope
the declaration
A a = { 1, 2 }, b = { 3, 4, 5 };
is identical to
int a[] = { 1, 2 }, b[] = { 3, 4, 5 };
due to the rules for incomplete types.
[#32] EXAMPLE 8 The declaration
char s[] = "abc", t[3] = "abc";
defines ``plain'' char array objects s and t whose elements
are initialized with character string literals. This
declaration is identical to
char s[] = { 'a', 'b', 'c', '\0' },
t[] = { 'a', 'b', 'c' };
The contents of the arrays are modifiable. On the other
hand, the declaration
char *p = "abc";
defines p with type ``pointer to char'' and initializes it |
to point to an object with type ``array of char'' with
length 4 whose elements are initialized with a character
string literal. If an attempt is made to use p to modify
the contents of the array, the behavior is undefined.
[#33] EXAMPLE 9 Arrays can be initialized to correspond to
the elements of an enumeration by using designators:
enum { member_one, member_two };
const char *nm[] = {
[member_two] = "member two",
[member_one] = "member one",
};
6.7.8 Language 6.7.8
WG14/N843 Committee Draft -- August 3, 1998 143
[#34] EXAMPLE 10 Structure members can be initialized to
nonzero values without depending on their order:
div_t answer = { .quot = 2, .rem = -1 };
[#35] EXAMPLE 11 Designators can be used to provide explicit
initialization when unadorned initializer lists might be
misunderstood:
struct { int a[3], b; } w[] =
{ [0].a = {1}, [1].a[0] = 2 };
[#36] EXAMPLE 12 Space can be ``allocated'' from both ends
of an array by using a single designator:
int a[MAX] = {
1, 3, 5, 7, 9, [MAX-5] = 8, 6, 4, 2, 0
};
[#37] In the above, if MAX is greater than ten, there will
be some zero-valued elements in the middle; if it is less
than ten, some of the values provided by the first five
initializers will be overridden by the second five.
[#38] EXAMPLE 13 Any member of a union can be initialized:
union { /* ... */ } u = { .any_member = 42 };
Forward references: common definitions <stddef.h> (7.17).
6.7.8 Language 6.7.8
144 Committee Draft -- August 3, 1998 WG14/N843
6.8 Statements
Syntax
[#1]
statement:
labeled-statement
compound-statement
expression-statement
selection-statement
iteration-statement
jump-statement
Semantics
[#2] A statement specifies an action to be performed.
Except as indicated, statements are executed in sequence.
[#3] A full expression is an expression that is not part of |
another expression or declarator. Each of the following is
a full expression: an initializer; the expression in an
expression statement; the controlling expression of a
selection statement (if or switch); the controlling
expression of a while or do statement; each of the
(optional) expressions of a for statement; the (optional)
expression in a return statement. The end of a full
expression is a sequence point.
Forward references: expression and null statements (6.8.3),
selection statements (6.8.4), iteration statements (6.8.5),
the return statement (6.8.6.4).
6.8.1 Labeled statements
Syntax
[#1]
labeled-statement:
identifier : statement
case constant-expr : statement
default : statement
Constraints
[#2] A case or default label shall appear only in a switch
statement. Further constraints on such labels are discussed
under the switch statement.
Semantics
[#3] Any statement may be preceded by a prefix that declares
an identifier as a label name. Labels in themselves do not
6.8 Language 6.8.1
WG14/N843 Committee Draft -- August 3, 1998 145
alter the flow of control, which continues unimpeded across
them.
Forward references: the goto statement (6.8.6.1), the
switch statement (6.8.4.2).
6.8.2 Compound statement, or block
Syntax
[#1]
compound-statement:
{ block-item-list-opt }
block-item-list:
block-item
block-item-list block-item
block-item:
declaration
statement
Semantics
[#2] A compound statement (also called a block) allows a set |
of declarations and statements to be grouped into one |
syntactic unit. The initializers of objects that have
automatic storage duration, and the variable length array
declarators of ordinary identifiers with block scope are
evaluated and the values are stored in the objects
(including storing an indeterminate value in objects without
an initializer) each time that the declaration is reached in
the order of execution, as if it were a statement, and
within each declaration in the order that declarators
appear.
6.8.3 Expression and null statements
Syntax
[#1]
expression-statement:
expression-opt ;
Semantics
[#2] The expression in an expression statement is evaluated
as a void expression for its side effects.121)
____________________
121Such as assignments, and function calls which have side
effects.
6.8.1 Language 6.8.3
146 Committee Draft -- August 3, 1998 WG14/N843
[#3] A null statement (consisting of just a semicolon)
performs no operations.
[#4] EXAMPLE 1 If a function call is evaluated as an
expression statement for its side effects only, the
discarding of its value may be made explicit by converting
the expression to a void expression by means of a cast:
int p(int);
/* ... */
(void)p(0);
[#5] EXAMPLE 2 In the program fragment
char *s;
/* ... */
while (*s++ != '\0')
;
a null statement is used to supply an empty loop body to the
iteration statement.
[#6] EXAMPLE 3 A null statement may also be used to carry a
label just before the closing } of a compound statement.
while (loop1) {
/* ... */
while (loop2) {
/* ... */
if (want_out)
goto end_loop1;
/* ... */
}
/* ... */
end_loop1: ;
}
Forward references: iteration statements (6.8.5).
6.8.4 Selection statements
Syntax
[#1]
6.8.3 Language 6.8.4
WG14/N843 Committee Draft -- August 3, 1998 147
selection-statement:
if ( expression ) statement
if ( expression ) statement else statement
switch ( expression ) statement
Semantics
[#2] A selection statement selects among a set of statements
depending on the value of a controlling expression.
6.8.4.1 The if statement
Constraints
[#1] The controlling expression of an if statement shall
have scalar type.
Semantics
[#2] In both forms, the first substatement is executed if
the expression compares unequal to 0. In the else form, the
second substatement is executed if the expression compares
equal to 0. If the first substatement is reached via a
label, the second substatement is not executed.
[#3] An else is associated with the lexically nearest
preceding if that is allowed by the syntax.
6.8.4.2 The switch statement
Constraints
[#1] The controlling expression of a switch statement shall |
have integer type. |
[#2] If the switch statement causes a jump to within the |
scope of an identifier with a variably modified type, the |
entire switch statement shall be within the scope of that |
identifier.122) |
[#3] The expression of each case label shall be an integer |
constant expression and no two of the case constant
expressions in the same switch statement shall have the same
value after conversion. There may be at most one default
label in a switch statement. (Any enclosed switch statement
may have a default label or case constant expressions with
values that duplicate case constant expressions in the
enclosing switch statement.)
____________________
122That is, the declaration either precedes the switch
statement, or it follows the last case or default label
associated with the switch that is in the block
containing the declaration.
6.8.4 Language 6.8.4.2
148 Committee Draft -- August 3, 1998 WG14/N843
Semantics
[#4] A switch statement causes control to jump to, into, or
past the statement that is the switch body, depending on the
value of a controlling expression, and on the presence of a
default label and the values of any case labels on or in the
switch body. A case or default label is accessible only
within the closest enclosing switch statement.
[#5] The integer promotions are performed on the controlling
expression. The constant expression in each case label is
converted to the promoted type of the controlling
expression. If a converted value matches that of the
promoted controlling expression, control jumps to the
statement following the matched case label. Otherwise, if
there is a default label, control jumps to the labeled
statement. If no converted case constant expression matches
and there is no default label, no part of the switch body is
executed.
Implementation limits
[#6] As discussed in 5.2.4.1, the implementation may limit |
the number of case values in a switch statement.
[#7] EXAMPLE In the artificial program fragment
switch (expr)
{
int i = 4;
f(i);
case 0:
i = 17;
/* falls through into default code */
default:
printf("%d\n", i);
}
the object whose identifier is i exists with automatic
storage duration (within the block) but is never
initialized, and thus if the controlling expression has a
nonzero value, the call to the printf function will access
an indeterminate value. Similarly, the call to the function
f cannot be reached.
6.8.4.2 Language 6.8.4.2
WG14/N843 Committee Draft -- August 3, 1998 149
6.8.5 Iteration statements
Syntax
[#1]
iteration-statement:
while ( expression ) statement
do statement while ( expression ) ;
for ( expr-opt ; expr-opt ; expr-opt ) statement
for ( declaration ; expr-opt ; expr-opt ) statement
Constraints
[#2] The controlling expression of an iteration statement
shall have scalar type.
[#3] The declaration part of a for statement shall only
declare identifiers for objects having storage class auto or
register.
Semantics
[#4] An iteration statement causes a statement called the
loop body to be executed repeatedly until the controlling
expression compares equal to 0.
6.8.5.1 The while statement
[#1] The evaluation of the controlling expression takes
place before each execution of the loop body.
6.8.5.2 The do statement
[#1] The evaluation of the controlling expression takes
place after each execution of the loop body.
6.8.5.3 The for statement
[#1] Except for the behavior of a continue statement in the |
loop body, the statement
for ( clause-1 ; expr-2 ; expr-3 ) statement
and the sequence of statements
{
clause-1 ;
while ( expr-2 ) {
statement
expr-3 ;
}
}
6.8.5 Language 6.8.5.3
150 Committee Draft -- August 3, 1998 WG14/N843
are equivalent (where clause-1 can be an expression or a
declaration).123) Unlike the other iteration statements, |
the for statement introduces new blocks that limit the scope |
of declarations and compound literals occurring in the loop.
[#2] Both clause-1 and expr-3 can be omitted. If clause-1 |
is an expression, it is evaluated as a void expression, as |
is expr-3. An omitted expr-2 is replaced by a nonzero
constant.
Forward references: the continue statement (6.8.6.2).
6.8.6 Jump statements
Syntax
[#1]
jump-statement:
goto identifier ;
continue ;
break ;
return expression-opt ;
Semantics
[#2] A jump statement causes an unconditional jump to
another place.
____________________
123Thus, clause-1 specifies initialization for the loop,
possibly declaring one or more variables for use in the
loop; expr-2, the controlling expression, specifies an
evaluation made before each iteration, such that
execution of the loop continues until the expression
compares equal to 0; expr-3 specifies an operation (such
as incrementing) that is performed after each iteration.
If clause-1 is a declaration, then the scope of any
variable it declares is the remainder of the declaration
and the entire loop, including the other two expressions.
6.8.5.3 Language 6.8.6
WG14/N843 Committee Draft -- August 3, 1998 151
6.8.6.1 The goto statement
Constraints
[#1] The identifier in a goto statement shall name a label
located somewhere in the enclosing function. A goto |
statement shall not jump from outside the scope of an |
identifier having a variably modified type to inside the |
scope of that identifier.
Semantics
[#2] A goto statement causes an unconditional jump to the
statement prefixed by the named label in the enclosing
function.
[#3] EXAMPLE 1 It is sometimes convenient to jump into the
middle of a complicated set of statements. The following
outline presents one possible approach to a problem based on
these three assumptions:
1. The general initialization code accesses objects only
visible to the current function.
2. The general initialization code is too large to
warrant duplication.
3. The code to determine the next operation is at the
head of the loop. (To allow it to be reached by
continue statements, for example.)
/* ... */
goto first_time;
for (;;) {
// determine next operation
/* ... */
if (need to reinitialize) {
// reinitialize-only code
/* ... */
first_time:
// general initialization code
/* ... */
continue;
}
// handle other operations
/* ... */
}
[#4] EXAMPLE 2 A goto statement is not allowed to jump past
any declarations of objects with variably modified types. A |
jump within the scope, however, is permitted.
6.8.6 Language 6.8.6.1
152 Committee Draft -- August 3, 1998 WG14/N843
goto lab3; // Error: going INTO scope of VLA.
{
double a[n];
a[j] = 4.4;
lab3:
a[j] = 3.3;
goto lab4; // OK, going WITHIN scope of VLA.
a[j] = 5.5;
lab4:
a[j] = 6.6;
}
goto lab4; // Error: going INTO scope of VLA.
6.8.6.2 The continue statement
Constraints
[#1] A continue statement shall appear only in or as a loop
body.
Semantics
[#2] A continue statement causes a jump to the loop-
continuation portion of the smallest enclosing iteration
statement; that is, to the end of the loop body. More
precisely, in each of the statements
while (/* ... */) { do { for (/* ... */) {
/* ... */ /* ... */ /* ... */
continue; continue; continue;
/* ... */ /* ... */ /* ... */
contin: ; contin: ; contin: ;
} } while (/* ... */); }
unless the continue statement shown is in an enclosed
iteration statement (in which case it is interpreted within
that statement), it is equivalent to goto contin;.124)
____________________
124Following the contin: label is a null statement.
6.8.6.1 Language 6.8.6.2
WG14/N843 Committee Draft -- August 3, 1998 153
6.8.6.3 The break statement
Constraints
[#1] A break statement shall appear only in or as a switch
body or loop body.
Semantics
[#2] A break statement terminates execution of the smallest
enclosing switch or iteration statement.
6.8.6.4 The return statement
Constraints
[#1] A return statement with an expression shall not appear
in a function whose return type is void. A return statement
without an expression shall only appear in a function whose
return type is void.
Semantics
[#2] A return statement terminates execution of the current
function and returns control to its caller. A function may
have any number of return statements.
[#3] If a return statement with an expression is executed,
the value of the expression is returned to the caller as the
value of the function call expression. If the expression
has a type different from the return type of the function in
which it appears, the value is converted as if by assignment
to an object having the return type of the function.125) *
[#4] EXAMPLE In:
____________________
125The return statement is not an assignment. The overlap
restriction of subclause 6.5.16.1 does not apply to the
case of function return.
6.8.6.2 Language 6.8.6.4
154 Committee Draft -- August 3, 1998 WG14/N843
struct s { double i; } f(void);
union {
struct {
int f1;
struct s f2;
} u1;
struct {
struct s f3;
int f4;
} u2;
} g;
struct s f(void)
{
return g.u1.f2;
}
/* ... */
g.u2.f3 = f();
there is no undefined behavior, although there would be if |
the assignment were done directly (without using a function |
call to fetch the value).
6.8.6.4 Language 6.8.6.4
WG14/N843 Committee Draft -- August 3, 1998 155
6.9 External definitions
Syntax
[#1]
translation-unit:
external-declaration
translation-unit external-declaration
external-declaration:
function-definition
declaration
Constraints
[#2] The storage-class specifiers auto and register shall
not appear in the declaration specifiers in an external
declaration.
[#3] There shall be no more than one external definition for
each identifier declared with internal linkage in a
translation unit. Moreover, if an identifier declared with
internal linkage is used in an expression (other than as a
part of the operand of a sizeof operator), there shall be
exactly one external definition for the identifier in the
translation unit.
Semantics
[#4] As discussed in 5.1.1.1, the unit of program text after
preprocessing is a translation unit, which consists of a
sequence of external declarations. These are described as
``external'' because they appear outside any function (and
hence have file scope). As discussed in 6.7, a declaration
that also causes storage to be reserved for an object or a
function named by the identifier is a definition.
