In the syntax notation used in the language section ($3), 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 so that
{ expression<opt> }
indicates
an optional expression enclosed in braces.
Syntax
token:
keyword
identifier
constant
string-literal
operator
punctuator
preprocessing-token:
header-name
identifier
pp-number
character-constant
string-literal
operator
punctuator
each
non-white-space character that cannot be one of the above
Constraints
Each preprocessing token that is converted to a token shall have the
lexical form of a keyword, an identifier, a constant, a string literal, an
operator, or a punctuator.
Semantics
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
,operators , 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 ,operators , punctuators , and single non-white-space
characters that do not lexically match the other preprocessing token categories.
If a ' or a character
matches the last category, the behavior is undefined.
Comments (described later) and the characters space, horizontal tab,
new-line, vertical tab, and form-feed---collectively called white space
---canseparate preprocessing tokens.
As described in $3.8, 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 token only as part of a header name or between the
quotation characters in a character constant or string literal.
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.
Examples
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.
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 ($3.1.3.4), comments ($3.1.9), expressions
($3.3), floating constants ($3.1.3.1), header names
($3.1.7), macro replacement ($3.8.3), postfix increment and decrement operators ($3.3.2.4), prefix increment
and decrement operators ($3.3.3.1), preprocessing directives
($3.8),
preprocessing numbers ($3.1.8), string literals ($3.1.4).
Syntax
keyword: one of
auto double
int struct
break else
long switch
case enum
register typedef
char extern
return union
const float
short unsigned
continue for
signed void
default goto sizeof
volatile
do if
static while
Semantics
The above tokens (entirely in lower-case) are reserved (in translation
phases 7 and 8) for use as keywords, and shall not be used otherwise.
Syntax
identifier:
nondigit
identifier nondigit
identifier digit
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
Description
An identifier is a sequence of nondigit characters (including the
underscore _ and the lower-case and upper-case letters) and digits.
The first character shall be a nondigit character.
Constraints
In translation phases 7 and 8, an identifier shall not consist of the
same sequence of characters as a keyword.
Semantics
An identifier denotes an object, a function, or one of the following
entities that will be described later: a tag or a member of a structure, union,
or enumeration; a typedef name; a label name; a macro name; or a macro
parameter. 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.
There is no specific limit on the maximum length of an identifier.
"Implementation
limits"
The implementation shall treat at least the first 31 characters of an
internal name (a macro name or an identifier that does not have external
linkage) as significant. Corresponding lower-case and upper-case letters are
different. The implementation may
further restrict the significance of an external name (an identifier that has
external linkage) to six characters and may ignore distinctions of alphabetical
case for such names./10/ These
limitations on identifiers are all implementation-defined.
Any identifiers that differ in a significant character are different
identifiers. If two identifiers differ in a non-significant character, the
behavior is undefined.
Forward
references: linkages of identifiers ($3.1.2.2), macro replacement ($3.8.3).
An identifier is visible (i.e., can be used) only within a region of
program text called its scope . 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.)
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.
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 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 outer declaration of a lexically identical identifier
exists in the same name space, it is hidden until the current scope terminates,
after which it again becomes visible.
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 ($3.6.2), declarations
($3.5),
enumeration specifiers ($3.5.2.2), function calls ($3.3.2.2), function
declarators (including prototypes) ($3.5.4.3), function definitions ($3.7.1),
the goto statement ($3.6.6.1), labeled statements ($3.6.1), name spaces of
identifiers ($3.1.2.3), scope of macro definitions ($3.8.3.5), source file
inclusion ($3.8.2), tags ($3.5.2.3), type specifiers
($3.5.2).
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.
In the set of translation units and libraries that constitutes an entire
program, each instance of a particular identifier with external linkage denotes
the same object or function. Within
one translation unit, each instance of an identifier with internal linkage
denotes the same object or function. Identifiers
with no linkage denote unique entities.
If the declaration of an identifier for an object or a function has file
scope and contains the storage-class specifier static , the identifier has
internal linkage.
If the declaration of an identifier for an object or a function contains
the storage-class specifier extern , the identifier has the same linkage as any
visible declaration of the identifier with file scope.
If there is no visible declaration with file scope, the identifier has
external linkage.
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.
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; an identifier declared to be an object inside a block
without the storage-class specifier extern .
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 ($3.6.2), declarations
($3.5),
expressions ($3.3), external definitions ($3.7).
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
any/11/ 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: declarators ($3.5.4), enumeration specifiers ($3.5.2.2), labeled
statements ($3.6.1), structure and union specifiers ($3.5.2.1), structure and
union members ($3.3.2.3), tags ($3.5.2.3).
An object has a storage duration that determines its lifetime.
There are two storage durations: static and automatic.
An object 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 and retains its last-stored value throughout the
execution of the entire program./12/
An object declared with no linkage and without the storage-class
specifier static has automatic storage duration
.Storage is guaranteed to be reserved for a new instance of such an
object on each normal entry into the block in which it is declared, or on a jump
from outside the block to a label in the block or in an enclosed block.
If an initialization is specified for the value stored in the object, it
is performed on each normal entry, but not if the block is entered by a jump to
a label. Storage for the object is
no longer guaranteed to be reserved when execution of the block ends in any way.
(Entering an enclosed block suspends but does not end execution of the
enclosing block. Calling a function
that returns suspends but does not end execution of the block containing the
call.) The value of a pointer that referred to an object with automatic storage
duration that is no longer guaranteed to be reserved is indeterminate.
Forward
references: compound statement, or block ($3.6.2), function calls
($3.3.2.2),
initialization ($3.5.7).
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).
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 $2.2.1 is stored in
a char object, its value is guaranteed to be positive.
If other quantities are stored in a char object, the behavior is
implementation-defined: the values are treated as either signed
or nonnegative integers.
There are four signed integer types
,designated as signed char , short int , int , and long int .
(The signed integer and other types may be designated in several
additional ways, as described in $3.5.2.)
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> ).
In the list of signed integer types above, the range of values of each
type is a subrange of the values of the next type in the list.
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 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. A
computation involving unsigned operands 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.
There are three 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 .
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.
There are three character types ,designated
as char , signed char , and unsigned char .
An enumeration comprises a set of named integer constant values.
Each distinct enumeration constitutes a different enumerated type .
The void type comprises an empty set of values; it is an incomplete type
that cannot be completed.
Any number of derived types can be constructed from the basic,
enumerated, and incomplete types, as follows:
*
An array type describes a contiguously allocated set of objects with a
particular member object type, called the element type
.Array types are characterized by their element type and by the number of
members of 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 set of member objects, each
of which has an optionally specified name and possibly distinct type.
*
A union type describes an overlapping 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
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.''
These methods of constructing derived types can be applied recursively.
The type char , the signed and unsigned integer types, and the enumerated
types are collectively called integral types
.The representations of integral types shall define values by use of a
pure binary numeration system./13/
American National Dictionary for Information Processing Systems .)The
representations of floating types are unspecified.
Integral 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
./14/
A pointer to void shall have the same representation and alignment
requirements as a pointer to a character type.
Other pointer types need not have the same representation or alignment
requirements.
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 $3.5.2.3)
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.
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, a function, or a pointer type derivation to T .