[#5] An external definition is an external declaration that
is also a definition of a function or an object. If an
identifier declared with external linkage is used in an
expression (other than as part of the operand of a sizeof
operator), somewhere in the entire program there shall be
exactly one external definition for the identifier;
otherwise, there shall be no more than one.126)
____________________
126Thus, if an identifier declared with external linkage is
not used in an expression, there need be no external
definition for it.
6.9 Language 6.9
156 Committee Draft -- August 3, 1998 WG14/N843
6.9.1 Function definitions
Syntax
[#1]
function-definition:
declaration-specifiers declarator declaration-list-opt compound-statement
declaration-list:
declaration
declaration-list declaration
Constraints
[#2] The identifier declared in a function definition (which
is the name of the function) shall have a function type, as
specified by the declarator portion of the function
definition.127)
[#3] The return type of a function shall be void or an
object type other than array type.
[#4] The storage-class specifier, if any, in the declaration
specifiers shall be either extern or static.
[#5] If the declarator includes a parameter type list, the |
declaration of each parameter shall include an identifier, |
except for the special case of a parameter list consisting
of a single parameter of type void, in which case there |
shall not be an identifier. No declaration list shall
follow.
[#6] If the declarator includes an identifier list, each
declaration in the declaration list shall have at least one
declarator, those declarators shall declare only identifiers
from the identifier list, and every identifier in the
identifier list shall be declared. An identifier declared
as a typedef name shall not be redeclared as a parameter.
The declarations in the declaration list shall contain no
____________________
127The intent is that the type category in a function
definition cannot be inherited from a typedef:
typedef int F(void); /* type F is ``function of no arguments returning int'' */
F f, g; /* f and g both have type compatible with F */
F f { /* ... */ } /* WRONG: syntax/constraint error */
F g() { /* ... */ } /* WRONG: declares that g returns a function */
int f(void) { /* ... */ } /* RIGHT: f has type compatible with F */
int g() { /* ... */ } /* RIGHT: g has type compatible with F */
F *e(void) { /* ... */ } /* e returns a pointer to a function */
F *((e))(void) { /* ... */ } /* same: parentheses irrelevant */
int (*fp)(void); /* fp points to a function that has type F */
F *Fp; /* Fp points to a function that has type F */
6.9.1 Language 6.9.1
WG14/N843 Committee Draft -- August 3, 1998 157
storage-class specifier other than register and no
initializations.
Semantics
[#7] The declarator in a function definition specifies the
name of the function being defined and the identifiers of
its parameters. If the declarator includes a parameter type
list, the list also specifies the types of all the
parameters; such a declarator also serves as a function
prototype for later calls to the same function in the same
translation unit. If the declarator includes an identifier
list,128) the types of the parameters shall be declared in a
following declaration list. In either case, the type of |
each parameter is adjusted as described in 6.7.5.3 for a |
parameter type list; the resulting type shall be an object |
type.
[#8] If a function that accepts a variable number of
arguments is defined without a parameter type list that ends
with the ellipsis notation, the behavior is undefined.
[#9] Each parameter has automatic storage duration. Its |
identifier is an lvalue, which is in effect declared at the
head of the compound statement that constitutes the function |
body (and therefore cannot be redeclared in the function |
body except in an enclosed block). The layout of the
storage for parameters is unspecified.
[#10] On entry to the function, all size expressions of |
variably modified parameters are evaluated and the value of |
each argument expression is converted to the type of the |
corresponding parameter as if by assignment. (Array |
expressions and function designators as arguments were |
converted to pointers before the call.) *
[#11] After all parameters have been assigned, the compound
statement that constitutes the body of the function
definition is executed.
[#12] If the } that terminates a function is reached, and
the value of the function call is used by the caller, the
behavior is undefined.
[#13] EXAMPLE 1 In the following:
extern int max(int a, int b)
{
return a > b ? a : b;
}
____________________
128See ``future language directions'' (6.11.4).
6.9.1 Language 6.9.1
158 Committee Draft -- August 3, 1998 WG14/N843
extern is the storage-class specifier and int is the type
specifier; max(int a, int b) is the function declarator; and
{ return a > b ? a : b; }
is the function body. The following similar definition uses
the identifier-list form for the parameter declarations:
extern int max(a, b)
int a, b;
{
return a > b ? a : b;
}
Here int a, b; is the declaration list for the parameters.
The difference between these two definitions is that the
first form acts as a prototype declaration that forces
conversion of the arguments of subsequent calls to the
function, whereas the second form does not. |
[#14] EXAMPLE 2 To pass one function to another, one might
say
int f(void);
/* ... */
g(f);
Then the definition of g might read
void g(int (*funcp)(void))
{
/* ... */ (*funcp)() /* or funcp() ... */
}
or, equivalently,
void g(int func(void))
{
/* ... */ func() /* or (*func)() ... */
}
6.9.2 External object definitions
Semantics
[#1] If the declaration of an identifier for an object has
file scope and an initializer, the declaration is an
external definition for the identifier.
[#2] A declaration of an identifier for an object that has
file scope without an initializer, and without a storage-
6.9.1 Language 6.9.2
WG14/N843 Committee Draft -- August 3, 1998 159
class specifier or with the storage-class specifier static,
constitutes a tentative definition. If a translation unit
contains one or more tentative definitions for an
identifier, and the translation unit contains no external
definition for that identifier, then the behavior is exactly
as if the translation unit contains a file scope declaration
of that identifier, with the composite type as of the end of
the translation unit, with an initializer equal to 0.
[#3] If the declaration of an identifier for an object is a
tentative definition and has internal linkage, the declared
type shall not be an incomplete type.
[#4] EXAMPLE 1
int i1 = 1; // definition, external linkage
static int i2 = 2; // definition, internal linkage
extern int i3 = 3; // definition, external linkage
int i4; // tentative definition, external linkage
static int i5; // tentative definition, internal linkage
int i1; // valid tentative definition, refers to previous
int i2; // 6.2.2 renders undefined, linkage disagreement
int i3; // valid tentative definition, refers to previous
int i4; // valid tentative definition, refers to previous
int i5; // 6.2.2 renders undefined, linkage disagreement
extern int i1; // refers to previous, whose linkage is external
extern int i2; // refers to previous, whose linkage is internal
extern int i3; // refers to previous, whose linkage is external
extern int i4; // refers to previous, whose linkage is external
extern int i5; // refers to previous, whose linkage is internal
[#5] EXAMPLE 2 If at the end of the translation unit
containing
int i[];
the array i still has incomplete type, the implicit |
initializer causes it to have one element, which is set to |
zero on program startup.
6.9.2 Language 6.9.2
160 Committee Draft -- August 3, 1998 WG14/N843
6.10 Preprocessing directives
Syntax
[#1]
preprocessing-file:
group-opt
group:
group-part
group group-part
group-part:
pp-tokens-opt new-line
if-section
control-line
if-section:
if-group elif-groups-opt else-group-opt endif-line
if-group:
# if constant-expr new-line group-opt
# ifdef identifier new-line group-opt
# ifndef identifier new-line group-opt
elif-groups:
elif-group
elif-groups elif-group
elif-group:
# elif constant-expr new-line group-opt
else-group:
# else new-line group-opt
endif-line:
# endif new-line
control-line:
# include pp-tokens new-line
# define identifier replacement-list new-line
# define identifier lparen identifier-list-opt )
replacement-list new-line
# define identifier lparen ... ) replacement-list new-line
# define identifier lparen identifier-list , ... )
replacement-list new-line
# undef identifier new-line
# line pp-tokens new-line
# error pp-tokens-opt new-line
# pragma pp-tokens-opt new-line
# new-line
6.10 Language 6.10
WG14/N843 Committee Draft -- August 3, 1998 161
lparen: |
a ( character not immediately preceded by white-space|
replacement-list:
pp-tokens-opt
pp-tokens:
preprocessing-token
pp-tokens preprocessing-token
new-line:
the new-line character
Description
[#2] A preprocessing directive consists of a sequence of
preprocessing tokens that begins with a # preprocessing
token that (at the start of translation phase 4) is either
the first character in the source file (optionally after
white space containing no new-line characters) or that
follows white space containing at least one new-line
character, and is ended by the next new-line character.129)
A new-line character ends the preprocessing directive even
if it occurs within what would otherwise be an invocation of
a function-like macro.
Constraints
[#3] The only white-space characters that shall appear
between preprocessing tokens within a preprocessing
directive (from just after the introducing # preprocessing
token through just before the terminating new-line
character) are space and horizontal-tab (including spaces
that have replaced comments or possibly other white-space
characters in translation phase 3). *
Semantics
[#4] The implementation can process and skip sections of
source files conditionally, include other source files, and
replace macros. These capabilities are called
preprocessing, because conceptually they occur before
translation of the resulting translation unit.
[#5] The preprocessing tokens within a preprocessing
directive are not subject to macro expansion unless
____________________
129Thus, preprocessing directives are commonly called
``lines''. These ``lines'' have no other syntactic
significance, as all white space is equivalent except in
certain situations during preprocessing (see the #
character string literal creation operator in 6.10.3.2,
for example).
6.10 Language 6.10
162 Committee Draft -- August 3, 1998 WG14/N843
otherwise stated.
[#6] EXAMPLE In:
#define EMPTY
EMPTY # include <file.h>
the sequence of preprocessing tokens on the second line is
not a preprocessing directive, because it does not begin
with a # at the start of translation phase 4, even though it
will do so after the macro EMPTY has been replaced.
6.10.1 Conditional inclusion
Constraints
[#1] The expression that controls conditional inclusion
shall be an integer constant expression except that: it
shall not contain a cast; identifiers (including those
lexically identical to keywords) are interpreted as
described below;130) and it may contain unary operator
expressions of the form
defined identifier
or
defined ( identifier )
which evaluate to 1 if the identifier is currently defined
as a macro name (that is, if it is predefined or if it has
been the subject of a #define preprocessing directive
without an intervening #undef directive with the same
subject identifier), 0 if it is not.
Semantics
[#2] Preprocessing directives of the forms
# if constant-expr new-line group-opt
# elif constant-expr new-line group-opt
check whether the controlling constant expression evaluates
to nonzero.
[#3] Prior to evaluation, macro invocations in the list of
preprocessing tokens that will become the controlling
constant expression are replaced (except for those macro
names modified by the defined unary operator), just as in
____________________
130Because the controlling constant expression is evaluated
during translation phase 4, all identifiers either are or
are not macro names -- there simply are no keywords,
enumeration constants, etc.
6.10 Language 6.10.1
WG14/N843 Committee Draft -- August 3, 1998 163
normal text. If the token defined is generated as a result
of this replacement process or use of the defined unary
operator does not match one of the two specified forms prior
to macro replacement, the behavior is undefined. After all
replacements due to macro expansion and the defined unary
operator have been performed, all remaining identifiers are
replaced with the pp-number 0, and then each preprocessing
token is converted into a token. The resulting tokens
compose the controlling constant expression which is
evaluated according to the rules of 6.6, except that all
signed integer types and all unsigned integer types act as
if they have the same representation as, respectively, the
types intmax_t and uintmax_t defined in the header
<stdint.h>. This includes interpreting character constants,
which may involve converting escape sequences into execution
character set members. Whether the numeric value for these
character constants matches the value obtained when an
identical character constant occurs in an expression (other
than within a #if or #elif directive) is
implementation-defined.131) Also, whether a single-
character character constant may have a negative value is
implementation-defined.
[#4] Preprocessing directives of the forms
# ifdef identifier new-line group-opt
# ifndef identifier new-line group-opt
check whether the identifier is or is not currently defined
as a macro name. Their conditions are equivalent to #if
defined identifier and #if !defined identifier respectively.
[#5] Each directive's condition is checked in order. If it
evaluates to false (zero), the group that it controls is
skipped: directives are processed only through the name that
determines the directive in order to keep track of the level
of nested conditionals; the rest of the directives'
preprocessing tokens are ignored, as are the other
preprocessing tokens in the group. Only the first group
whose control condition evaluates to true (nonzero) is
processed. If none of the conditions evaluates to true, and
there is a #else directive, the group controlled by the
#else is processed; lacking a #else directive, all the
groups until the #endif are skipped.132)
____________________
131Thus, the constant expression in the following #if
directive and if statement is not guaranteed to evaluate
to the same value in these two contexts.
#if 'z' - 'a' == 25
if ('z' - 'a' == 25)
6.10.1 Language 6.10.1
164 Committee Draft -- August 3, 1998 WG14/N843
Forward references: macro replacement (6.10.3), source file
inclusion (6.10.2), largest integer types (7.18.1.5).
6.10.2 Source file inclusion
Constraints
[#1] A #include directive shall identify a header or source
file that can be processed by the implementation.
Semantics
[#2] A preprocessing directive of the form
# include <h-char-sequence> new-line
searches a sequence of implementation-defined places for a
header identified uniquely by the specified sequence between
the < and > delimiters, and causes the replacement of that
directive by the entire contents of the header. How the
places are specified or the header identified is
implementation-defined.
[#3] A preprocessing directive of the form
# include "q-char-sequence" new-line
causes the replacement of that directive by the entire
contents of the source file identified by the specified
sequence between the " delimiters. The named source file is
searched for in an implementation-defined manner. If this
search is not supported, or if the search fails, the
directive is reprocessed as if it read
# include <h-char-sequence> new-line
with the identical contained sequence (including >
characters, if any) from the original directive.
[#4] A preprocessing directive of the form
# include pp-tokens new-line
(that does not match one of the two previous forms) is
permitted. The preprocessing tokens after include in the
directive are processed just as in normal text. (Each
identifier currently defined as a macro name is replaced by
____________________
132As indicated by the syntax, a preprocessing token shall
not follow a #else or #endif directive before the
terminating new-line character. However, comments may
appear anywhere in a source file, including within a
preprocessing directive.
6.10.1 Language 6.10.2
WG14/N843 Committee Draft -- August 3, 1998 165
its replacement list of preprocessing tokens.) The
directive resulting after all replacements shall match one
of the two previous forms.133) The method by which a
sequence of preprocessing tokens between a < and a >
preprocessing token pair or a pair of " characters is
combined into a single header name preprocessing token is
implementation-defined.
[#5] The implementation shall provide unique mappings for
sequences consisting of one or more letters or digits (as
defined in 5.2.1) followed by a period (.) and a single
letter. The first character shall be a letter. The
implementation may ignore the distinctions of alphabetical
case and restrict the mapping to eight significant
characters before the period.
[#6] A #include preprocessing directive may appear in a
source file that has been read because of a #include
directive in another file, up to an implementation-defined
nesting limit (see 5.2.4.1).
[#7] EXAMPLE 1 The most common uses of #include
preprocessing directives are as in the following:
#include <stdio.h>
#include "myprog.h"
[#8] EXAMPLE 2 This illustrates macro-replaced #include
directives:
#if VERSION == 1
#define INCFILE "vers1.h"
#elif VERSION == 2
#define INCFILE "vers2.h" // and so on
#else
#define INCFILE "versN.h"
#endif
#include INCFILE
Forward references: macro replacement (6.10.3).
____________________
133Note that adjacent string literals are not concatenated
into a single string literal (see the translation phases
in 5.1.1.2); thus, an expansion that results in two
string literals is an invalid directive.