A type is characterized by its top type
,which is either the first type named in describing a derived type (as
noted above in the construction of derived types), or the type itself if the
type consists of no derived types.
A type has qualified type if its top type is specified with a type
qualifier; otherwise it has unqualified type
.The type qualifiers const and volatile respectively designate
const-qualified type and volatile-qualified type
./15/ For each
qualified type there is an unqualified type that is specified the same way as
the qualified type, but without any type qualifiers in its top type.
This type is known as the unqualified version of the qualified type.
Similarly, there are appropriately qualified versions of types (such as a
const-qualified version of a type), just as there are appropriately
non-qualified versions of types (such as a non-const-qualified version of a
type).
Examples
The type designated as `` float * '' is called ``pointer to float '' and
its top type is a pointer type, 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 --- it is called ``pointer to const float '' and is a pointer to a
qualified type.
Finally, the type designated as `` struct tag (*[5])(float) '' is called
``array of pointer to function returning struct tag .'' Its top type is array
type. The array has length five and
the function has a single parameter of type float .
Forward
references: character constants ($3.1.3.4), declarations
($3.5), tags ($3.5.2.3), type qualifiers
($3.5.3).
Two types have compatible type if their types are the same.
Additional rules for determining whether two types are compatible are
described in $3.5.2 for type specifiers, in $3.5.3 for type qualifiers, and in
$3.5.4 for declarators./16/ Moreover,
two structure, union, or enumeration types declared in separate translation
units are compatible if they have the same number of members, the same member
names, and compatible member types; for two structures, the members shall be in
the same order; for two enumerations, the members shall have the same values.
All declarations that refer to the same object or function shall have
compatible type; otherwise the behavior is undefined.
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 has the following
additions:
*
If one type is an array of known size, the composite type is an array of that
size.
*
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.
*
If both types have 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.
For an identifier with external or internal linkage declared in the same
scope as another declaration for that identifier, the type of the identifier
becomes the composite type.
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 ($3.5.4), enumeration specifiers ($3.5.2.2), structure
and union specifiers ($3.5.2.1), type definitions ($3.5.6), type qualifiers
($3.5.3), type specifiers ($3.5.2).
Syntax
constant:
floating-constant
integer-constant
enumeration-constant
character-constant
Constraints
The value of a constant shall be in the range of representable values for
its type.
Semantics
Each constant has a type, determined by its form and value, as detailed
later.
Syntax
floating-constant:
fractional-constant exponent-part<opt> floating-suffix<opt>
digit-sequence 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
floating-suffix: one of
f l F
L
Description
A floating constant has a value part that may be followed by an exponent
part and a suffix that specifies its type.
The components of the value 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 or E followed by an exponent
consisting of an optionally signed digit sequence.
Either the whole-number part or the fraction part shall be present;
either the period or the exponent part shall be present.
Semantics
The value part is interpreted as a decimal rational number; the digit
sequence in the exponent part is interpreted as a decimal integer.
The exponent indicates the power of 10 by which the value part is to be
scaled. If the scaled value is in
the range of representable values (for its type) but cannot be represented
exactly, the result is either the nearest higher or nearest lower value, chosen
in an implementation-defined manner.
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 .
Syntax
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:
0x hexadecimal-digit
0X hexadecimal-digit
hexadecimal-constant hexadecimal-digit
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
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>
long-suffix unsigned-suffix<opt>
unsigned-suffix: one of
u U
long-suffix: one of
l L
Description
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.
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
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.
The type of an integer constant is the first of the corresponding list in
which its value can be represented. Unsuffixed
decimal: int , long int , unsigned long int ; unsuffixed octal or hexadecimal:
int , unsigned int , long int , unsigned long int ; suffixed by the letter u or
U : unsigned int , unsigned long int ; suffixed by the letter l or L : long int
, unsigned long int ; suffixed by both the letters u or U and l or L : unsigned
long int .
Syntax
enumeration-constant:
identifier
Semantics
An identifier declared as an enumeration constant has type int .
Forward
references: enumeration specifiers ($3.5.2.2).
Syntax
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
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
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.
The single-quote ', the double-quote
, the question-mark ? , the backslash \ , and arbitrary integral values,
are representable according to the following table of escape sequences:
single-quote ' \'
double-quote " \"
question-mark ? \?
backslash \ \\
octal integer \
octal digits
hexadecimal integer
\x hexadecimal digits
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
\\ .
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.
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.
Each octal or hexadecimal escape sequence is the longest sequence of
characters that can constitute the escape sequence.
In addition, certain nongraphic characters are representable by escape
sequences consisting of the backslash \ followed by a lower-case letter: \a , \b
, \f , \n , \r , \t , and \v ./17/
If any other escape sequence is encountered, the behavior is undefined./18/
Constraints
The value of an octal or hexadecimal escape sequence shall be in the
range of representable values for the unsigned type corresponding to its type.
Semantics
An integer character constant has type int .
The value of an integer character constant containing a single character
that maps into 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.
In particular, in an implementation in which type char has the same range
of values as signed char , the high-order bit position of a single-character
integer character constant is treated as a sign bit.
A wide character constant has type wchar_t , an integral type defined in
the <stddef.h> header. The
value of a wide character constant containing a single multibyte character that
maps into 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.
Examples
The construction '\0' is commonly used to represent the null character.
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 if type char
has the same range of values as unsigned char , the character constant '\xFF'
has the value
Even if eight bits are used for objects that have type char , the
construction '\x123' specifies an integer character constant containing only one
character. (The value of this
single-character integer character constant is implementation-defined and
violates the above constraint.) To specify an integer character constant
containing the two characters whose values are 0x12 and '3', the construction
'\0223' may be used, since a hexadecimal escape sequence is terminated only by a
non-hexadecimal character. (The
value of this two-character integer character constant is implementation-defined
also.)
Even if 12 or more bits are used for objects 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: characters and integers ($3.2.1.1) common definitions
<stddef.h> ($4.1.5), the mbtowc function ($4.10.7.2).
Syntax
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
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 .
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
A character string literal has static storage duration and type ``array
of char ,'' and is initialized with the given characters.
A wide string literal has static storage duration and type ``array of
wchar_t ,'' and is initialized with the wide characters corresponding to the
given multibyte characters. Character
string literals that are adjacent tokens are concatenated into a single
character string literal. A null
character is then appended./19/
Likewise, adjacent wide string literal tokens are concatenated into a
single wide string literal to which a code with value zero is then appended.
If a character string literal token is adjacent to a wide string literal
token, the behavior is undefined.
Identical string literals of either form need not be distinct.
If the program attempts to modify a string literal of either form, the
behavior is undefined.
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> ($4.1.5).
Syntax
operator: one of
[ ] (
) .
->
++
-- &
* + - ~
! sizeof
/ % <<
>> <
> <=
>= =^=
!= ^
| && ||
? :
= *= /=
%= +=
-= <<= >>=
&= ^=
|=
, # ##
Constraints
The operators [ ] , ( ) , and ? : shall occur in pairs, possibly
separated by expressions. The operators # and ## shall occur in macro-defining
preprocessing directives only.
Semantics
An operator specifies an operation to be performed (an evaluation ) that
yields a value, or yields a designator, or produces a side effect, or a
combination thereof. An operand is an entity on which an operator acts.
Forward
references: expressions ($3.3), macro replacement ($3.8.3).
Syntax
punctuator: one of
[ ] (
) {
} *
, : =
; ...
#
Constraints
The punctuators [ ] , ( ) , and { } shall occur in pairs, possibly
separated by expressions, declarations, or statements.
The punctuator # shall occur in preprocessing directives only.
Semantics
A punctuator is a symbol that has independent syntactic and semantic
significance but does not specify an operation to be performed that yields a
value. Depending on context, the
same symbol may also represent an operator or part of an operator.
Forward
references: expressions ($3.3), declarations ($3.5), preprocessing directives
($3.8), statements ($3.6).
Syntax
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 "
Constraints
Header name preprocessing tokens shall only appear within a #include
preprocessing directive.
Semantics
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 $3.8.2.
If the characters ', \ , ,
or /* occur in the sequence between the < and > delimiters, the behavior
is undefined. Similarly, if the characters ', \ , or /* occur in the
sequence between the delimiters,
the behavior is undefined./20/
Example
The following sequence of characters:
0x3<1/a.h>1e2
#include <1/a.h>
#define const.member@$
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 ($3.8.2).
Syntax
pp-number:
digit
. digit
pp-number digit
pp-number nondigit
pp-number
e sign
pp-number E sign
pp-number .
Description
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+ , or E- character sequences.
Preprocessing number tokens lexically include all floating and integer
constant tokens.
Semantics
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.
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./21/
Several operators convert operand values from one type to another
automatically. This section
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 $3.2.1.5 summarizes the conversions performed
by most ordinary operators; it is supplemented as required by the discussion of
each operator in $3.3.
Conversion of an operand value to a compatible type causes no change.
Forward
references: cast operators ($3.3.4).
A char , a short int , or an int bit-field, or their signed or unsigned
varieties, or an object that has enumeration type, may be used in an expression
wherever an int or unsigned int may be used.
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 integral promotions
.
The integral promotions preserve value including sign.
As discussed earlier, whether a ``plain'' char is treated as signed is
implementation-defined.
Forward
references: enumeration specifiers ($3.5.2.2), structure and union specifiers
($3.5.2.1).
When an unsigned integer is converted to another integral type, if the
value can be represented by the new type, its value is unchanged.
When a signed integer is converted to an unsigned integer with equal or
greater size, if the value of the signed integer is nonnegative, its value is
unchanged. Otherwise: if the
unsigned integer has greater size, the signed integer is first promoted to the
signed integer corresponding to the unsigned integer; the value is converted to
unsigned by adding to it one greater than the largest number that can be
represented in the unsigned integer type./22/
When an integer is demoted to an unsigned integer with smaller size, the
result is the nonnegative remainder on division by the number one greater than
the largest unsigned number that can be represented in the type with smaller
size. When an integer is demoted to
a signed integer with smaller size, or an unsigned integer is converted to its
corresponding signed integer, if the value cannot be represented the result is
implementation-defined.
When a value of floating type is converted to integral type, the
fractional part is discarded. If the value of the integral part cannot be represented by
the integral type, the behavior is undefined./23/
When a value of integral type is converted to 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.
When a float is promoted to double or long double , or a double is
promoted to long double , its value is unchanged.
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.
Many binary operators that expect operands of arithmetic type cause
conversions and yield result types in a similar way.
The purpose is to yield a common type, which is also the type of the
result. This pattern is called the
usual arithmetic conversions :First,
if either operand has type long double , the other operand is converted to long
double . Otherwise, if either
operand has type double , the other operand is converted to double .
Otherwise, if either operand has type float , the other operand is
converted to float . Otherwise, the
integral promotions are performed on both operands. Then the following rules are applied: If either operand has
type unsigned long int , the other operand is converted to unsigned long int .
Otherwise, if one operand has type long int and the other has type
unsigned int , if a long int can represent all values of an unsigned int , the
operand of type unsigned int is converted to long int ; if a long int cannot
represent all the values of an unsigned int , both operands are converted to
unsigned long int . Otherwise, if
either operand has type long int , the other operand is converted to long int .
Otherwise, if either operand has type unsigned int , the other operand is
converted to unsigned int . Otherwise,
both operands have type int .
The values of operands and of the results of expressions may be
represented in greater precision and range than that required by the type; the
types are not changed thereby.
An lvalue is an expression (with an object type or an incomplete type
other than void ) that designates an object./24/
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
of all contained structures or unions) with a const-qualified type.
Except when it is the operand of the sizeof operator, the unary &
operator, the ++ operator, the -- operator, or 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.
Except when it is the operand of the sizeof operator or the unary &
operator, or is a character string literal used to initialize an array of
character type, or is a wide string literal used to initialize an array with
element type compatible with wchar_t , an lvalue that has type ``array of type
'' is converted to an expression that has type ``pointer to type '' that points
to the initial member of the array object and is not an lvalue.
A function designator is an expression that has function type.
Except when it is the operand of the sizeof operator/25/ 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 ($3.3.3.2), assignment operators
($3.3.16), common definitions <stddef.h> ($4.1.5), initialization
($3.5.7), postfix increment and decrement operators ($3.3.2.4), prefix increment
and decrement operators ($3.3.3.1), the sizeof operator ($3.3.3.4), structure
and union members ($3.3.2.3).
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 occurs in a context where a void
expression is required, its value or designator is discarded.
(A void expression is evaluated for its side effects.)
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.
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.
An integral constant expression with the value 0, or such an expression
cast to type void * , is called a null pointer constant
.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.
Two null pointers, converted through possibly different sequences of
casts to pointer types, shall compare equal.
Forward
references: cast operators ($3.3.4), equality operators ($3.3.9), simple
assignment ($3.3.16.1).
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.
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./26/
Except as indicated by the syntax/27/ or otherwise specified later (for
the function-call operator () , && , || , ?: , and comma operators), the
order of evaluation of subexpressions and the order in which side effects take
place are both unspecified.
Some operators (the unary operator ~ , and the binary operators <<
, >> , & , ^ , and | , collectively described as bitwise operators
)shall have operands that have integral type.
These operators return values that depend on the internal representations
of integers, and thus have implementation-defined aspects for signed types.
If an exception occurs during the evaluation of an expression (that is,
if the result is not mathematically defined or not representable), the behavior
is undefined.
An object shall have its stored value accessed only by an lvalue that has
one of the following types:/28/
*
the declared type of the object,
*
a qualified version of the declared type of the object,
*
a type that is the signed or unsigned type corresponding to the declared type of
the object,
*
a type that is the signed or unsigned type corresponding to a qualified version
of the declared 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.
Syntax
primary-expression:
identifier
constant
string-literal
( expression )
Semantics
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).
A constant is a primary expression.
Its type depends on its form, as detailed in $3.1.3.
A string literal is a primary expression.
It is an lvalue with type as detailed in $3.1.4.
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.
Forward
references: declarations ($3.5).
Syntax
postfix-expression:
primary-expression
postfix-expression [ expression
]
postfix-expression
( argument-expression-list<opt>
)
postfix-expression . identifier
postfix-expression -> identifier
postfix-expression ++
postfix-expression --
argument-expression-list:
assignment-expression
argument-expression-list , assignment-expression
Constraints
One of the expressions shall have type ``pointer to object type ,'' the
other expression shall have integral type, and the result has type `` type .''