6.10.2 Language 6.10.2
166 Committee Draft -- August 3, 1998 WG14/N843
6.10.3 Macro replacement
Constraints
[#1] Two replacement lists are identical if and only if the
preprocessing tokens in both have the same number, ordering,
spelling, and white-space separation, where all white-space
separations are considered identical.
[#2] An identifier currently defined as a macro without use
of lparen (an object-like macro) shall not be redefined by
another #define preprocessing directive unless the second
definition is an object-like macro definition and the two
replacement lists are identical.
[#3] An identifier currently defined as a macro using lparen
(a function-like macro) shall not be redefined by another
#define preprocessing directive unless the second definition
is a function-like macro definition that has the same number
and spelling of parameters, and the two replacement lists
are identical.
[#4] If the identifier-list in the macro definition does not
end with an ellipsis, the number of arguments, including
those arguments consisting of no preprocessing tokens, in an
invocation of a function-like macro shall agree with the
number of parameters in the macro definition. Otherwise,
there shall be more arguments in the invocation than there
are parameters in the macro definition (excluding the ...).
There shall exist a ) preprocessing token that terminates
the invocation.
[#5] The identifier __VA_ARGS__ shall only occur in the
replacement-list of a #define preprocessing directive using
the ellipsis notation in the arguments.
[#6] A parameter identifier in a function-like macro shall
be uniquely declared within its scope.
Semantics
[#7] The identifier immediately following the define is
called the macro name. There is one name space for macro
names. Any white-space characters preceding or following
the replacement list of preprocessing tokens are not
considered part of the replacement list for either form of
macro.
[#8] If a # preprocessing token, followed by an identifier,
occurs lexically at the point at which a preprocessing
directive could begin, the identifier is not subject to
macro replacement.
[#9] A preprocessing directive of the form
6.10.3 Language 6.10.3
WG14/N843 Committee Draft -- August 3, 1998 167
# define identifier replacement-list new-line
defines an object-like macro that causes each subsequent
instance of the macro name134) to be replaced by the
replacement list of preprocessing tokens that constitute the
remainder of the directive. *
[#10] A preprocessing directive of the form
# define identifier lparen identifier-list-opt ) replacement-list new-line
# define identifier lparen ... ) replacement-list new-line
# define identifier lparen identifier-list , ... ) replacement-list new-line
defines a function-like macro with arguments, similar
syntactically to a function call. The parameters are
specified by the optional list of identifiers, whose scope
extends from their declaration in the identifier list until
the new-line character that terminates the #define
preprocessing directive. Each subsequent instance of the
function-like macro name followed by a ( as the next
preprocessing token introduces the sequence of preprocessing
tokens that is replaced by the replacement list in the
definition (an invocation of the macro). The replaced
sequence of preprocessing tokens is terminated by the
matching ) preprocessing token, skipping intervening matched
pairs of left and right parenthesis preprocessing tokens.
Within the sequence of preprocessing tokens making up an
invocation of a function-like macro, new-line is considered
a normal white-space character.
[#11] The sequence of preprocessing tokens bounded by the
outside-most matching parentheses forms the list of
arguments for the function-like macro. The individual
arguments within the list are separated by comma
preprocessing tokens, but comma preprocessing tokens between
matching inner parentheses do not separate arguments. If
there are sequences of preprocessing tokens within the list
of arguments that would otherwise act as preprocessing
directives, the behavior is undefined.
[#12] If there is a ... in the identifier-list in the macro
definition, then the trailing arguments, including any
separating comma preprocessing tokens, are merged to form a
single item: the variable arguments. The number of arguments
so combined is such that, following merger, the number of
arguments is one more than the number of parameters in the
macro definition (excluding the ...).
____________________
134Since, by macro-replacement time, all character constants
and string literals are preprocessing tokens, not
sequences possibly containing identifier-like
subsequences (see 5.1.1.2, translation phases), they are
never scanned for macro names or parameters.
6.10.3 Language 6.10.3
168 Committee Draft -- August 3, 1998 WG14/N843
6.10.3.1 Argument substitution
[#1] After the arguments for the invocation of a function-
like macro have been identified, argument substitution takes
place. A parameter in the replacement list, unless preceded
by a # or ## preprocessing token or followed by a ##
preprocessing token (see below), is replaced by the
corresponding argument after all macros contained therein
have been expanded. Before being substituted, each
argument's preprocessing tokens are completely macro
replaced as if they formed the rest of the preprocessing
file; no other preprocessing tokens are available.
[#2] An identifier __VA_ARGS__ that occurs in the
replacement list shall be treated as if it were a parameter,
and the variable arguments shall form the preprocessing
tokens used to replace it.
6.10.3.2 The # operator
Constraints
[#1] Each # preprocessing token in the replacement list for
a function-like macro shall be followed by a parameter as
the next preprocessing token in the replacement list.
Semantics
[#2] If, in the replacement list, a parameter is immediately
preceded by a # preprocessing token, both are replaced by a
single character string literal preprocessing token that
contains the spelling of the preprocessing token sequence
for the corresponding argument. Each occurrence of white
space between the argument's preprocessing tokens becomes a
single space character in the character string literal.
White space before the first preprocessing token and after
the last preprocessing token composing the argument is |
deleted. Otherwise, the original spelling of each
preprocessing token in the argument is retained in the
character string literal, except for special handling for
producing the spelling of string literals and character
constants: a \ character is inserted before each " and \
character of a character constant or string literal
(including the delimiting " characters), except that it is |
unspecified whether a \ character is inserted before the \ |
character beginning a universal character name. If the
replacement that results is not a valid character string
literal, the behavior is undefined. The character string
literal corresponding to an empty argument is "". The order
of evaluation of # and ## operators is unspecified.
6.10.3 Language 6.10.3.2
WG14/N843 Committee Draft -- August 3, 1998 169
6.10.3.3 The ## operator
Constraints
[#1] A ## preprocessing token shall not occur at the
beginning or at the end of a replacement list for either
form of macro definition.
Semantics
[#2] If, in the replacement list of a function-like macro, a |
parameter is immediately preceded or followed by a ##
preprocessing token, the parameter is replaced by the
corresponding argument's preprocessing token sequence;
however, if an argument consists of no preprocessing tokens,
the parameter is replaced by a placemarker preprocessing
token instead.
[#3] For both object-like and function-like macro
invocations, before the replacement list is reexamined for
more macro names to replace, each instance of a ##
preprocessing token in the replacement list (not from an
argument) is deleted and the preceding preprocessing token
is concatenated with the following preprocessing token. |
Placemarker preprocessing tokens are handled specially:
concatenation of two placemarkers results in a single
placemarker preprocessing token, and concatenation of a |
placemarker with a non-placemarker preprocessing token
results in the non-placemarker preprocessing token. If the |
result is not a valid preprocessing token, the behavior is
undefined. The resulting token is available for further
macro replacement. The order of evaluation of ## operators
is unspecified.
[#4] EXAMPLE In the following fragment: |
#define hash_hash # ## #
#define mkstr(a) # a
#define in_between(a) mkstr(a)
#define join(c, d) in_between(c hash_hash d)
char p[] = join(x, y); // equivalent to
// char p[] = "x ## y";
The expansion produces, at various stages:
6.10.3.2 Language 6.10.3.3
170 Committee Draft -- August 3, 1998 WG14/N843
join(x, y)
in_between(x hash_hash y)
in_between(x ## y)
mkstr(x ## y)
"x ## y"
In other words, expanding hash_hash produces a new token,
consisting of two adjacent sharp signs, but this new token |
is not the ## operator.
6.10.3.4 Rescanning and further replacement
[#1] After all parameters in the replacement list have been
substituted and # and ## processing has taken place, all
placemarker preprocessing tokens are removed. Then, the |
resulting preprocessing token sequence is rescanned, along |
with all subsequent preprocessing tokens of the source file, |
for more macro names to replace.
[#2] If the name of the macro being replaced is found during
this scan of the replacement list (not including the rest of
the source file's preprocessing tokens), it is not replaced.
Further, if any nested replacements encounter the name of
the macro being replaced, it is not replaced. These
nonreplaced macro name preprocessing tokens are no longer
available for further replacement even if they are later
(re)examined in contexts in which that macro name
preprocessing token would otherwise have been replaced.
[#3] The resulting completely macro-replaced preprocessing
token sequence is not processed as a preprocessing directive
even if it resembles one, but all pragma unary operator
expressions within it are then processed as specified in
6.10.9 below.
6.10.3.5 Scope of macro definitions
[#1] A macro definition lasts (independent of block
structure) until a corresponding #undef directive is
encountered or (if none is encountered) until the end of
translation phase 4.
[#2] A preprocessing directive of the form
# undef identifier new-line
causes the specified identifier no longer to be defined as a
macro name. It is ignored if the specified identifier is
not currently defined as a macro name.
6.10.3.3 Language 6.10.3.5
WG14/N843 Committee Draft -- August 3, 1998 171
[#3] EXAMPLE 1 The simplest use of this facility is to
define a ``manifest constant'', as in
#define TABSIZE 100
int table[TABSIZE];
[#4] EXAMPLE 2 The following defines a function-like macro
whose value is the maximum of its arguments. It has the
advantages of working for any compatible types of the
arguments and of generating in-line code without the
overhead of function calling. It has the disadvantages of
evaluating one or the other of its arguments a second time
(including side effects) and generating more code than a
function if invoked several times. It also cannot have its
address taken, as it has none.
#define max(a, b) ((a) > (b) ? (a) : (b))
The parentheses ensure that the arguments and the resulting
expression are bound properly.
[#5] EXAMPLE 3 To illustrate the rules for redefinition and
reexamination, the sequence
#define x 3
#define f(a) f(x * (a))
#undef x
#define x 2
#define g f
#define z z[0]
#define h g(~
#define m(a) a(w)
#define w 0,1
#define t(a) a
#define p() int
#define q(x) x
#define r(x,y) x ## y
#define str(x) # x
f(y+1) + f(f(z)) % t(t(g)(0) + t)(1);
g(x+(3,4)-w) | h 5) & m
(f)^m(m);
p() i[q()] = { q(1), r(2,3), r(4,), r(,5), r(,) };
char c[2][6] = { str(hello), str() };
results in
6.10.3.5 Language 6.10.3.5
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f(2 * (y+1)) + f(2 * (f(2 * (z[0])))) % f(2 * (0)) + t(1);
f(2 * (2+(3,4)-0,1)) | f(2 * (~ 5)) & f(2 * (0,1))^m(0,1);
int i[] = { 1, 23, 4, 5, };
char c[2][6] = { "hello", "" };
[#6] EXAMPLE 4 To illustrate the rules for creating
character string literals and concatenating tokens, the
sequence
#define str(s) # s
#define xstr(s) str(s)
#define debug(s, t) printf("x" # s "= %d, x" # t "= %s", \
x ## s, x ## t)
#define INCFILE(n) vers ## n // from previous #include example
#define glue(a, b) a ## b
#define xglue(a, b) glue(a, b)
#define HIGHLOW "hello"
#define LOW LOW ", world"
debug(1, 2);
fputs(str(strncmp("abc\0d", "abc", '\4') // this goes away
== 0) str(: @\n), s);
#include xstr(INCFILE(2).h)
glue(HIGH, LOW);
xglue(HIGH, LOW)
results in
printf("x" "1" "= %d, x" "2" "= %s", x1, x2);
fputs(
"strncmp(\"abc\\0d\", \"abc\", '\\4') == 0" ": @\n",
s);
#include "vers2.h" (after macro replacement, before file access)
"hello";
"hello" ", world"
or, after concatenation of the character string literals,
printf("x1= %d, x2= %s", x1, x2);
fputs(
"strncmp(\"abc\\0d\", \"abc\", '\\4') == 0: @\n",
s);
#include "vers2.h" (after macro replacement, before file access)
"hello";
"hello, world"
Space around the # and ## tokens in the macro definition is
optional.
[#7] EXAMPLE 5 To illustrate the rules for
6.10.3.5 Language 6.10.3.5
WG14/N843 Committee Draft -- August 3, 1998 173
placemarker ## placemarker
the sequence
#define t(x,y,z) x ## y ## z
int j[] = { t(1,2,3), t(,4,5), t(6,,7), t(8,9,),
t(10,,), t(,11,), t(,,12), t(,,) };
results in
int j[] = { 123, 45, 67, 89,
10, 11, 12, };
[#8] EXAMPLE 6 To demonstrate the redefinition rules, the
following sequence is valid.
#define OBJ_LIKE (1-1)
#define OBJ_LIKE /* white space */ (1-1) /* other */
#define FUNC_LIKE(a) ( a )
#define FUNC_LIKE( a )( /* note the white space */ \
a /* other stuff on this line
*/ )
But the following redefinitions are invalid:
#define OBJ_LIKE (0) /* different token sequence */
#define OBJ_LIKE (1 - 1) /* different white space */
#define FUNC_LIKE(b) ( a ) /* different parameter usage */
#define FUNC_LIKE(b) ( b ) /* different parameter spelling */
[#9] EXAMPLE 7 Finally, to show the variable argument list
macro facilities:
#define debug(...) fprintf(stderr, __VA_ARGS__)
#define showlist(...) puts(#__VA_ARGS__)
#define report(test, ...) ((test)?puts(#test):\
printf(__VA_ARGS__))
debug("Flag");
debug("X = %d\n", x);
showlist(The first, second, and third items.);
report(x>y, "x is %d but y is %d", x, y);
results in
fprintf(stderr, "Flag" );
fprintf(stderr, "X = %d\n", x );
puts( "The first, second, and third items." );
((x>y)?puts("x>y"):
printf("x is %d but y is %d", x, y));
6.10.3.5 Language 6.10.3.5
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6.10.4 Line control
Constraints
[#1] The string literal of a #line directive, if present,
shall be a character string literal.
Semantics
[#2] The line number of the current source line is one
greater than the number of new-line characters read or
introduced in translation phase 1 (5.1.1.2) while processing
the source file to the current token.
[#3] A preprocessing directive of the form
# line digit-sequence new-line
causes the implementation to behave as if the following
sequence of source lines begins with a source line that has
a line number as specified by the digit sequence
(interpreted as a decimal integer). The digit sequence
shall not specify zero, nor a number greater than
2147483647.
[#4] A preprocessing directive of the form
# line digit-sequence "s-char-sequence-opt" new-line
sets the presumed line number similarly and changes the |
presumed name of the source file to be the contents of the
character string literal.
[#5] A preprocessing directive of the form
# line pp-tokens new-line
(that does not match one of the two previous forms) is
permitted. The preprocessing tokens after line on the
directive are processed just as in normal text (each
identifier currently defined as a macro name is replaced by
its replacement list of preprocessing tokens). The
directive resulting after all replacements shall match one
of the two previous forms and is then processed as
appropriate.
6.10.3.5 Language 6.10.4
WG14/N843 Committee Draft -- August 3, 1998 175
6.10.5 Error directive
Semantics
[#1] A preprocessing directive of the form
# error pp-tokens-opt new-line
causes the implementation to produce a diagnostic message
that includes the specified sequence of preprocessing
tokens.