Semantics
A postfix expression followed by an expression in square brackets [] is a
subscripted designation of a member 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 member of an array object) and E2 is an
integer, E1[E2] designates the E2 -th member of E1 (counting from zero).
Successive subscript operators designate a member of a multi-dimensional
array object. If E is an n
-dimensional array ( n >=2) with dimensions i x j "x ... x" k ,
then E (used as other than an lvalue) is converted to a pointer to an ( n
-1)-dimensional array with dimensions j "x ... x" 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).
Example
Consider the array object defined by the declaration
int x[3][5];
Here
x is a 3x5 array of int s; more precisely, x is an array of three member
objects, each of which is an array of five int s.
In the expression x[i] , which is equivalent to (*(x+(i))) , x is first
converted to a pointer to the initial array of five int s.
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 int s.
When used in the expression x[i][j] , that in turn is converted to a
pointer to the first of the int s, so x[i][j] yields an int .
Forward
references: additive operators ($3.3.6), address and indirection operators
($3.3.3.2), array declarators ($3.5.4.2).
Constraints
The expression that denotes the called function/29/ shall have type
pointer to function returning void or returning an object type other than array.
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
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.
If the expression that precedes the parenthesized argument list in a
function call consists solely of an identifier, and if no declaration is visible
for this identifier, the identifier is implicitly declared exactly as if, in the
innermost block containing the function call, the declaration
extern int identifier();
appeared./30/
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./31/
The value of the function call expression is specified in $3.6.6.4.
If the expression that denotes the called function has a type that does
not include a prototype, the integral 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 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.
If the function is defined with a type that includes a prototype, and the
types of the arguments after promotion are not compatible with the types of the
parameters, or if the prototype ends with an ellipsis ( ", ..." ), the
behavior is undefined.
If the expression that denotes the called function has a type that
includes a prototype, the arguments are implicitly converted, as if by
assignment, to the types of the corresponding parameters.
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. 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.
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 declarator.
The order of evaluation of the function designator, the arguments, and
subexpressions within the arguments is unspecified, but there is a sequence
point before the actual call.
Recursive function calls shall be permitted, both directly and indirectly
through any chain of other functions.
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 shall be completed before the function pointed to by
pf[f1()] is entered.
Forward
references: function declarators (including prototypes) ($3.5.4.3), function
definitions ($3.7.1), the return statement ($3.6.6.4), simple assignment
($3.3.16.1).
Constraints
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.
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
A postfix expression followed by a dot .
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.
A postfix expression followed by an arrow -> and an identifier
designates a member of a structure or union object.
The value is that of the named member of the object to which the first
expression points, and is an lvalue./32/
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.
With one exception, if a member of a union object is accessed after a
value has been stored in a different member of the object, the behavior is
implementation-defined./33/ 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, and if the union object currently contains one of these
structures, it is permitted to inspect the common initial part of any of them.
Two structures share a common initial sequence if corresponding members
have compatible types for a sequence of one or more initial members.
Example
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.
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)
/*...*/ sin(u.nf.doublenode) /*...*/
Forward
references: address and indirection operators ($3.3.3.2), structure and union
specifiers ($3.5.2.1).
Constraints
The operand of the postfix increment or decrement operator shall have
qualified or unqualified scalar type and shall be a modifiable lvalue.
Semantics
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.
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 ($3.3.6), compound assignment
($3.3.16.2).
Syntax
unary-expression:
postfix-expression
++ unary-expression
-- unary-expression
unary-operator cast-expression
sizeof unary-expression
sizeof ( type-name )
unary-operator: one of
& *
+ - ~ !
Constraints
The operand of the prefix increment or decrement operator shall have
qualified or unqualified scalar type and shall be a modifiable lvalue.
Semantics
The value of the operand of the prefix ++ operator is 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.
The prefix -- operator is analogous to the prefix ++ operator, except
that the value of the operand is decremented.
Forward
references: additive operators ($3.3.6), compound assignment
($3.3.16.2).
Constraints
The operand of the unary & operator shall be either a function
designator or an lvalue that designates an object that is not a bit-field and is
not declared with the register storage-class specifier.
The operand of the unary * operator shall have pointer type.
Semantics
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
.''
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./34/
Forward
references: storage-class specifiers ($3.5.1), structure and union specifiers
($3.5.2.1).
Constraints
The operand of the unary + or - operator shall have arithmetic type; of
the ~ operator, integral type; of the ! operator, scalar type.
Semantics
The result of the unary + operator is the value of its operand.
The integral promotion is performed on the operand, and the result has
the promoted type.
The result of the unary - operator is the negative of its operand.
The integral promotion is performed on the operand, and the result has
the promoted type.
The result of the ~ operator is the bitwise complement of its 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
integral promotion is performed on the operand, and the result has the promoted
type. The expression ~E is
equivalent to (ULONG_MAX-E) if E is promoted to type unsigned long , to
(UINT_MAX-E) if E is promoted to type unsigned int .
(The constants ULONG_MAX and UINT_MAX are defined in the header
<limits.h> .)
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: limits <float.h> and <limits.h> ($4.1.4).
Constraints
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
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, which is not itself
evaluated. The result is an integer constant.
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./35/
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.
The value of the result is implementation-defined, and its type (an
unsigned integral type) is size_t defined in the <stddef.h> header.
Examples
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();
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 .
Another use of the sizeof operator is to compute the number of members in
an array:
sizeof array / sizeof array[0]
Forward
references: common definitions <stddef.h> ($4.1.5), declarations
($3.5),
structure and union specifiers ($3.5.2.1), type names ($3.5.5).
Syntax
cast-expression:
unary-expression
( type-name ) cast-expression
Constraints
Unless the type name specifies void type, the type name shall specify
qualified or unqualified scalar type and the operand shall have scalar type.
Semantics
Preceding an expression by a parenthesized type name converts the value
of the expression to the named type. This
construction is called a cast ./36/
A cast that specifies an implicit conversion or no conversion has no
effect on the type or value of an expression.
Conversions that involve pointers (other than as permitted by the
constraints of $3.3.16.1) shall be specified by means of an explicit cast; they
have implementation-defined aspects: A pointer may be converted to an integral
type. The size of integer required
and the result are implementation-defined.
If the space provided is not long enough, the behavior is undefined.
An arbitrary integer may be converted to a pointer.
The result is implementation-defined./37/ A pointer to an object or incomplete type may be
converted to a pointer to a different object type or a different incomplete
type. The resulting pointer might
not be valid if it is improperly aligned for the type pointed to. It is guaranteed, however, that a pointer to an object of a
given alignment may be converted to a pointer to an object of the same alignment
or a less strict alignment and back again; the result shall compare equal to the
original pointer. (An object that
has character type has the least strict alignment.) 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 that has a type that is
not compatible with the type of the called function, the behavior is undefined.
Forward
references: equality operators ($3.3.9), function declarators (including
prototypes) ($3.5.4.3), simple assignment ($3.3.16.1), type names
($3.5.5).
Syntax
multiplicative-expression:
cast-expression
multiplicative-expression * cast-expression
multiplicative-expression / cast-expression
multiplicative-expression % cast-expression
Constraints
Each of the operands shall have arithmetic type.
The operands of the % operator shall have integral type.
Semantics
The usual arithmetic conversions are performed on the operands.
The result of the binary * operator is the product of the operands.
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.
When integers are divided and the division is inexact, if both operands
are positive the result of the / operator is the largest integer less than the
algebraic quotient and the result of the % operator is positive.
If either operand is negative, whether the result of the / operator is
the largest integer less than the algebraic quotient or the smallest integer
greater than the algebraic quotient is implementation-defined, as is the sign of
the result of the % operator. If
the quotient a/b is representable, the expression (a/b)*b + a%b shall equal a .
Syntax
additive-expression:
multiplicative-expression
additive-expression + multiplicative-expression
additive-expression - multiplicative-expression
Constraints
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 integral
type. (Incrementing is equivalent
to adding 1.)
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
integral type. (Decrementing is
equivalent to subtracting 1.)
Semantics
If both operands have arithmetic type, the usual arithmetic conversions
are performed on them.
The result of the binary + operator is the sum of the operands.
The result of the binary - operator is the difference resulting from the
subtraction of the second operand from the first.
When an expression that has integral type is added to or subtracted from
a pointer, the integral value is first multiplied by the size of the object
pointed to. The result has the type of the pointer operand.
If the pointer operand points to a member of an array object, and the
array object is large enough, the result points to a member of the same array
object, appropriately offset from the original member.
Thus if P points to a member of an array object, the expression P+1
points to the next member of the array object.
Unless both the pointer operand and the result point to a member of the
same array object, or one past the last member of the array object, the behavior
is undefined. Unless both the
pointer operand and the result point to a member of the same array object, or
the pointer operand points one past the last member of an array object and the
result points to a member of the same array object, the behavior is undefined if
the result is used as the operand of a unary * operator.
When two pointers to members of the same array object are subtracted, the
difference is divided by the size of a member.
The result represents the difference of the subscripts of the two array
members. The size of the result is
implementation-defined, and its type (a signed integral type) is ptrdiff_t
defined in the <stddef.h> header. As
with any other arithmetic overflow, if the result does not fit in the space
provided, the behavior is undefined. If
two pointers that do not point to members of the same array object are
subtracted, the behavior is undefined. However,
if P points either to a member of an array object or one past the last member of
an array object, and Q points to the last member of the same array object, the
expression (Q+1) - P has the same value as (Q-P) + 1 , even though Q+1 does not
point to a member of the array object.
Forward
references: common definitions <stddef.h> ($4.1.5).
Syntax
shift-expression:
additive-expression
shift-expression << additive-expression
shift-expression >> additive-expression
Constraints
Each of the operands shall have integral type.
Semantics
The integral 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 in bits of the promoted left operand, the behavior is undefined.
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 multiplied by the quantity,
2 raised to the power E2 , reduced modulo ULONG_MAX+1 if E1 has type unsigned
long , UINT_MAX+1 otherwise. (The
constants ULONG_MAX and UINT_MAX are defined in the header <limits.h> .)
The result of E1 >> E2 is E1 right-shifted E2 bit positions.
If E1 has an unsigned type or if E1 has a signed 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.
Syntax
relational-expression:
shift-expression
relational-expression < shift-expression
relational-expression > shift-expression
relational-expression <=
shift-expression
relational-expression >= shift-expression
Constraints
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
*
both operands are pointers to qualified or unqualified versions of compatible
incomplete types.
Semantics
If both of the operands have arithmetic type, the usual arithmetic
conversions are performed.
When two pointers are compared, the result depends on the relative
locations in the address space of the objects pointed to.
If the objects pointed to are members of the same aggregate object,
pointers to structure members declared later compare higher than pointers to
members declared earlier in the structure, and pointers to array elements with
larger subscript values compare higher 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 objects pointed to are not members of the same aggregate or union
object, the result is undefined, with the following exception.
If P points to the last member of an array object and Q points to a
member of the same array object, the pointer expression P+1 compares higher than
Q , even though P+1 does not point to a member of the array object.
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./38/
The result has type int .
Syntax
equality-expression:
relational-expression
equality-expression =^= relational-expression
equality-expression != relational-expression
Constraints
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
qualified or unqualified version of void ; or
*
one operand is a pointer and the other is a null pointer constant.
Semantics
The =^= (equal to) and the != (not equal to) operators are analogous to
the relational operators except for their lower precedence./39/
If two pointers to object or incomplete types compare equal, they point
to the same object. If two pointers to functions compare equal, they point to the
same function. If two pointers
point to the same object or function, they compare equal./40/ If one of the operands is a pointer to an object
or incomplete type and the other has type pointer to a qualified or unqualified
version of void , the pointer to an object or incomplete type is converted to
the type of the other operand.
Syntax
AND-expression:
equality-expression
AND-expression &
equality-expression
Constraints
Each of the operands shall have integral type.
Semantics
The usual arithmetic conversions are performed on the operands.
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).
Syntax
exclusive-OR-expression:
AND-expression
exclusive-OR-expression ^ AND-expression
Constraints
Each of the operands shall have integral type.
Semantics
The usual arithmetic conversions are performed on the operands.
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).
Syntax
inclusive-OR-expression:
exclusive-OR-expression
inclusive-OR-expression | exclusive-OR-expression
Constraints
Each of the operands shall have integral type.
Semantics
The usual arithmetic conversions are performed on the operands.
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).
Syntax
logical-AND-expression:
inclusive-OR-expression
logical-AND-expression && inclusive-OR-expression
Constraints
Each of the operands shall have scalar type.
Semantics
The && operator shall yield 1 if both of its operands compare
unequal to 0, otherwise it yields 0. The
result has type int .
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.
Syntax
logical-OR-expression:
logical-AND-expression
logical-OR-expression || logical-AND-expression
Constraints
Each of the operands shall have scalar type.
Semantics
The || operator shall yield 1 if either of its operands compare unequal
to 0, otherwise it yields 0. The
result has type int .
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.
Syntax
conditional-expression:
logical-OR-expression
logical-OR-expression ? expression
: conditional-expression
Constraints
The first operand shall have scalar type.
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
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 value of the second or
third operand (whichever is evaluated) is the result./41/
If both the second and third operands have arithmetic type, the usual
arithmetic conversions are performed to bring them to a common type and the
result has that type. 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.
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 differently qualified versions of a
compatible type, the result has 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
other operand is converted to type pointer to void , and the result has that
type.
Syntax
assignment-expression:
conditional-expression
unary-expression assignment-operator assignment-expression
assignment-operator: one of
= *= /=
%= +=
-= <<= >>=
&= ^=
|=
Constraints
An assignment operator shall have a modifiable lvalue as its left
operand.
Semantics
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.
The order of evaluation of the operands is unspecified.
Constraints
One of the following shall hold:/42/
*
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 by the right; or
*
the left operand is a pointer and the right is a null pointer constant.
Semantics
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.
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.
Example
In the program fragment
int f(void);
char c;
/*...*/
/*...*/ ((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
.
Constraints
For the operators += and -= only, either the left operand shall be a
pointer to an object type and the right shall have integral type, or the left
operand shall have qualified or unqualified arithmetic type and the right shall
have arithmetic type.