6.10.6 Pragma directive
Semantics
[#1] A preprocessing directive of the form
# pragma pp-tokens-opt new-line
where the preprocessing token STDC does not immediately
follow pragma in the directive (prior to any macro
replacement)135) causes the implementation to behave in a
manner which it shall document. The behavior might cause |
translation to fail or cause the translator or the resulting |
program to behave in a non-conforming manner. Any such
pragma that is not recognized by the implementation is
ignored.
[#2] If the preprocessing token STDC does immediately follow
pragma in the directive (prior to any macro replacement),
then no macro replacement is performed on the directive, and
the directive shall have one of the following forms whose
meanings are described elsewhere:
#pragma STDC FP_CONTRACT on-off-switch
#pragma STDC FENV_ACCESS on-off-switch
#pragma STDC CX_LIMITED_RANGE on-off-switch
on-off-switch: one of
ON OFF DEFAULT
____________________
135An implementation is not required to perform macro
replacement in pragmas, but it is permitted except for in
standard pragmas (where STDC immediately follows pragma).
If the result of macro replacement in a non-standard
pragma has the same form as a standard pragma, the
behavior is still implementation-defined; an
implementation is permitted to behave as if it were the
standard pragma, but is not required to.
6.10.5 Language 6.10.6
176 Committee Draft -- August 3, 1998 WG14/N843
Forward references: the FP_CONTRACT pragma (7.12.2), the
FENV_ACCESS pragma (7.6.1), the CX_LIMITED_RANGE pragma
(7.3.4).
6.10.7 Null directive
Semantics
[#1] A preprocessing directive of the form
# new-line
has no effect.
6.10.8 Predefined macro names
[#1] The following macro names shall be defined by the
implementation:
__LINE__ The presumed line number (within the current source |
file) of the current source line (a decimal |
constant).136)
__FILE__ The presumed name of the current source file (a |
character string literal).136)
__DATE__ The date of translation of the source file: a |
character string literal of the form "Mmm dd yyyy",
where the names of the months are the same as those
generated by the asctime function, and the first
character of dd is a space character if the value |
is less than 10. If the date of translation is not
available, an implementation-defined valid date
shall be supplied.
__TIME__ The time of translation of the source file: a |
character string literal of the form "hh:mm:ss" as
in the time generated by the asctime function. If |
the time of translation is not available, an
implementation-defined valid time shall be
supplied.
__STDC__ The decimal constant 1, intended to indicate a
conforming implementation.
__STDC_VERSION__ The decimal constant 199901L.137)
____________________
136The presumed line number and source file name can be
changed by the #line directive.
137This macro was not specified in ISO/IEC 9899:1990 and was |
specified as 199409L in ISO/IEC 9899:AMD1:1995
6.10.6 Language 6.10.8
WG14/N843 Committee Draft -- August 3, 1998 177
[#2] The following macro names are conditionally defined by
the implementation: |
__STDC_ISO_10646__ A decimal constant of the form yyyymmL |
(for example, 199712L), intended to |
indicate that values of type wchar_t are |
the coded representations of the |
characters defined by ISO/IEC 10646, |
along with all amendments and technical |
corrigenda as of the specified year and |
month.
__STDC_IEC_559__ The decimal constant 1, intended to
indicate conformance to the
specifications in annex F (IEC 60559
floating-point arithmetic).
__STDC_IEC_559_COMPLEX__ The decimal constant 1, intended to
indicate adherence to the specifications
in informative annex G (IEC 60559
compatible complex arithmetic).
[#3] The values of the predefined macros (except for
__LINE__ and __FILE__) remain constant throughout the
translation unit.
[#4] None of these macro names, nor the identifier defined,
shall be the subject of a #define or a #undef preprocessing
directive. Any other predefined macro names shall begin |
with a leading underscore followed by an uppercase letter or
a second underscore.
Forward references: the asctime function (7.23.3.1).
6.10.9 Pragma operator
Semantics
[#1] A unary operator expression of the form:
_Pragma ( string-literal )
is processed as follows: The string literal is destringized
by deleting the L prefix, if present, deleting the leading
and trailing double-quotes, replacing each escape sequence |
\" by a double-quote, and replacing each escape sequence \\ |
by a single backslash. The resulting sequence of characters
is processed through translation phase 3 to produce
preprocessing tokens that are executed as if they were the
pp-tokens in a pragma directive. The original four
preprocessing tokens in the unary operator expression are
removed.
[#2] EXAMPLE A directive of the form:
6.10.8 Language 6.10.9
178 Committee Draft -- August 3, 1998 WG14/N843
#pragma listing on "..\listing.dir" |
can also be expressed as:
_Pragma ( "listing on \"..\\listing.dir\"" ) |
The latter form is processed in the same way whether it
appears literally as shown, or results from macro
replacement, as in:
#define LISTING(x) PRAGMA(listing on #x)
#define PRAGMA(x) _Pragma(#x)
LISTING ( ..\listing.dir ) |
6.10.9 Language 6.10.9
WG14/N843 Committee Draft -- August 3, 1998 179
6.11 Future language directions
6.11.1 Character escape sequences
[#1] Lowercase letters as escape sequences are reserved for
future standardization. Other characters may be used in
extensions.
6.11.2 Storage-class specifiers
[#1] The placement of a storage-class specifier other than
at the beginning of the declaration specifiers in a
declaration is an obsolescent feature.
6.11.3 Function declarators
[#1] The use of function declarators with empty parentheses
(not prototype-format parameter type declarators) is an
obsolescent feature.
6.11.4 Function definitions
[#1] The use of function definitions with separate parameter
identifier and declaration lists (not prototype-format
parameter type and identifier declarators) is an obsolescent
feature.
6.11.5 Pragma directives
[#1] Pragmas whose first pp-token is STDC are reserved for
future standardization.
6.11 Language 6.11.5
180 Committee Draft -- August 3, 1998 WG14/N843
7. Library
7.1 Introduction
7.1.1 Definitions of terms
[#1] A string is a contiguous sequence of characters
terminated by and including the first null character. The |
term multibyte string is sometimes used instead to emphasize |
special processing given to multibyte characters contained |
in the string or to avoid confusion with a wide string. A
pointer to a string is a pointer to its initial (lowest
addressed) character. The length of a string is the number
of characters preceding the null character and the value of
a string is the sequence of the values of the contained
characters, in order.
[#2] A letter is a printing character in the execution
character set corresponding to any of the 52 required
lowercase and uppercase letters in the source character set,
listed in 5.2.1.
[#3] The decimal-point character is the character used by
functions that convert floating-point numbers to or from
character sequences to denote the beginning of the
fractional part of such character sequences.138) It is
represented in the text and examples by a period, but may be
changed by the setlocale function.
[#4] A wide character is a code value (a binary encoded
integer) of an object of type wchar_t that corresponds to a
member of the extended character set.139)
[#5] A null wide character is a wide character with code
value zero.
[#6] A wide string is a contiguous sequence of wide
characters terminated by and including the first null wide
character. A pointer to a wide string is a pointer to its
initial (lowest addressed) wide character. The length of a
wide string is the number of wide characters preceding the
null wide character and the value of a wide string is the
sequence of code values of the contained wide characters, in
____________________
138The functions that make use of the decimal-point
character are the string conversion functions (7.20.1),
the wide-string numeric conversion functions (7.24.4.1),
the formatted input/output functions (7.19.6), and the
formatted wide-character input/output functions (7.24.2).
139An equivalent definition can be found in 6.4.4.4.
7 Library 7.1.1
WG14/N843 Committee Draft -- August 3, 1998 181
order.
[#7] A shift sequence is a contiguous sequence of bytes
within a multibyte string that (potentially) causes a change
in shift state (see 5.2.1.2). A shift sequence shall not
have a corresponding wide character; it is instead taken to
be an adjunct to an adjacent multibyte character.140)
Forward references: character handling (7.4), the setlocale
function (7.11.1.1).
7.1.2 Standard headers
[#1] Each library function is declared, with a type that
includes a prototype, in a header,141) whose contents are
made available by the #include preprocessing directive. The
header declares a set of related functions, plus any
necessary types and additional macros needed to facilitate
their use. Declarations of types described in this clause
shall not include type qualifiers, unless explicitly stated
otherwise.
[#2] The standard headers are
<assert.h> <inttypes.h> <signal.h> <stdlib.h>
<complex.h> <iso646.h> <stdarg.h> <string.h>
<ctype.h> <limits.h> <stdbool.h> <tgmath.h>
<errno.h> <locale.h> <stddef.h> <time.h>
<fenv.h> <math.h> <stdint.h> <wchar.h>
<float.h> <setjmp.h> <stdio.h> <wctype.h>
[#3] If a file with the same name as one of the above < and
> delimited sequences, not provided as part of the
implementation, is placed in any of the standard places that |
are searched for included source files, the behavior is |
undefined.
[#4] Standard headers may be included in any order; each may
be included more than once in a given scope, with no effect
different from being included only once, except that the
effect of including <assert.h> depends on the definition of |
NDEBUG (see 7.2). If used, a header shall be included
____________________
140For state-dependent encodings, the values for MB_CUR_MAX
and MB_LEN_MAX shall thus be large enough to count all
the bytes in any complete multibyte character plus at
least one adjacent shift sequence of maximum length.
Whether these counts provide for more than one shift
sequence is the implementation's choice.
141A header is not necessarily a source file, nor are the <
and > delimited sequences in header names necessarily
valid source file names.
7.1.1 Library 7.1.2
182 Committee Draft -- August 3, 1998 WG14/N843
outside of any external declaration or definition, and it
shall first be included before the first reference to any of
the functions or objects it declares, or to any of the types
or macros it defines. However, if an identifier is declared
or defined in more than one header, the second and
subsequent associated headers may be included after the
initial reference to the identifier. The program shall not
have any macros with names lexically identical to keywords
currently defined prior to the inclusion.
[#5] Any definition of an object-like macro described in
this clause shall expand to code that is fully protected by
parentheses where necessary, so that it groups in an
arbitrary expression as if it were a single identifier.
[#6] Any declaration of a library function shall have
external linkage.
[#7] A summary of the contents of the standard headers is
given in annex B.
Forward references: diagnostics (7.2).
7.1.3 Reserved identifiers
[#1] Each header declares or defines all identifiers listed
in its associated subclause, and optionally declares or
defines identifiers listed in its associated future library
directions subclause and identifiers which are always
reserved either for any use or for use as file scope
identifiers.
-- All identifiers that begin with an underscore and
either an uppercase letter or another underscore are
always reserved for any use.
-- All identifiers that begin with an underscore are
always reserved for use as identifiers with file scope
in both the ordinary and tag name spaces.
-- Each macro name in any of the following subclauses
(including the future library directions) is reserved
for use as specified if any of its associated headers
is included; unless explicitly stated otherwise (see
7.1.4).
-- All identifiers with external linkage in any of the
following subclauses (including the future library
directions) are always reserved for use as identifiers
with external linkage.142)
____________________
142The list of reserved identifiers with external linkage
includes errno, setjmp, and va_end.
7.1.2 Library 7.1.3
WG14/N843 Committee Draft -- August 3, 1998 183
-- Each identifier with file scope listed in any of the
following subclauses (including the future library
directions) is reserved for use as macro and as an
identifier with file scope in the same name space if
any of its associated headers is included.
[#2] No other identifiers are reserved. If the program
declares or defines an identifier in a context in which it |
is reserved (other than as allowed by 7.1.4), or defines a
reserved identifier as a macro name, the behavior is
undefined.
[#3] If the program removes (with #undef) any macro
definition of an identifier in the first group listed above,
the behavior is undefined.
7.1.4 Use of library functions
[#1] Each of the following statements applies unless
explicitly stated otherwise in the detailed descriptions |
that follow: If an argument to a function has an invalid
value (such as a value outside the domain of the function,
or a pointer outside the address space of the program, or a
null pointer) or a type (after promotion) not expected by a
function with variable number of arguments, the behavior is
undefined. If a function argument is described as being an
array, the pointer actually passed to the function shall
have a value such that all address computations and accesses
to objects (that would be valid if the pointer did point to
the first element of such an array) are in fact valid. Any
function declared in a header may be additionally
implemented as a function-like macro defined in the header,
so if a library function is declared explicitly when its
header is included, one of the techniques shown below can be
used to ensure the declaration is not affected by such a
macro. Any macro definition of a function can be suppressed
locally by enclosing the name of the function in
parentheses, because the name is then not followed by the
left parenthesis that indicates expansion of a macro
function name. For the same syntactic reason, it is
permitted to take the address of a library function even if
it is also defined as a macro.143) The use of #undef to
remove any macro definition will also ensure that an actual
function is referred to. Any invocation of a library
function that is implemented as a macro shall expand to code
that evaluates each of its arguments exactly once, fully
protected by parentheses where necessary, so it is generally
safe to use arbitrary expressions as arguments.144)
Likewise, those function-like macros described in the
____________________
143This means that an implementation shall provide an actual
function for each library function, even if it also
provides a macro for that function.
7.1.3 Library 7.1.4
184 Committee Draft -- August 3, 1998 WG14/N843
following subclauses may be invoked in an expression
anywhere a function with a compatible return type could be
called.145) All object-like macros listed as expanding to
integer constant expressions shall additionally be suitable
for use in #if preprocessing directives.
[#2] Provided that a library function can be declared
without reference to any type defined in a header, it is
also permissible to declare the function and use it without
including its associated header.
[#3] There is a sequence point immediately before a library
function returns.
[#4] The functions in the standard library are not
guaranteed to be reentrant and may modify objects with
static storage duration.146)
[#5] EXAMPLE The function atoi may be used in any of
several ways:
-- by use of its associated header (possibly generating a
macro expansion)
#include <stdlib.h>
const char *str;
/* ... */
i = atoi(str);
____________________
144Such macros might not contain the sequence points that
the corresponding function calls do.
145Because external identifiers and some macro names
beginning with an underscore are reserved,
implementations may provide special semantics for such
names. For example, the identifier _BUILTIN_abs could be
used to indicate generation of in-line code for the abs
function. Thus, the appropriate header could specify
#define abs(x) _BUILTIN_abs(x)
for a compiler whose code generator will accept it.
In this manner, a user desiring to guarantee that a given
library function such as abs will be a genuine function
may write
#undef abs
whether the implementation's header provides a macro
implementation of abs or a built-in implementation. The
prototype for the function, which precedes and is hidden
by any macro definition, is thereby revealed also.
146Thus, a signal handler cannot, in general, call standard
library functions.
7.1.4 Library 7.1.4
WG14/N843 Committee Draft -- August 3, 1998 185
-- by use of its associated header (assuredly generating a
true function reference)
#include <stdlib.h>
#undef atoi
const char *str;
/* ... */
i = atoi(str);
or
#include <stdlib.h>
const char *str;
/* ... */
i = (atoi)(str);
-- by explicit declaration
extern int atoi(const char *);
const char *str;
/* ... */
i = atoi(str);
7.1.4 Library 7.1.4
186 Committee Draft -- August 3, 1998 WG14/N843
7.2 Diagnostics <assert.h>
[#1] The header <assert.h> defines the assert macro and
refers to another macro,
NDEBUG
which is not defined by <assert.h>. If NDEBUG is defined as
a macro name at the point in the source file where
<assert.h> is included, the assert macro is defined simply
as
#define assert(ignore) ((void)0)
The assert macro is redefined according to the current state |
of NDEBUG each time that <assert.h> is included.