For the other operators, each operand shall have arithmetic type
consistent with those allowed by the corresponding binary operator.
Semantics
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.
Syntax
expression:
assignment-expression
expression , assignment-expression
Semantics
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./43/
Example
As indicated by the syntax, in contexts where a comma is a punctuator (in
lists of arguments to functions and lists of initializers) the comma operator as
described in this section cannot appear. 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 ($3.5.7).
Syntax
constant-expression:
conditional-expression
Description
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
Constant expressions shall not contain assignment, increment, decrement,
function-call, or comma operators, except when they are contained within the
operand of a sizeof operator./44/
Each constant expression shall evaluate to a constant that is in the
range of representable values for its type.
Semantics
An expression that evaluates to a constant is required in several
contexts./45/ If the
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.
An integral constant expression shall have integral type and shall only
have operands that are integer constants, enumeration constants, character
constants, sizeof expressions, and floating constants that are the immediate
operands of casts. Cast operators in an integral constant expression shall only
convert arithmetic types to integral types, except as part of an operand to the
sizeof operator.
More latitude is permitted for constant expressions in initializers.
Such a constant expression shall evaluate to one of the following:
*
an arithmetic constant expression,
*
an address constant, or
*
an address constant for an object type plus or minus an integral constant
expression.
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.
An address constant is 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 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 an address constant, but the value of
an object shall not be accessed by use of these operators.
The semantic rules for the evaluation of a constant expression are the
same as for non-constant expressions./46/
Forward
references: initialization ($3.5.7).
Syntax
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>
init-declarator-list:
init-declarator
init-declarator-list , init-declarator
init-declarator:
declarator
declarator = initializer
Constraints
A declaration shall declare at least a declarator, a tag, or the members
of an enumeration.
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 $3.5.2.3.
All declarations in the same scope that refer to the same object or
function shall specify compatible types.
Semantics
A declaration specifies the interpretation and attributes of a set of
identifiers. A declaration that
also causes storage to be reserved for an object or function named by an
identifier is a definition ./47/
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.
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 ($3.5.4), enumeration specifiers ($3.5.2.2),
initialization ($3.5.7), tags ($3.5.2.3).
Syntax
storage-class-specifier:
typedef
extern
static
auto
register
Constraints
At most one storage-class specifier may be given in the declaration
specifiers in a declaration./48/
Semantics
The typedef specifier is called a ``storage-class specifier'' for
syntactic convenience only; it is discussed in $3.5.6.
The meanings of the various linkages and storage durations were discussed
in $3.1.2.2 and $3.1.2.4.
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./49/
The declaration of an identifier for a function that has block scope
shall have no explicit storage-class specifier other than extern .
Forward
references: type definitions ($3.5.6).
Syntax
type-specifier:
void
char
short
int
long
float
double
signed
unsigned
struct-or-union-specifier
enum-specifier
typedef-name
Constraints
Each
list of type specifiers shall be one of the following sets; 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 , signed int , or no type specifiers
*
unsigned , or unsigned int
*
long , signed long , long int , or signed long int
*
unsigned long , or unsigned long int
*
float
*
double
*
long double
*
struct-or-union specifier
*
enum-specifier
*
typedef-name
Semantics
Specifiers for structures, unions, and enumerations are discussed in
$3.5.2.1 through $3.5.2.3. Declarations
of typedef names are discussed in $3.5.6. The
characteristics of the other types are discussed in $3.1.2.5.
Each of the above comma-separated lists designates the same type, except
that for bit-field declarations, signed int (or signed ) may differ from int (or
no type specifiers).
Forward
references: enumeration specifiers ($3.5.2.2), structure and union specifiers
($3.5.2.1), tags ($3.5.2.3), type definitions
($3.5.6).
Syntax
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>
struct-declarator-list:
struct-declarator
struct-declarator-list
, struct-declarator
struct-declarator:
declarator
declarator<opt> : constant-expression
Constraints
A structure or union shall not contain a member with incomplete or
function type. Hence it shall not
contain an instance of itself (but may contain a pointer to an instance of
itself).
The expression that specifies the width of a bit-field shall be an
integral constant expression that has nonnegative value that shall not exceed
the number of bits in an ordinary object of compatible type.
If the value is zero, the declaration shall have no declarator.
Semantics
As discussed in $3.1.2.5, a structure is a type consisting of a sequence
of named members, whose storage is allocated in an ordered sequence, and a union
is a type consisting of a sequence of named members, whose storage overlap.
Structure and union specifiers have the same form.
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. The type
is incomplete until after the } that terminates the list.
A member of a structure or union may have any object type.
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 ;/50/ its width is preceded by a colon.
A bit-field may have type int , unsigned int , or signed int .
Whether the high-order bit position of a ``plain'' int bit-field is
treated as a sign bit is implementation-defined.
A bit-field is interpreted as an integral type consisting of the
specified number of bits.
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.
A bit-field declaration with no declarator, but only a colon and a width,
indicates an unnamed bit-field./51/
As a special case of this, a bit-field 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.
Each non-bit-field member of a structure or union object is aligned in an
implementation-defined manner appropriate to its type.
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 cast, 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 therefore be unnamed holes within a structure object, but not
at its beginning, as necessary to achieve the appropriate alignment.
The size of a union is sufficient to contain the largest of its members.
The value of at most one of the members can be stored in a union object
at any time. A pointer to a union
object, suitably cast, 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.
There may also be unnamed padding at the end of a structure or union, as
necessary to achieve the appropriate alignment were the structure or union to be
a member of an array.
Syntax
enum-specifier:
enum identifier<opt> {
enumerator-list }
enum identifier
enumerator-list:
enumerator
enumerator-list , enumerator
enumerator:
enumeration-constant
enumeration-constant = constant-expression
Constraints
The expression that defines the value of an enumeration constant shall be
an integral constant expression that has a value representable as an int .
Semantics
The identifiers in an enumerator list are declared as constants that have
type int and may appear wherever such are permitted./52/
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. (A
combination of both forms of enumerators 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.
Each enumerated type shall be compatible with an integer type; the choice
of type is implementation-defined.
Example
enum hue { chartreuse, burgundy, claret=20, winedark };
/*...*/
enum hue col, *cp;
/*...*/
col = claret;
cp = &col;
/*...*/
/*...*/ (*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}.
A type specifier of the form
struct-or-union identifier { struct-declaration-list
}
enum identifier {
enumerator-list }
declares
the identifier to be the tag of the structure, union, or enumeration specified
by the list. The list defines the
structure content ,union content
,or enumeration content .If
this declaration of the tag is visible, a subsequent declaration that uses the
tag and that omits the bracketed list specifies the declared structure, union,
or enumerated type. Subsequent
declarations in the same scope shall omit the bracketed list.
If a type specifier of the form
struct-or-union identifier
occurs
prior to the declaration that defines the content, the structure or union is an
incomplete type./53/ It
declares a tag that specifies a type that may be used only when the size of an
object of the specified type is not needed./54/
If the type is to be completed, another declaration of the tag in the
same scope (but not in an enclosed block, which declares a new type known only
within that block) shall define the content.