[#2] The assert macro shall be implemented as a macro, not
as an actual function. If the macro definition is
suppressed in order to access an actual function, the
behavior is undefined.
7.2.1 Program diagnostics
7.2.1.1 The assert macro
Synopsis
[#1]
#include <assert.h>
void assert(_Bool expression); |
Description
[#2] The assert macro puts diagnostic tests into programs.
When it is executed, if expression is false (that is,
compares equal to 0), the assert macro writes information
about the particular call that failed (including the text of
the argument, the name of the source file, the source line
number, and the name of the enclosing function -- the
latter are respectively the values of the preprocessing
macros __FILE__ and __LINE__ and of the identifier __func__) |
on the standard error file in an implementation-defined
format.147) It then calls the abort function.
Returns
____________________
147The message written might be of the form: |
Assertion failed: expression, function abc, file xyz, |
line nnn . |
7.2 Library 7.2.1.1
WG14/N843 Committee Draft -- August 3, 1998 187
[#3] The assert macro returns no value.
Forward references: the abort function (7.20.4.1).
7.2.1.1 Library 7.2.1.1
188 Committee Draft -- August 3, 1998 WG14/N843
7.3 Complex arithmetic <complex.h>
7.3.1 Introduction
[#1] The header <complex.h> defines macros and declares
functions that support complex arithmetic.148) Each
synopsis specifies a family of functions consisting of a
principal function with one or more double complex
parameters and a double complex or double return value; and
other functions with same name but with f and l suffixes
which are corresponding functions with float and long double
parameters and return values.
[#2] The macro
complex
expands to _Complex; the macro
_Complex_I
expands to a constant expression of type const float
_Complex, with the value of the imaginary unit.149)
[#3] The macros
imaginary
and
_Imaginary_I
are defined if and only if the implementation supports
imaginary types;150) if defined, they expand to _Imaginary
and a constant expression of type const float _Imaginary
with the value of the imaginary unit.
[#4] The macro
I
expands to either _Imaginary_I or _Complex_I. If
_Imaginary_I is not defined, I shall expand to _Complex_I.
[#5] Notwithstanding the provisions of 7.1.3, a program is
permitted to undefine and perhaps then redefine the macros
____________________
148See ``future library directions'' (7.26.1).
149The imaginary unit is a number i such that i2=-1. |
150A specification for imaginary types is in informative
annex G.
7.3 Library 7.3.1
WG14/N843 Committee Draft -- August 3, 1998 189
complex, imaginary, and I. |
Forward references: IEC 60559-compatible complex arithmetic |
(annex G).
7.3.2 Conventions
[#1] Values are interpreted as radians, not degrees. An
implementation may set errno but is not required to.
7.3.3 Branch cuts
[#1] Some of the functions below have branch cuts, across
which the function is discontinuous. For implementations
with a signed zero (including all IEC 60559 implementations)
that follow the specification of annex G, the sign of zero
distinguishes one side of a cut from another so the function
is continuous (except for format limitations) as the cut is
approached from either side. For example, for the square
root function, which has a branch cut along the negative
real axis, the top of the cut, with imaginary part +0, maps
to the positive imaginary axis, and the bottom of the cut,
with imaginary part -0, maps to the negative imaginary axis.
[#2] Implementations that do not support a signed zero (see
annex F) cannot distinguish the sides of branch cuts. These
implementations shall map a cut so the function is
continuous as the cut is approached coming around the finite
endpoint of the cut in a counter clockwise direction.
(Branch cuts for the functions specified here have just one
finite endpoint.) For example, for the square root
function, coming counter clockwise around the finite
endpoint of the cut along the negative real axis approaches
the cut from above, so the cut maps to the positive
imaginary axis.
7.3.4 The CX_LIMITED_RANGE pragma
Synopsis
[#1]
#include <complex.h>
#pragma STDC CX_LIMITED_RANGE on-off-switch
Description
[#2] The usual mathematical formula for complex multiply, |
divide, and absolute value are problematic because of their |
treatment of infinities and because of undue overflow and
underflow. The CX_LIMITED_RANGE pragma can be used to
inform the implementation that (where the state is on) the |
usual mathematical formulas are acceptable.151) The pragma
can occur either outside external declarations or preceding
7.3.1 Library 7.3.4
190 Committee Draft -- August 3, 1998 WG14/N843
all explicit declarations and statements inside a compound
statement. When outside external declarations, the pragma
takes effect from its occurrence until another
CX_LIMITED_RANGE pragma is encountered, or until the end of
the translation unit. When inside a compound statement, the
pragma takes effect from its occurrence until another
CX_LIMITED_RANGE pragma is encountered (within a nested
compound statement), or until the end of the compound
statement; at the end of a compound statement the state for
the pragma is restored to its condition just before the
compound statement. If this pragma is used in any other
context, the behavior is undefined. The default state for
the pragma is off.
7.3.5 Trigonometric functions
7.3.5.1 The cacos functions
Synopsis
[#1]
#include <complex.h>
double complex cacos(double complex z);
float complex cacosf(float complex z);
long double complex cacosl(long double complex z);
Description
[#2] The cacos functions compute the complex arc cosine of
z, with branch cuts outside the interval [-1, 1] along the
real axis.
Returns
[#3] The cacos functions return the complex arc cosine
value, in the range of a strip mathematically unbounded
along the imaginary axis and in the interval [0, pi] along
the real axis.
____________________
151The purpose of the pragma is to allow the implementation
to use the formulas: |
(x+iy)×(u+iv)=(xu-yv)+i(yu+xv) |
(x+iy)/(u+iv)=[(xu+yv)+i(yu-xv)]/(u2+v2) |
|x+iy|=x2+y2 |
where the programmer can determine they are safe.
7.3.4 Library 7.3.5.1
WG14/N843 Committee Draft -- August 3, 1998 191
7.3.5.2 The casin functions
Synopsis
[#1]
#include <complex.h>
double complex casin(double complex z);
float complex casinf(float complex z);
long double complex casinl(long double complex z);
Description
[#2] The casin functions compute the complex arc sine of z,
with branch cuts outside the interval [-1, 1] along the real
axis.
Returns
[#3] The casin functions return the complex arc sine value,
in the range of a strip mathematically unbounded along the
imaginary axis and in the interval [-pi/2, pi/2] along the
real axis.
7.3.5.3 The catan functions
Synopsis
[#1]
#include <complex.h>
double complex catan(double complex z);
float complex catanf(float complex z);
long double complex catanl(long double complex z);
Description
[#2] The catan functions compute the complex arc tangent of
z, with branch cuts outside the interval [-i, i] along the
imaginary axis.
Returns
[#3] The catan functions return the complex arc tangent
value, in the range of a strip mathematically unbounded
along the imaginary axis and in the interval [-pi/2, pi/2]
along the real axis.
7.3.5.1 Library 7.3.5.3
192 Committee Draft -- August 3, 1998 WG14/N843
7.3.5.4 The ccos functions
Synopsis
[#1]
#include <complex.h>
double complex ccos(double complex z);
float complex ccosf(float complex z);
long double complex ccosl(long double complex z);
Description
[#2] The ccos function computes the complex cosine of z.
Returns
[#3] The ccos functions return the complex cosine value.
7.3.5.5 The csin functions
Synopsis
[#1]
#include <complex.h>
double complex csin(double complex z);
float complex csinf(float complex z);
long double complex csinl(long double complex z);
Description
[#2] The csin functions compute the complex sine of z.
Returns
[#3] The csin functions return the complex sine value.
7.3.5.6 The ctan functions
Synopsis
[#1]
#include <complex.h>
double complex ctan(double complex z);
float complex ctanf(float complex z);
long double complex ctanl(long double complex z);
Description
[#2] The ctan functions compute the complex tangent of z.
7.3.5.3 Library 7.3.5.6
WG14/N843 Committee Draft -- August 3, 1998 193
Returns
[#3] The ctan functions return the complex tangent value.
7.3.6 Hyperbolic functions
7.3.6.1 The cacosh functions
Synopsis
[#1]
#include <complex.h>
double complex cacosh(double complex z);
float complex cacoshf(float complex z);
long double complex cacoshl(long double complex z);
Description
[#2] The cacosh functions compute the complex arc hyperbolic
cosine of z, with a branch cut at values less than 1 along
the real axis.
Returns
[#3] The cacosh functions return the complex arc hyperbolic
cosine value, in the range of a half-strip of non-negative
values along the real axis and in the interval [-ipi, ipi]
along the imaginary axis.
7.3.6.2 The casinh functions
Synopsis
[#1]
#include <complex.h>
double complex casinh(double complex z);
float complex casinhf(float complex z);
long double complex casinhl(long double complex z);
Description
[#2] The casinh functions compute the complex arc hyperbolic
sine of z, with branch cuts outside the interval [-i, i]
along the imaginary axis.
Returns
[#3] The casinh functions return the complex arc hyperbolic
sine value, in the range of a strip mathematically unbounded
along the real axis and in the interval [-ipi/2, ipi/2]
along the imaginary axis.
7.3.5.6 Library 7.3.6.2
194 Committee Draft -- August 3, 1998 WG14/N843
7.3.6.3 The catanh functions
Synopsis
[#1]
#include <complex.h>
double complex catanh(double complex z);
float complex catanhf(float complex z);
long double complex catanhl(long double complex z);
Description
[#2] The catanh functions compute the complex arc hyperbolic
tangent of z, with branch cuts outside the interval [-1, 1]
along the real axis.
Returns
[#3] The catanh functions return the complex arc hyperbolic
tangent value, in the range of a strip mathematically
unbounded along the real axis and in the interval
[-ipi/2, ipi/2] along the imaginary axis.
7.3.6.4 The ccosh functions
Synopsis
[#1]
#include <complex.h>
double complex ccosh(double complex z);
float complex ccoshf(float complex z);
long double complex ccoshl(long double complex z);
Description
[#2] The ccosh functions compute the complex hyperbolic
cosine of z.
Returns
[#3] The ccosh functions return the complex hyperbolic
cosine value.
7.3.6.2 Library 7.3.6.4
WG14/N843 Committee Draft -- August 3, 1998 195
7.3.6.5 The csinh functions
Synopsis
[#1]
#include <complex.h>
double complex csinh(double complex z);
float complex csinhf(float complex z);
long double complex csinhl(long double complex z);
Description
[#2] The csinh functions compute the complex hyperbolic sine
of z.
Returns
[#3] The csinh functions return the complex hyperbolic sine
value.
7.3.6.6 The ctanh functions
Synopsis
[#1]
#include <complex.h>
double complex ctanh(double complex z);
float complex ctanhf(float complex z);
long double complex ctanhl(long double complex z);
Description
[#2] The ctanh functions compute the complex hyperbolic
tangent of z.
Returns
[#3] The ctanh functions return the complex hyperbolic
tangent value.
7.3.7 Exponential and logarithmic functions
7.3.6.4 Library 7.3.7
196 Committee Draft -- August 3, 1998 WG14/N843
7.3.7.1 The cexp functions
Synopsis
[#1]
#include <complex.h>
double complex cexp(double complex z);
float complex cexpf(float complex z);
long double complex cexpl(long double complex z);
Description
[#2] The cexp functions compute the complex base-e
exponential of z.
Returns
[#3] The cexp functions return the complex base-e
exponential value.
7.3.7.2 The clog functions
Synopsis
[#1]
#include <complex.h>
double complex clog(double complex z);
float complex clogf(float complex z);
long double complex clogl(long double complex z);
Description
[#2] The clog functions compute the complex natural (base-e)
logarithm of z, with a branch cut along the negative real
axis.
Returns
[#3] The clog functions return the complex natural logarithm
value, in the range of a strip mathematically unbounded
along the real axis and in the interval [-ipi, ipi] along
the imaginary axis.
7.3.8 Power and absolute-value functions
7.3.7 Library 7.3.8
WG14/N843 Committee Draft -- August 3, 1998 197
7.3.8.1 The cabs functions
Synopsis
[#1]
#include <complex.h>
double cabs(double complex z);
float cabsf(float complex z);
long double cabsl(long double complex z);
Description
[#2] The cabs functions compute the complex absolute value
(also called norm, modulus, or magnitude) of z.
Returns
[#3] The cabs functions return the complex absolute value.
7.3.8.2 The cpow functions
Synopsis
[#1]
#include <complex.h>
double complex cpow(double complex x, double complex y);
float complex cpowf(float complex x, float complex y);
long double complex cpowl(long double complex x,
long double complex y);
Description
[#2] The cpow functions compute the complex power function
xy, with a branch cut for the first parameter along the
negative real axis.
Returns
[#3] The cpow functions return the complex power function
value.
7.3.8 Library 7.3.8.2
198 Committee Draft -- August 3, 1998 WG14/N843
7.3.8.3 The csqrt functions
Synopsis
[#1]
#include <complex.h>
double complex csqrt(double complex z);
float complex csqrtf(float complex z);
long double complex csqrtl(long double complex z);
Description
[#2] The csqrt functions compute the complex square root of
z, with a branch cut along the negative real axis.
Returns
[#3] The csqrt functions return the complex square root
value, in the range of the right half-plane (including the
imaginary axis).
7.3.9 Manipulation functions
7.3.9.1 The carg functions
Synopsis
[#1]
#include <complex.h>
double carg(double complex z);
float cargf(float complex z);
long double cargl(long double complex z);
Description
[#2] The carg functions compute the argument (also called
phase angle) of z, with a branch cut along the negative real
axis.
Returns
[#3] The carg functions return the value of the argument in
the range [-pi, pi].
7.3.8.2 Library 7.3.9.1
WG14/N843 Committee Draft -- August 3, 1998 199
7.3.9.2 The cimag functions
Synopsis
[#1]
#include <complex.h>
double cimag(double complex z);
float cimagf(float complex z);
long double cimagl(long double complex z);
Description
[#2] The cimag functions compute the imaginary part of
z.152)
Returns
[#3] The cimag functions return the imaginary part value (as
a real).
7.3.9.3 The conj functions
Synopsis
[#1]
#include <complex.h>
double complex conj(double complex z);
float complex conjf(float complex z);
long double complex conjl(long double complex z);
Description
[#2] The conj functions compute the complex conjugate of z,
by reversing the sign of its imaginary part.
Returns
[#3] The conj functions return the complex conjugate value.
____________________
152For a variable z of complex type, z == creal(z) +
cimag(z)*I.
7.3.9.1 Library 7.3.9.3
200 Committee Draft -- August 3, 1998 WG14/N843
7.3.9.4 The cproj functions
Synopsis
[#1]
#include <complex.h>
double complex cproj(double complex z);
float complex cprojf(float complex z);
long double complex cprojl(long double complex z);
Description
[#2] The cproj functions compute a projection of z onto the
Riemann sphere: z projects to z except that all complex
infinities (even those with one infinite part and one NaN
part) project to positive infinity on the real axis. If z
has an infinite part, then cproj(z) is equivalent to
INFINITY + I * copysign(0.0, cimag(z))
Returns
[#3] The cproj functions return the value of the projection
onto the Riemann sphere.