A declaration of the form
struct-or-union identifier ;
specifies
a structure or union type and declares a tag, both visible only within the scope
in which the declaration occurs. It
specifies a new type distinct from any type with the same tag in an enclosing
scope (if any).
A type specifier of the form
struct-or-union { struct-declaration-list
}
enum { enumerator-list }
specifies
a new structure, union, or enumerated type, within the translation unit, that
can only be referred to by the declaration of which it is a part./55/
Examples
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 .
The following alternative formulation uses the typedef mechanism:
typedef struct tnode TNODE;
struct tnode {
int count;
TNODE *left, *right;
};
TNODE s, *sp;
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 otherwise vacuous 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: type definitions ($3.5.6).
Syntax
type-qualifier:
const
volatile
Constraints
The same type qualifier shall not appear more than once in the same
specifier list or qualifier list, either directly or via one or more typedef s.
Semantics
The properties associated with qualified types are meaningful only for
expressions that are lvalues./56/
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./57/
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
$2.1.2.3. 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./58/
What constitutes an access to an object that has volatile-qualified type
is implementation-defined.
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./59/
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.
Examples
An object declared
extern const volatile int real_time_clock;
may
be modifiable by hardware, but cannot be assigned to, incremented, or
decremented.
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 * '' */
Syntax
declarator:
pointer<opt> direct-declarator
direct-declarator:
identifier
( declarator )
direct-declarator [ constant-expression<opt>
]
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
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
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.
In the following subsections, 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.
If, in the declaration `` T D1 ,'' D1 has the form
identifier
then
the type specified for ident is T .
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
complex declarators may be altered by parentheses.
"Implementation
limits"
The implementation shall allow the specification of types that have at
least 12 pointer, array, and function declarators (in any valid combinations)
modifying an arithmetic, a structure, a union, or an incomplete type, either
directly or via one or more typedef s.
Forward
references: type definitions ($3.5.6).
Semantics
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.
For two pointer types to be compatible, both shall be identically
qualified and both shall be pointers to compatible types.
Examples
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 the const int pointed to by ptr_to_constant shall not be modified,
but ptr_to_constant itself may be changed to point to another const int .
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.
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 .''
Constraints
The expression that specifies the size of an array shall be an integral
constant expression that has a value greater than zero.
Semantics
If, in the declaration `` T D1 ,'' D1 has the form
D[ constant-expression<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 array of T .''/60/
If the size is not present, the array type is an incomplete type.
For two array types to be compatible, both shall have compatible element
types, and if both size specifiers are present, they shall have the same value.
Examples
float fa[11], *afp[17];
declares
an array of float numbers and an array of pointers to float numbers.
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.
Forward
references: function definitions ($3.7.1), initialization ($3.5.7).
Constraints
A function declarator shall not specify a return type that is a function
type or an array type.
The only storage-class specifier that shall occur in a parameter
declaration is register .
An identifier list in a function declarator that is not part of a
function definition shall be empty.
Semantics
If, in the declaration `` T D1 ,'' D1 has the form
D( parameter-type-list)
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 .''
A parameter type list specifies the types of, and may declare identifiers
for, the parameters of the function. If
the list terminates with an ellipsis ( , ... ), no information about the number
or types of the parameters after the comma is supplied./61/
The special case of void as the only item in the list specifies that the
function has no parameters.
In a parameter declaration, a single typedef name in parentheses is taken
to be an abstract declarator that specifies a function with a single parameter,
not as redundant parentheses around the identifier for a declarator.
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.
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 function definition specifies that the
function has no parameters. The
empty list in a function declarator that is not part of a function definition
specifies that no information about the number or types of the parameters is
supplied./62/
For two function types to be compatible, both shall specify compatible
return types./63/ 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 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.
(For each parameter declared with function or array type, its type for
these comparisons is the one that results from conversion to a pointer type, as
in $3.7.1. For each parameter
declared with qualified type, its type for these comparisons is the unqualified
version of its declared type.)
Examples
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 .
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 external linkage, and the identifier of the pointer pfi
has block scope and no linkage.
Here are two more intricate examples.
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 at the end of the declaration of apfi. 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 ), and an int . The pointer returned by fpfi points to a function that has at least one
parameter, which has type int .
Forward
references: function definitions ($3.7.1), type names ($3.5.5).
Syntax
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> [
constant-expression<opt> ]
direct-abstract-declarator<opt> (
parameter-type-list<opt> )
Semantics
In several contexts it is desired 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./64/
Examples
The constructions
(a) int
(b) int *
(c) int *[3]
(d) int (*)[3]
(e) int *()
(f) int (*)(void)
(g) 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 int s, (e) function with no
parameter specification returning a pointer to int , (f) pointer to function
with no parameters returning an int , and (g) 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 .
Syntax
typedef-name:
identifier
Semantics
In a declaration whose storage-class specifier is typedef , each
declarator defines an identifier to be a typedef name that specifies the type
specified for the identifier in the way described in $3.5.4.
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. If the identifier is redeclared in an inner scope or is
declared as a member of a structure or union in the same or an inner scope, the
type specifiers shall not be omitted in the inner declaration.
Examples
After
typedef int MILES, KLICKSP();
typedef struct { double re, im; } complex;
the
constructions
MILES distance;
extern KLICKSP *metricp;
complex x;
complex 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. The
object distance has a type compatible with any other int object.
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 , and int .
The following 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.) 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 .
Syntax
initializer:
assignment-expression
{ initializer-list }
{ initializer-list , }
initializer-list:
initializer
initializer-list , initializer
Constraints
There shall be no more initializers in an initializer list than there are
objects to be initialized.
The type of the entity to be initialized shall be an object type or an
array of unknown size.
All the expressions in an initializer for an object that has static
storage duration or in an initializer list for an object that has aggregate or
union type shall be constant expressions.
If the declaration of an identifier has block scope, and the identifier
has external or internal linkage, there shall be no initializer for the
identifier.
Semantics
An initializer specifies the initial value stored in an object.
All unnamed structure or union members are ignored during initialization.
If an object that has static storage duration is not initialized
explicitly, it is initialized implicitly as if every member that has arithmetic
type were assigned 0 and every member that has pointer type were assigned a null
pointer constant. If an object that has automatic storage duration is not
initialized explicitly, its value is indeterminate./65/
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;
the same type constraints and conversions as for simple assignment apply.
A brace-enclosed initializer for a union object initializes the member
that appears first in the declaration list of the union type.
The initializer for a structure or union object that has automatic
storage duration either shall be an initializer list as described below, or
shall be a single expression that has compatible structure or union type.
In the latter case, the initial value of the object is that of the
expression.
The rest of this section deals with initializers for objects that have
aggregate or union type.
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
members of the array.
An array with element type compatible with wchar_t may be initialized by
a wide string literal, optionally enclosed in braces.
Successive codes of the wide string literal (including the terminating
zero-valued code if there is room or if the array is of unknown size) initialize
the members of the array.
Otherwise, the initializer for an object that has aggregate type shall be
a brace-enclosed list of initializers for the members of the aggregate, written
in increasing subscript or member order; and the initializer for an object that
has union type shall be a brace-enclosed initializer for the first member of the
union.