7.3.9.5 The creal functions
Synopsis
[#1]
#include <complex.h>
double creal(double complex z);
float crealf(float complex z);
long double creall(long double complex z);
Description
[#2] The creal functions compute the real part of z.153)
Returns
[#3] The creal functions return the real part value.
____________________
153For a variable z of complex type, z == creal(z) +
cimag(z)*I.
7.3.9.3 Library 7.3.9.5
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7.4 Character handling <ctype.h>
[#1] The header <ctype.h> declares several functions useful
for testing and mapping characters.154) In all cases the
argument is an int, the value of which shall be
representable as an unsigned char or shall equal the value
of the macro EOF. If the argument has any other value, the
behavior is undefined.
[#2] The behavior of these functions is affected by the
current locale. Those functions that have locale-specific
aspects only when not in the "C" locale are noted below.
[#3] The term printing character refers to a member of a
locale-specific set of characters, each of which occupies
one printing position on a display device; the term control
character refers to a member of a locale-specific set of
characters that are not printing characters.155)
Forward references: EOF (7.19.1), localization (7.11).
7.4.1 Character testing functions
[#1] The functions in this subclause return nonzero (true)
if and only if the value of the argument c conforms to that
in the description of the function.
7.4.1.1 The isalnum function
Synopsis
[#1]
#include <ctype.h>
int isalnum(int c);
Description
[#2] The isalnum function tests for any character for which
isalpha or isdigit is true.
____________________
154See ``future library directions'' (7.26.2).
155In an implementation that uses the seven-bit US ASCII |
character set, the printing characters are those whose
values lie from 0x20 (space) through 0x7E (tilde); the
control characters are those whose values lie from 0
(NUL) through 0x1F (US), and the character 0x7F (DEL).
7.4 Library 7.4.1.1
202 Committee Draft -- August 3, 1998 WG14/N843
7.4.1.2 The isalpha function
Synopsis
[#1]
#include <ctype.h>
int isalpha(int c);
Description
[#2] The isalpha function tests for any character for which
isupper or islower is true, or any character that is one of
a locale-specific set of alphabetic characters for which
none of iscntrl, isdigit, ispunct, or isspace is true.156)
In the "C" locale, isalpha returns true only for the
characters for which isupper or islower is true.
7.4.1.3 The iscntrl function
Synopsis
[#1]
#include <ctype.h>
int iscntrl(int c);
Description
[#2] The iscntrl function tests for any control character.
7.4.1.4 The isdigit function
Synopsis
[#1]
#include <ctype.h>
int isdigit(int c);
Description
[#2] The isdigit function tests for any decimal-digit
character (as defined in 5.2.1).
____________________
156The functions islower and isupper test true or false
separately for each of these additional characters; all
four combinations are possible.
7.4.1.1 Library 7.4.1.4
WG14/N843 Committee Draft -- August 3, 1998 203
7.4.1.5 The isgraph function
Synopsis
[#1]
#include <ctype.h>
int isgraph(int c);
Description
[#2] The isgraph function tests for any printing character
except space (' ').
7.4.1.6 The islower function
Synopsis
[#1]
#include <ctype.h>
int islower(int c);
Description
[#2] The islower function tests for any character that is a
lowercase letter or is one of a locale-specific set of
characters for which none of iscntrl, isdigit, ispunct, or
isspace is true. In the "C" locale, islower returns true
only for the characters defined as lowercase letters (as
defined in 5.2.1).
7.4.1.7 The isprint function
Synopsis
[#1]
#include <ctype.h>
int isprint(int c);
Description
[#2] The isprint function tests for any printing character
including space (' ').
7.4.1.4 Library 7.4.1.7
204 Committee Draft -- August 3, 1998 WG14/N843
7.4.1.8 The ispunct function
Synopsis
[#1]
#include <ctype.h>
int ispunct(int c);
Description
[#2] The ispunct function tests for any printing character
that is one of a locale-specific set of punctuation
characters for which neither isspace nor isalnum is true.
7.4.1.9 The isspace function
Synopsis
[#1]
#include <ctype.h>
int isspace(int c);
Description
[#2] The isspace function tests for any character that is a
standard white-space character or is one of a locale-
specific set of characters for which isalnum is false. The
standard white-space characters are the following: space
(' '), form feed ('\f'), new-line ('\n'), carriage return
('\r'), horizontal tab ('\t'), and vertical tab ('\v'). In
the "C" locale, isspace returns true only for the standard
white-space characters.
7.4.1.10 The isupper function
Synopsis
[#1]
#include <ctype.h>
int isupper(int c);
Description
[#2] The isupper function tests for any character that is an
uppercase letter or is one of a locale-specific set of
characters for which none of iscntrl, isdigit, ispunct, or
isspace is true. In the "C" locale, isupper returns true
only for the characters defined as uppercase letters (as
defined in 5.2.1).
7.4.1.7 Library 7.4.1.10
WG14/N843 Committee Draft -- August 3, 1998 205
7.4.1.11 The isxdigit function
Synopsis
[#1]
#include <ctype.h>
int isxdigit(int c);
Description
[#2] The isxdigit function tests for any hexadecimal-digit
character (as defined in 6.4.4.2).
7.4.2 Character case mapping functions
7.4.2.1 The tolower function
Synopsis
[#1]
#include <ctype.h>
int tolower(int c);
Description
[#2] The tolower function converts an uppercase letter to a
corresponding lowercase letter.
Returns
[#3] If the argument is a character for which isupper is
true and there are one or more corresponding characters, as
specified by the current locale, for which islower is true,
the tolower function returns one of the corresponding
characters (always the same one for any given locale);
otherwise, the argument is returned unchanged.
7.4.2.2 The toupper function
Synopsis
[#1]
#include <ctype.h>
int toupper(int c);
Description
[#2] The toupper function converts a lowercase letter to a
corresponding uppercase letter.
7.4.1.10 Library 7.4.2.2
206 Committee Draft -- August 3, 1998 WG14/N843
Returns
[#3] If the argument is a character for which islower is
true and there are one or more corresponding characters, as
specified by the current locale, for which isupper is true,
the toupper function returns one of the corresponding
characters (always the same one for any given locale);
otherwise, the argument is returned unchanged.
7.4.2.2 Library 7.4.2.2
WG14/N843 Committee Draft -- August 3, 1998 207
7.5 Errors <errno.h>
[#1] The header <errno.h> defines several macros, all
relating to the reporting of error conditions.
[#2] The macros are
EDOM
EILSEQ
ERANGE
which expand to integer constant expressions with type int,
distinct positive values, and which are suitable for use in
#if preprocessing directives; and
errno
which expands to a modifiable lvalue157) that has type int,
the value of which is set to a positive error number by
several library functions. It is unspecified whether errno
is a macro or an identifier declared with external linkage.
If a macro definition is suppressed in order to access an
actual object, or a program defines an identifier with the
name errno, the behavior is undefined.
[#3] The value of errno is zero at program startup, but is
never set to zero by any library function.158) The value of
errno may be set to nonzero by a library function call
whether or not there is an error, provided the use of errno
is not documented in the description of the function in this
International Standard.
[#4] Additional macro definitions, beginning with E and a
digit or E and an uppercase letter,159) may also be
specified by the implementation.
____________________
157The macro errno need not be the identifier of an object.
It might expand to a modifiable lvalue resulting from a
function call (for example, *errno()).
158Thus, a program that uses errno for error checking should
set it to zero before a library function call, then
inspect it before a subsequent library function call. Of
course, a library function can save the value of errno on
entry and then set it to zero, as long as the original
value is restored if errno's value is still zero just
before the return.
159See ``future library directions'' (7.26.3).
7.5 Library 7.5
208 Committee Draft -- August 3, 1998 WG14/N843
7.6 Floating-point environment <fenv.h>
[#1] The header <fenv.h> declares two types and several
macros and functions to provide access to the floating-point
environment. The floating-point environment refers
collectively to any floating-point status flags and control
modes supported by the implementation.160) A floating-point
status flag is a system variable whose value is set as a
side effect of floating-point arithmetic to provide |
auxiliary information. A floating-point control mode is a
system variable whose value may be set by the user to affect
the subsequent behavior of floating-point arithmetic. |
[#2] Certain programming conventions support the intended
model of use for the floating-point environment:161)
-- a function call does not alter its caller's modes,
clear its caller's flags, nor depend on the state of
its caller's flags unless the function is so
documented;
-- a function call is assumed to require default modes,
unless its documentation promises otherwise or unless
the function is known not to use floating-point;
-- a function call is assumed to have the potential for
raising floating-point exceptions, unless its
documentation promises otherwise, or unless the
function is known not to use floating-point.
[#3] The type
fenv_t
represents the entire floating-point environment.
[#4] The type
fexcept_t
represents the floating-point exception flags collectively,
____________________
160This header is designed to support the exception status
flags and directed-rounding control modes required by IEC
60559, and other similar floating-point state
information. Also it is designed to facilitate code
portability among all systems.
161With these conventions, a programmer can safely assume
default modes (or be unaware of them). The
responsibilities associated with accessing the floating-
point environment fall on the programmer or program that
does so explicitly.
7.6 Library 7.6
WG14/N843 Committee Draft -- August 3, 1998 209
including any status the implementation associates with the
flags.
[#5] Each of the macros
FE_DIVBYZERO
FE_INEXACT
FE_INVALID
FE_OVERFLOW
FE_UNDERFLOW
is defined if and only if the implementation supports the
exception by means of the functions in 7.6.2. Additional |
floating-point exceptions, with macro definitions beginning |
with FE_ and an uppercase letter, may also be specified by |
the implementation. The defined macros expand to integer
constant expressions with values such that bitwise ORs of
all combinations of the macros result in distinct values.
[#6] The macro
FE_ALL_EXCEPT
is simply the bitwise OR of all exception macros defined by
the implementation.
[#7] Each of the macros
FE_DOWNWARD
FE_TONEAREST
FE_TOWARDZERO
FE_UPWARD
is defined if and only if the implementation supports
getting and setting the represented rounding direction by
means of the fegetround and fesetround functions. |
Additional rounding directions, with macro definitions |
beginning with FE_ and an uppercase letter, may also be |
specified by the implementation. The defined macros expand
to integer constant expressions whose values are distinct
nonnegative values.162)
[#8] The macro
FE_DFL_ENV
represents the default floating-point environment -- the
one installed at program startup -- and has type pointer
to const-qualified fenv_t. It can be used as an argument to
____________________
162Even though the rounding direction macros may expand to
constants corresponding to the values of FLT_ROUNDS, they
are not required to do so.
7.6 Library 7.6
210 Committee Draft -- August 3, 1998 WG14/N843
<fenv.h> functions that manage the floating-point
environment.
[#9] Additional macro definitions, beginning with FE_ and
having type pointer to const-qualified fenv_t, may also be
specified by the implementation.
7.6.1 The FENV_ACCESS pragma
Synopsis
[#1]
#include <fenv.h>
#pragma STDC FENV_ACCESS on-off-switch
Description
[#2] The FENV_ACCESS pragma provides a means to inform the
implementation when a program might access the floating-
point environment to test flags or run under non-default
modes.163) The pragma shall occur either outside external
declarations or preceding all explicit declarations and
statements inside a compound statement. When outside
external declarations, the pragma takes effect from its
occurrence until another FENV_ACCESS pragma is encountered,
or until the end of the translation unit. When inside a
compound statement, the pragma takes effect from its
occurrence until another FENV_ACCESS pragma is encountered
(within a nested compound statement), or until the end of
the compound statement; at the end of a compound statement
the state for the pragma is restored to its condition just
before the compound statement. If this pragma is used in
any other context, the behavior is undefined. If part of a
program tests flags or runs under non-default mode settings,
but was translated with the state for the FENV_ACCESS pragma
off, then the behavior is undefined. The default state (on
or off) for the pragma is implementation-defined.
[#3] EXAMPLE
____________________
163The purpose of the FENV_ACCESS pragma is to allow certain
optimizations, for example global common subexpression
elimination, code motion, and constant folding, that
could subvert flag tests and mode changes. In general,
if the state of FENV_ACCESS is off then the translator
can assume that default modes are in effect and the flags
are not tested.
7.6 Library 7.6.1
WG14/N843 Committee Draft -- August 3, 1998 211
#include <fenv.h>
void f(double x)
{
#pragma STDC FENV_ACCESS ON
void g(double);
void h(double);
/* ... */
g(x + 1);
h(x + 1);
/* ... */
}
[#4] If the function g might depend on status flags set as a
side effect of the first x + 1, or if the second x + 1 might
depend on control modes set as a side effect of the call to
function g, then the program shall contain an appropriately
placed invocation of #pragma STDC FENV_ACCESS ON.164)
7.6.2 Exceptions
[#1] The following functions provide access to the exception
flags.165) The int input argument for the functions
represents a subset of floating-point exceptions, and can be |
zero or the bitwise OR of one or more exception macros, for |
example FE_OVERFLOW | FE_INEXACT. For other argument values
the behavior of these functions is undefined.
____________________
164The side effects impose a temporal ordering that requires
two evaluations of x + 1. On the other hand, without the
#pragma STDC FENV_ACCESS ON pragma, and assuming the
default state is off, just one evaluation of x + 1 would
suffice.
165The functions fetestexcept, feraiseexcept, and
feclearexcept support the basic abstraction of flags that
are either set or clear. An implementation may endow
exception flags with more information -- for example,
the address of the code which first raised the exception;
the functions fegetexceptflag and fesetexceptflag deal
with the full content of flags.
7.6.1 Library 7.6.2
212 Committee Draft -- August 3, 1998 WG14/N843
7.6.2.1 The feclearexcept function
Synopsis
[#1]
#include <fenv.h>
void feclearexcept(int excepts);
Description
[#2] The feclearexcept function clears the supported
exceptions represented by its argument.
7.6.2.2 The fegetexceptflag function
Synopsis
[#1]
#include <fenv.h>
void fegetexceptflag(fexcept_t *flagp,
int excepts);
Description
[#2] The fegetexceptflag function stores an implementation-
defined representation of the exception flags indicated by
the argument excepts in the object pointed to by the
argument flagp.
7.6.2.3 The feraiseexcept function
Synopsis
[#1]
#include <fenv.h>
void feraiseexcept(int excepts);
Description
[#2] The feraiseexcept function raises the supported
exceptions represented by its argument.166) The order in
which these exceptions are raised is unspecified, except as
stated in F.7.6. Whether the feraiseexcept function
additionally raises the inexact exception whenever it raises
the overflow or underflow exception is implementation-
____________________
166The effect is intended to be similar to that of
exceptions raised by arithmetic operations. Hence,
enabled traps for exceptions raised by this function are
taken. The specification in F.7.6 is in the same spirit.
7.6.2 Library 7.6.2.3
WG14/N843 Committee Draft -- August 3, 1998 213
defined.
7.6.2.4 The fesetexceptflag function
Synopsis
[#1]
#include <fenv.h>
void fesetexceptflag(const fexcept_t *flagp,
int excepts);
Description
[#2] The fesetexceptflag function sets the complete status
for those exception flags indicated by the argument excepts,
according to the representation in the object pointed to by
flagp. The value of *flagp shall have been set by a
previous call to fegetexceptflag whose second argument
represented at least those exceptions represented by the
argument excepts. This function does not raise exceptions, |
but only sets the state of the flags.
7.6.2.5 The fetestexcept function
Synopsis
[#1]
#include <fenv.h>
int fetestexcept(int excepts);
Description
[#2] The fetestexcept function determines which of a
specified subset of the exception flags are currently set.
The excepts argument specifies the exception flags to be
queried.167)
Returns
[#3] The fetestexcept function returns the value of the
bitwise OR of the exception macros corresponding to the
currently set exceptions included in excepts.
[#4] EXAMPLE Call f if invalid is set, then g if overflow
is set:
____________________
167This mechanism allows testing several exceptions with
just one function call.
7.6.2.3 Library 7.6.2.5
214 Committee Draft -- August 3, 1998 WG14/N843
#include <fenv.h>
/* ... */
{
#pragma STDC FENV_ACCESS ON
int set_excepts;
// maybe raise exceptions
set_excepts =
fetestexcept(FE_INVALID | FE_OVERFLOW);
if (set_excepts & FE_INVALID) f();
if (set_excepts & FE_OVERFLOW) g();
/* ... */
}
7.6.3 Rounding
[#1] The fegetround and fesetround functions provide control
of rounding direction modes.
7.6.3.1 The fegetround function
Synopsis
[#1]
#include <fenv.h>
int fegetround(void);
Description
[#2] The fegetround function gets the current rounding
direction.
Returns
[#3] The fegetround function returns the value of the
rounding direction macro representing the current rounding
direction.
7.6.3.2 The fesetround function
Synopsis
[#1]
#include <fenv.h>
int fesetround(int round);
Description
[#2] The fesetround function establishes the rounding
direction represented by its argument round. If the |
argument is not equal to the value of a rounding direction
7.6.2.5 Library 7.6.3.2
WG14/N843 Committee Draft -- August 3, 1998 215
macro, the rounding direction is not changed.
Returns
[#3] The fesetround function returns a zero value if and |
only if the argument is equal to a rounding direction macro |
(that is, if and only if the requested rounding direction
can be established).
[#4] EXAMPLE 1 Save, set, and restore the rounding
direction. Report an error and abort if setting the
rounding direction fails.
#include <fenv.h>
#include <assert.h>
/* ... */
{
#pragma STDC FENV_ACCESS ON
int save_round;
int setround_ok;
save_round = fegetround();
setround_ok = fesetround(FE_UPWARD);
assert(setround_ok);
/* ... */
fesetround(save_round);
/* ... */
}
7.6.4 Environment
[#1] The functions in this section manage the floating-point
environment -- status flags and control modes -- as one
entity.
7.6.4.1 The fegetenv function
Synopsis
[#1]
#include <fenv.h>
void fegetenv(fenv_t *envp);
Description
[#2] The fegetenv function stores the current floating-point
environment in the object pointed to by envp.
7.6.3.2 Library 7.6.4.1
216 Committee Draft -- August 3, 1998 WG14/N843
7.6.4.2 The feholdexcept function
Synopsis
[#1]
#include <fenv.h>
int feholdexcept(fenv_t *envp);
Description
[#2] The feholdexcept function saves the current floating-
point environment in the object pointed to by envp, clears
the exception flags, and then installs a non-stop (continue
on exceptions) mode, if available, for all exceptions.168)
Returns
[#3] The feholdexcept function returns zero if and only if |
non-stop exception handling was successfully installed.
7.6.4.3 The fesetenv function
Synopsis
[#1]
#include <fenv.h>
void fesetenv(const fenv_t *envp);
Description
[#2] The fesetenv function establishes the floating-point
environment represented by the object pointed to by envp.
The argument envp shall point to an object set by a call to
fegetenv or feholdexcept, or equal the macro FE_DFL_ENV or
an implementation-defined environment macro. Note that
fesetenv merely installs the state of the exception flags
represented through its argument, and does not raise these
exceptions.
____________________
168IEC 60559 systems have a default non-stop mode, and
typically at least one other mode for trap handling or
aborting; if the system provides only the non-stop mode
then installing it is trivial. For such systems, the
feholdexcept function can be used in conjunction with the
feupdateenv function to write routines that hide spurious
exceptions from their callers.
7.6.4.1 Library 7.6.4.3
WG14/N843 Committee Draft -- August 3, 1998 217
7.6.4.4 The feupdateenv function
Synopsis
[#1]
#include <fenv.h>
void feupdateenv(const fenv_t *envp);
Description
[#2] The feupdateenv function saves the currently raised
exceptions in its automatic storage, installs the floating-
point environment represented by the object pointed to by
envp, and then raises the saved exceptions. The argument
envp shall point to an object set by a call to feholdexcept
or fegetenv, or equal the macro FE_DFL_ENV or an
implementation-defined environment macro.
[#3] EXAMPLE 1 Hide spurious underflow exceptions:
#include <fenv.h>
double f(double x)
{
#pragma STDC FENV_ACCESS ON
double result;
fenv_t save_env;
feholdexcept(&save_env);
// compute result
if (/* test spurious underflow */)
feclearexcept(FE_UNDERFLOW);
feupdateenv(&save_env);
return result;
}
7.6.4.3 Library 7.6.4.4
218 Committee Draft -- August 3, 1998 WG14/N843
7.7 Characteristics of floating types <float.h>
[#1] The header <float.h> defines several macros that expand
to various limits and parameters of the standard floating-
point types.
[#2] The macros, their meanings, and the constraints (or
restrictions) on their values are listed in 5.2.4.2.2.
7.7 Library 7.7
WG14/N843 Committee Draft -- August 3, 1998 219
7.8 Format conversion of integer types <inttypes.h>
[#1] The header <inttypes.h> includes the header <stdint.h>
and extends it with additional facilities provided by hosted
implementations.
[#2] It declares four functions for converting numeric
character strings to greatest-width integers and, for each
type declared in <stdint.h>, it defines corresponding macros
for conversion specifiers for use with the formatted
input/output functions.169)
Forward references: integer types <stdint.h> (7.18).
7.8.1 Macros for format specifiers
[#1] Each of the following object-like macros170) expands to |
a character string literal containing a conversion
specifier, possibly modified by a length modifier, suitable
for use within the format argument of a formatted
input/output function when converting the corresponding
integer type. These macro names have the general form of
PRI (character string literals for the fprintf family) or
SCN (character string literals for the fscanf family),171)
followed by the conversion specifier, followed by a name
corresponding to a similar type name in 7.18.1. For
example, PRIdFAST32 can be used in a format string to print
the value of an integer of type int_fast32_t.
[#2] The fprintf macros for signed integers are:
PRId8 PRId16 PRId32 PRId64
PRIdLEAST8 PRIdLEAST16 PRIdLEAST32 PRIdLEAST64
PRIdFAST8 PRIdFAST16 PRIdFAST32 PRIdFAST64
PRIdMAX PRIdPTR
PRIi8 PRIi16 PRIi32 PRIi64
PRIiLEAST8 PRIiLEAST16 PRIiLEAST32 PRIiLEAST64
PRIiFAST8 PRIiFAST16 PRIiFAST32 PRIiFAST64
PRIiMAX PRIiPTR
[#3] The fprintf macros for unsigned integers are:
____________________
169See ``future library directions'' (7.26.4).
170C++ implementations should define these macros only when
__STDC_FORMAT_MACROS is defined before <inttypes.h> is
included.
171Separate macros are given for use with fprintf and fscanf
functions because, in the general case, different format |
specifiers may be required for fprintf and fscanf, even |
when the type is the same.
7.8 Library 7.8.1
220 Committee Draft -- August 3, 1998 WG14/N843
PRIo8 PRIo16 PRIo32 PRIo64
PRIoLEAST8 PRIoLEAST16 PRIoLEAST32 PRIoLEAST64
PRIoFAST8 PRIoFAST16 PRIoFAST32 PRIoFAST64
PRIoMAX PRIoPTR
PRIu8 PRIu16 PRIu32 PRIu64
PRIuLEAST8 PRIuLEAST16 PRIuLEAST32 PRIuLEAST64
PRIuFAST8 PRIuFAST16 PRIuFAST32 PRIuFAST64
PRIuMAX PRIuPTR
PRIx8 PRIx16 PRIx32 PRIx64
PRIxLEAST8 PRIxLEAST16 PRIxLEAST32 PRIxLEAST64
PRIxFAST8 PRIxFAST16 PRIxFAST32 PRIxFAST64
PRIxMAX PRIxPTR
PRIX8 PRIX16 PRIX32 PRIX64
PRIXLEAST8 PRIXLEAST16 PRIXLEAST32 PRIXLEAST64
PRIXFAST8 PRIXFAST16 PRIXFAST32 PRIXFAST64
PRIXMAX PRIXPTR
[#4] The fscanf macros for signed integers are:
SCNd8 SCNd16 SCNd32 SCNd64
SCNdLEAST8 SCNdLEAST16 SCNdLEAST32 SCNdLEAST64
SCNdFAST8 SCNdFAST16 SCNdFAST32 SCNdFAST64
SCNdMAX SCNdPTR
SCNi8 SCNi16 SCNi32 SCNi64
SCNiLEAST8 SCNiLEAST16 SCNiLEAST32 SCNiLEAST64
SCNiFAST8 SCNiFAST16 SCNiFAST32 SCNiFAST64
SCNiMAX SCNiPTR
[#5] The fscanf macros for unsigned integers are:
SCNo8 SCNo16 SCNo32 SCNo64
SCNoLEAST8 SCNoLEAST16 SCNoLEAST32 SCNoLEAST64
SCNoFAST8 SCNoFAST16 SCNoFAST32 SCNoFAST64
SCNoMAX SCNoPTR
SCNu8 SCNu16 SCNu32 SCNu64
SCNuLEAST8 SCNuLEAST16 SCNuLEAST32 SCNuLEAST64
SCNuFAST8 SCNuFAST16 SCNuFAST32 SCNuFAST64
SCNuMAX SCNuPTR
SCNx8 SCNx16 SCNx32 SCNx64
SCNxLEAST8 SCNxLEAST16 SCNxLEAST32 SCNxLEAST64
SCNxFAST8 SCNxFAST16 SCNxFAST32 SCNxFAST64
SCNxMAX SCNxPTR
[#6] Because the default argument promotions do not affect
pointer parameters, there might not exist suitable fscanf
format specifiers for some of the types defined in this
header. Consequently, as a special exception to the
requirement that the implementation define all macros
7.8.1 Library 7.8.1
WG14/N843 Committee Draft -- August 3, 1998 221
associated with each type defined by this header, in such a
case the problematic fscanf macros may be left undefined.
[#7] EXAMPLE
#include <inttypes.h>
#include <wchar.h>
int main(void)
{
uintmax_t i = UINTMAX_MAX; // this type always exists
wprintf(L"The largest integer value is %020"
PRIxMAX "\n", i);
return 0;
}
7.8.2 Conversion functions for greatest-width integer types
7.8.2.1 The strtoimax and strtoumax functions |
Synopsis
[#1]
#include <inttypes.h>
intmax_t strtoimax(const char * restrict nptr,
char ** restrict endptr, int base);
uintmax_t strtoumax(const char * restrict nptr, |
char ** restrict endptr, int base); |
Description
[#2] The strtoimax and strtoumax functions are equivalent to |
the strtol, strtoll, strtoul, and strtoull functions, except |
that the initial portion of the string is converted to
intmax_t and uintmax_t representation, respectively. |
Returns
[#3] The strtoimax and strtoumax functions return the |
converted value, if any. If no conversion could be |
performed, zero is returned. If the correct value is
outside the range of representable values, INTMAX_MAX, |
INTMAX_MIN, or UINTMAX_MAX is returned (according to the |
return type and sign of the value, if any), and the value of
the macro ERANGE is stored in errno. |
7.8.2.2 The wcstoimax and wcstoumax functions |
Synopsis
[#1]
7.8.1 Library 7.8.2.2
222 Committee Draft -- August 3, 1998 WG14/N843
#include <stddef.h> // for wchar_t
#include <inttypes.h>
intmax_t wcstoimax(const wchar_t * restrict nptr,
wchar_t ** restrict endptr, int base);
uintmax_t wcstoumax(const wchar_t * restrict nptr, |
wchar_t ** restrict endptr, int base); |
Description
[#2] The wcstoimax and wcstoumax functions are equivalent to |
the wcstol, wcstoll, wcstoul, and wcstoull functions except |
that the initial portion of the wide string is converted to
intmax_t and uintmax_t representation, respectively. |
Returns
[#3] The wcstoimax function returns the converted value, if
any. If no conversion could be performed, zero is returned. |
If the correct value is outside the range of representable
values, INTMAX_MAX, INTMAX_MIN, or UINTMAX_MAX is returned |
(according to the return type and sign of the value, if |
any), and the value of the macro ERANGE is stored in errno. *
7.8.2.2 Library 7.8.2.2
WG14/N843 Committee Draft -- August 3, 1998 223
7.9 Alternative spellings <iso646.h>
[#1] The header <iso646.h> defines the following eleven
macros (on the left) that expand to the corresponding tokens
(on the right):
and &&
and_eq &=
bitand &
bitor |
compl ~
not !
not_eq !=
or ||
or_eq |=
xor ^
xor_eq ^=
7.9 Library 7.9
224 Committee Draft -- August 3, 1998 WG14/N843
7.10 Sizes of integer types <limits.h>
[#1] The header <limits.h> defines several macros that
expand to various limits and parameters of the standard
integer types.
[#2] The macros, their meanings, and the constraints (or
restrictions) on their values are listed in 5.2.4.2.1.
7.10 Library 7.10
WG14/N843 Committee Draft -- August 3, 1998 225
7.11 Localization <locale.h>
[#1] The header <locale.h> declares two functions, one type,
and defines several macros.
[#2] The type is
struct lconv
which contains members related to the formatting of numeric
values. The structure shall contain at least the following
members, in any order. The semantics of the members and
their normal ranges are explained in 7.11.2.1. In the "C" |
locale, the members shall have the values specified in the
comments.
char *decimal_point; // "."
char *thousands_sep; // ""
char *grouping; // ""
char *mon_decimal_point; // "" *
char *mon_thousands_sep; // ""
char *mon_grouping; // ""
char *positive_sign; // ""
char *negative_sign; // ""
char *currency_symbol; // "" |
char frac_digits; // CHAR_MAX
char p_cs_precedes; // CHAR_MAX
char n_cs_precedes; // CHAR_MAX *
char p_sep_by_space; // CHAR_MAX |
char n_sep_by_space; // CHAR_MAX
char p_sign_posn; // CHAR_MAX
char n_sign_posn; // CHAR_MAX
char *int_curr_symbol; // "" |
char int_frac_digits; // CHAR_MAX |
char int_p_cs_precedes; // CHAR_MAX |
char int_n_cs_precedes; // CHAR_MAX |
char int_p_sep_by_space; // CHAR_MAX |
char int_n_sep_by_space; // CHAR_MAX |
char int_p_sign_posn; // CHAR_MAX |
char int_n_sign_posn; // CHAR_MAX |
[#3] The macros defined are NULL (described in 7.17); and
LC_ALL
LC_COLLATE
LC_CTYPE
LC_MONETARY
LC_NUMERIC
LC_TIME
which expand to integer constant expressions with distinct
values, suitable for use as the first argument to the
setlocale function.172) Additional macro definitions,
beginning with the characters LC_ and an uppercase
7.11 Library 7.11
226 Committee Draft -- August 3, 1998 WG14/N843
letter,173) may also be specified by the implementation.
7.11.1 Locale control
7.11.1.1 The setlocale function
Synopsis
[#1]
#include <locale.h>
char *setlocale(int category, const char *locale);
Description
[#2] The setlocale function selects the appropriate portion
of the program's locale as specified by the category and
locale arguments. The setlocale function may be used to
change or query the program's entire current locale or
portions thereof. The value LC_ALL for category names the
program's entire locale; the other values for category name
only a portion of the program's locale. LC_COLLATE affects
the behavior of the strcoll and strxfrm functions. LC_CTYPE
affects the behavior of the character handling functions174)
and the multibyte and wide-character functions. LC_MONETARY
affects the monetary formatting information returned by the
localeconv function. LC_NUMERIC affects the decimal-point
character for the formatted input/output functions and the
string conversion functions, as well as the nonmonetary
formatting information returned by the localeconv function.
LC_TIME affects the behavior of the strftime and strfxtime
functions.
[#3] A value of "C" for locale specifies the minimal
environment for C translation; a value of "" for locale
specifies the locale-specific native environment. Other
implementation-defined strings may be passed as the second
argument to setlocale.
[#4] At program startup, the equivalent of
setlocale(LC_ALL, "C");
is executed.
____________________
172ISO/IEC 9945-2 specifies locale and charmap formats that
may be used to specify locales for C.
173See ``future library directions'' (7.26.5).
174The only functions in 7.4 whose behavior is not affected
by the current locale are isdigit and isxdigit.
7.11 Library 7.11.1.1
WG14/N843 Committee Draft -- August 3, 1998 227
[#5] The implementation shall behave as if no library
function calls the setlocale function.
Returns
[#6] If a pointer to a string is given for locale and the
selection can be honored, the setlocale function returns a
pointer to the string associated with the specified category
for the new locale. If the selection cannot be honored, the
setlocale function returns a null pointer and the program's
locale is not changed.
[#7] A null pointer for locale causes the setlocale function
to return a pointer to the string associated with the
category for the program's current locale; the program's
locale is not changed.175)
[#8] The pointer to string returned by the setlocale
function is such that a subsequent call with that string
value and its associated category will restore that part of
the program's locale. The string pointed to shall not be
modified by the program, but may be overwritten by a
subsequent call to the setlocale function.
Forward references: formatted input/output functions
(7.19.6), the multibyte character functions (7.20.7), the
multibyte string functions (7.20.8), string conversion
functions (7.20.1), the strcoll function (7.21.4.3), the
strftime function (7.23.3.5), the strfxtime function
(7.23.3.6), the strxfrm function (7.21.4.5).
7.11.2 Numeric formatting convention inquiry
7.11.2.1 The localeconv function
Synopsis
[#1]
#include <locale.h>
struct lconv *localeconv(void);
Description
[#2] The localeconv function sets the components of an
object with type struct lconv with values appropriate for
the formatting of numeric quantities (monetary and
otherwise) according to the rules of the current locale.
____________________
175The implementation shall arrange to encode in a string
the various categories due to a heterogeneous locale when
category has the value LC_ALL.
7.11.1.1 Library 7.11.2.1
228 Committee Draft -- August 3, 1998 WG14/N843
[#3] The members of the structure with type char * are
pointers to strings, any of which (except decimal_point) can
point to "", to indicate that the value is not available in
the current locale or is of zero length. Apart from
grouping and mon_grouping, the strings shall start and end
in the initial shift state. The members with type char are
nonnegative numbers, any of which can be CHAR_MAX to
indicate that the value is not available in the current
locale. The members include the following:
char *decimal_point
The decimal-point character used to format
nonmonetary quantities.
char *thousands_sep
The character used to separate groups of digits
before the decimal-point character in formatted
nonmonetary quantities.
char *grouping
A string whose elements indicate the size of each
group of digits in formatted nonmonetary quantities. *
char *mon_decimal_point
The decimal-point used to format monetary quantities.
char *mon_thousands_sep
The separator for groups of digits before the
decimal-point in formatted monetary quantities.
char *mon_grouping
A string whose elements indicate the size of each
group of digits in formatted monetary quantities.
char *positive_sign
The string used to indicate a nonnegative-valued
formatted monetary quantity.
char *negative_sign
The string used to indicate a negative-valued
formatted monetary quantity. |
char *currency_symbol
The local currency symbol applicable to the current |
locale.
char frac_digits
The number of fractional digits (those after the
decimal-point) to be displayed in a locally formatted |
monetary quantity.
char p_cs_precedes
Set to 1 or 0 if the currency_symbol respectively
precedes or succeeds the value for a nonnegative |
7.11.2.1 Library 7.11.2.1
WG14/N843 Committee Draft -- August 3, 1998 229
locally formatted monetary quantity.
char n_cs_precedes
Set to 1 or 0 if the currency_symbol respectively
precedes or succeeds the value for a negative locally |
formatted monetary quantity. |
char p_sep_by_space |
Set to a value indicating the separation of the |
currency_symbol, the sign string, and the value for a |
nonnegative locally formatted monetary quantity.
char n_sep_by_space
Set to a value indicating the separation of the |
currency_symbol, the sign string, and the value for a |
negative locally formatted monetary quantity.
char p_sign_posn
Set to a value indicating the positioning of the
positive_sign for a nonnegative locally formatted |
monetary quantity.
char n_sign_posn
Set to a value indicating the positioning of the
negative_sign for a negative locally formatted |
monetary quantity. |
char *int_curr_symbol |
The international currency symbol applicable to the |
current locale. The first three characters contain |
the alphabetic international currency symbol in |
accordance with those specified in ISO 4217:1995. |
The fourth character (immediately preceding the null |
character) is the character used to separate the |
international currency symbol from the monetary |
quantity. |
char int_frac_digits |
The number of fractional digits (those after the |
decimal-point) to be displayed in an internationally |
formatted monetary quantity. |
char int_p_cs_precedes |
Set to 1 or 0 if the int_currency_symbol respectively |
precedes or succeeds the value for a nonnegative |
internationally formatted monetary quantity. |
char int_n_cs_precedes |
Set to 1 or 0 if the int_currency_symbol respectively |
precedes or succeeds the value for a negative |
internationally formatted monetary quantity. |
char int_p_sep_by_space |
Set to a value indicating the separation of the |
7.11.2.1 Library 7.11.2.1
230 Committee Draft -- August 3, 1998 WG14/N843
int_currency_symbol, the sign string, and the value |
for a nonnegative internationally formatted monetary |
quantity. |
char int_n_sep_by_space |
Set to a value indicating the separation of the |
int_currency_symbol, the sign string, and the value |
for a negative internationally formatted monetary |
quantity. |
char int_p_sign_posn |
Set to a value indicating the positioning of the |
positive_sign for a nonnegative internationally |
formatted monetary quantity. |
char int_n_sign_posn |
Set to a value indicating the positioning of the |
negative_sign for a negative internationally |
formatted monetary quantity.
[#4] The elements of grouping and mon_grouping are
interpreted according to the following:
CHAR_MAX No further grouping is to be performed.
0 The previous element is to be repeatedly used for
the remainder of the digits.
other The integer value is the number of digits that |
compose the current group. The next element is
examined to determine the size of the next group
of digits before the current group.
[#5] The values of p_sep_by_space, n_sep_by_space, |
int_p_sep_by_space, and int_n_sep_by_space are interpreted |
according to the following: |
0 No space separates the currency symbol and value. |
1 A space separates the currency symbol and value. |
2 A space separates the currency symbol and the sign |
string, if adjacent. |
[#6] The values of p_sign_posn, n_sign_posn, |
int_p_sign_posn, and int_n_sign_posn are interpreted |
according to the following:
0 Parentheses surround the quantity and currency symbol. |
1 The sign string precedes the quantity and currency |
symbol.
7.11.2.1 Library 7.11.2.1
WG14/N843 Committee Draft -- August 3, 1998 231
2 The sign string succeeds the quantity and currency |
symbol.
3 The sign string immediately precedes the currency symbol. |
4 The sign string immediately succeeds the currency symbol. |
[#7] The implementation shall behave as if no library
function calls the localeconv function.
Returns
[#8] The localeconv function returns a pointer to the
filled-in object. The structure pointed to by the return
value shall not be modified by the program, but may be
overwritten by a subsequent call to the localeconv function.
In addition, calls to the setlocale function with categories
LC_ALL, LC_MONETARY, or LC_NUMERIC may overwrite the
contents of the structure.
[#9] EXAMPLE The following table illustrates the rules
which may well be used by four countries to format monetary
quantities. |
|| |
|| Local format | International format |
|+--------------+----------------+--------------+--------------|
Country ||Positive | Negative | Positive | Negative|
------------++--------------+----------------+--------------+--------------|
Finland ||1.234,56 mk | -1.234,56 mk | FIM 1.234,56 | FIM -1.234,56|
Italy ||L.1.234 | -L.1.234 | ITL 1.234 | -ITL 1.234|
Netherlands ||f 1.234,56 | f -1.234,56 | NLG 1.234,56 | NLG -1.234,56|
Switzerland ||SFrs.1,234.56 | SFrs.1,234.56C | CHF 1,234.56 | CHF 1,234.56C|
[#10] For these four countries, the respective values for
the monetary members of the structure returned by localeconv
are:
7.11.2.1 Library 7.11.2.1
232 Committee Draft -- August 3, 1998 WG14/N843
|| | | |
||Finland | Italy | Netherlands | Switzerland|
-------------------++------------+-------------+-------------+------------|
mon_decimal_point ||"," | "" | "," | "."|
mon_thousands_sep ||"." | "." | "." | ","
mon_grouping ||"\3" | "\3" | "\3" | "\3"
positive_sign ||"" | "" | "" | ""
negative_sign ||"-" | "-" | "-" | "C"
currency_symbol ||"mk" | "L." | "\u0192 " | "SFrs."|
frac_digits ||2 | 0 | 2 | 2|
p_cs_precedes ||0 | 1 | 1 | 1|
n_cs_precedes ||0 | 1 | 1 | 1|
p_sep_by_space ||1 | 0 | 1 | 0|
n_sep_by_space ||1 | 0 | 1 | 0|
p_sign_posn ||1 | 1 | 1 | 1
n_sign_posn ||1 | 1 | 4 | 2|
int_curr_symbol ||"FIM " | "ITL " | "NLG " | "CHF "|
int_frac_digits ||2 | 0 | 2 | 2|
int_p_cs_precedes ||1 | 1 | 1 | 1|
int_n_cs_precedes ||1 | 1 | 1 | 1|
int_p_sep_by_space ||1 | 1 | 1 | 1|
int_n_sep_by_space ||1 | 1 | 1 | 1|
int_p_sign_posn ||1 | 1 | 1 | 1|
int_n_sign_posn ||4 | 1 | 4 | 2|
7.11.2.1 Library 7.11.2.1
WG14/N843 Committee Draft -- August 3, 1998 233
7.12 Mathematics <math.h>
[#1] The header <math.h> declares two types and several
mathematical functions and defines several macros. Most
synopses specify a family of functions consisting of a
principal function with one or more double parameters, a
double return value, or both; and other functions with the
same name but with f and l suffixes which are corresponding
functions with float and long double parameters, return
values, or both.176) Integer arithmetic functions and
conversion functions are discussed later.
[#2] The types
float_t
double_t
are floating types at least as wide as float and double,
respectively, and such that double_t is at least as wide as
float_t. If FLT_EVAL_METHOD equals 0, float_t and double_t
are float and double, respectively; if FLT_EVAL_METHOD
equals 1, they are both double; if FLT_EVAL_METHOD equals 2,
they are both long double; and for other values of
FLT_EVAL_METHOD, they are otherwise
implementation-defined.177)
[#3] The macro
HUGE_VAL
expands to a positive double constant expression, not
necessarily representable as a float. The macros
HUGE_VALF
HUGE_VALL
are respectively float and long double analogs of
HUGE_VAL.178)
____________________
176Particularly on systems with wide expression evaluation,
a <math.h> function might pass arguments and return
values in wider format than the synopsis prototype
indicates.
177The types float_t and double_t are intended to be the
implementation's most efficient types at least as wide as
float and double, respectively. For FLT_EVAL_METHOD
equal 0, 1, or 2, the type float_t is the narrowest type
used by the implementation to evaluate floating
expressions.
178HUGE_VAL, HUGE_VALF, and HUGE_VALL can be positive
infinities in an implementation that supports infinities.
7.12 Library 7.12
234 Committee Draft -- August 3, 1998 WG14/N843
[#4] The macro
INFINITY
expands to a constant expression of type float representing
an implementation-defined positive or unsigned infinity, if |
available; else to a positive constant of type float that |
overflows at translation time.179)
[#5] The macro
NAN
is defined if and only if the implementation supports quiet
NaNs for the float type. It expands to a constant
expression of type float representing an implementation-
defined quiet NaN.
[#6] The macros
FP_INFINITE
FP_NAN
FP_NORMAL
FP_SUBNORMAL
FP_ZERO
are for number classification. They represent the mutually
exclusive kinds of floating-point values. They expand to
integer constant expressions with distinct values.
[#7] The macro
FP_FAST_FMA
is optionally defined. If defined, it indicates that the |
fma function generally executes about as fast as, or faster |
than, a multiply and an add of double operands.180) The
macros
FP_FAST_FMAF
FP_FAST_FMAL
are, respectively, float and long double analogs of
FP_FAST_FMA.
____________________
179In this case, using INFINITY will violate the constraint
in 6.4.4 and thus require a diagnostic.
180Typically, the FP_FAST_FMA macro is defined if and only
if the fma function is implemented directly with a
hardware multiply-add instruction. Software
implementations are expected to be substantially slower.
7.12 Library 7.12
WG14/N843 Committee Draft -- August 3, 1998 235
[#8] The macros
FP_ILOGB0
FP_ILOGBNAN
expand to integer constant expressions whose values are
returned by ilogb(x) if x is zero or NaN, respectively. The
value of FP_ILOGB0 shall be either INT_MIN or -INT_MAX. The
value of FP_ILOGBNAN shall be either INT_MAX or INT_MIN. *
Recommended practice
[#9] Conversion from (at least) double to decimal with |
DECIMAL_DIG digits and back should be the identity function |
(which assures that conversion from the widest supported IEC |
60559 format to decimal with DECIMAL_DIG digits and back is |
the identity function).
7