If the aggregate contains members that are aggregates or unions, or if
the first member of a union is an aggregate or union, the 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 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 members of the first
subaggregate or the first member of the contained union; any remaining
initializers are left to initialize the next member of the aggregate of which
the current subaggregate or contained union is a part.
If there are fewer initializers in a list than there are members of an
aggregate, the remainder of the aggregate shall be initialized implicitly the
same as objects that have static storage duration.
If an array of unknown size is initialized, its size is determined by the
number of initializers provided for its members.
At the end of its initializer list, the array no longer has incomplete
type.
Examples
The declaration
int x[] = { 1, 3, 5 };
defines
and initializes x as a one-dimensional array object that has three members, as
no size was specified and there are three initializers.
float 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 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
float 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] .
Also,
float z[4][3] = {
{ 1 }, { 2 }, { 3 }, { 4 }
};
initializes
the first column of z as specified and initializes the rest with zeros.
struct { int a[3], b; } w[] = { { 1 }, 2 };
is
a definition with an inconsistently bracketed initialization.
It defines an array with two member structures: w[0].a[0] is 1 and
w[1].a[0] is 2; all the other elements are zero.
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][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 members are initialized with
zero. Likewise, the initializers
for q[1][0][0] and q[2][0][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 occur. 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.
Note that the fully-bracketed and minimally-bracketed forms of
initialization are, in general, less likely to cause confusion.
Finally, the declaration
char s[] = "abc", t[3] = "abc";
defines
``plain'' char array objects s and t whose members 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 '' that is initialized to point to an object with
type ``array of char '' whose members 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.
Forward references: common definitions <stddef.h> ($4.1.5).
Syntax
statement:
labeled-statement
compound-statement
expression-statement
selection-statement
iteration-statement
jump-statement
Semantics
A statement specifies an action to be performed.
Except as indicated, statements are executed in sequence.
A full expression is an expression that is not part of another
expression. 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 three expressions
of a for statement; the expression in a return statement.
The end of a full expression is a sequence point.
Forward
references: expression and null statements ($3.6.3), selection statements
($3.6.4), iteration statements ($3.6.5), the return statement
($3.6.6.4).
Syntax
labeled-statement:
identifier : statement
case constant-expression :
statement
default : statement
Constraints
A case or default label shall appear only in a switch statement.
Further constraints on such labels are discussed under the switch
statement.
Semantics
Any statement may be preceded by a prefix that declares an identifier as
a label name. Labels in themselves
do not alter the flow of control, which continues unimpeded across them.
Forward
references: the goto statement ($3.6.6.1), the switch statement
($3.6.4.2).
Syntax
compound-statement:
{
declaration-list<opt> statement-list<opt> }
declaration-list:
declaration
declaration-list declaration
statement-list:
statement
statement-list statement
Semantics
A compound statement (also called a block
)allows a set of statements to be grouped into one syntactic unit, which
may have its own set of declarations and initializations (as discussed in $3.1.2.4). The initializers of
objects that have automatic storage duration are evaluated and the values are
stored in the objects in the order their declarators appear in the translation
unit.
Syntax
expression-statement:
expression<opt> ;
Semantics
The expression in an expression statement is evaluated as a void
expression for its side effects./66/
A null statement (consisting of just a semicolon) performs no operations.
Examples
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);
In the program fragment
char *s;
/*...*/
while (*s++ != '\0')
;
a
null statement is used to supply an empty loop body to the iteration statement.
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 ($3.6.5).
Syntax
selection-statement:
if ( expression ) statement
if ( expression ) statement else statement
switch ( expression ) statement
Semantics
A selection statement selects among a set of statements depending on the
value of a controlling expression.
Constraints
The controlling expression of an if statement shall have scalar type.
Semantics
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.
An else is associated with the lexically immediately preceding else -less
if that is in the same block (but not in an enclosed block).
Constraints
The controlling expression of a switch statement shall have integral
type. The expression of each case
label shall be an integral constant expression.
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.)
Semantics
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.
The integral 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"
As discussed previously ($2.2.4.1), the implementation may limit the
number of case values in a switch statement.
Syntax
iteration-statement:
while ( expression ) statement
do statement while ( expression
) ;
for ( expression<opt>
; expression<opt> ;
expression<opt> ) statement
Constraints
The controlling expression of an iteration statement shall have scalar
type.
Semantics
An iteration statement causes a statement called the loop body to be
executed repeatedly until the controlling expression compares equal to 0.
The evaluation of the controlling expression takes place before each
execution of the loop body.
The evaluation of the controlling expression takes place after each
execution of the loop body.
Except for the behavior of a continue statement in the loop body, the
statement
for ( expression-1 ; expression-2
; expression-3 )
statement
and
the sequence of statements
expression-1 ;
while ( expression-2) {
statement
expression-3 ;
}
are
equivalent./67/ expression-1 expression-2 , expression-3
Both expression-1 and expression-3 may be omitted.
Each is evaluated as a void expression.
An omitted expression-2 is replaced by a nonzero constant.
Forward
references: the continue statement ($3.6.6.2).
Syntax
jump-statement:
goto identifier ;
continue
;
break ;
return expression<opt>
;
Semantics
A jump statement causes an unconditional jump to another place.
Constraints
The identifier in a goto statement shall name a label located somewhere
in the current function.
Semantics
A
goto statement causes an unconditional jump to the statement prefixed by the
named label in the current function.
Constraints
A continue statement shall appear only in or as a loop body.
Semantics
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;
./68/
Constraints
A break statement shall appear only in or as a switch body or loop body.
Semantics
A break statement terminates execution of the smallest enclosing switch
or iteration statement.
Constraints
A return statement with an expression shall not appear in a function
whose return type is void .
Semantics
A return statement terminates execution of the current function and
returns control to its caller. A
function may have any number of return statements, with and without expressions.
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 that of the function in which it appears, it is converted as
if it were assigned to an object of that type.
If a return statement without an expression is executed, and the value of
the function call is used by the caller, the behavior is undefined.
Reaching the } that terminates a function is equivalent to executing a
return statement without an expression.
Syntax
translation-unit:
external-declaration
translation-unit external-declaration
external-declaration:
function-definition
declaration
Constraints
The storage-class specifiers auto and register shall not appear in the
declaration specifiers in an external declaration.
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
As discussed in $2.1.1.1, 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 $3.5, a declaration that also causes storage to be reserved for an
object or a function named by the identifier is a definition.
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./69/
Syntax
function-definition:
declaration-specifiers<opt> declarator declaration-list<opt>
compound-statement
Constraints
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./70/
The return type of a function shall be void or an object type other than
array.
The storage-class specifier, if any, in the declaration specifiers shall
be either extern or static .
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 there
shall not be an identifier). No declaration list shall follow.
If the declarator includes an identifier list, only the identifiers it
names shall be declared in the declaration list.
An identifier declared as a typedef name shall not be redeclared as a
parameter. The declarations in the declaration list shall contain no
storage-class specifier other than register and no initializations.
Semantics
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,/71/ the types of the
parameters may be declared in a following declaration list.
Any parameter that is not declared has type int .
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.
On entry to the function the value of each argument expression shall be
converted to the type of its corresponding parameter, as if by assignment to the
parameter. Array expressions and function designators as arguments are
converted to pointers before the call. 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 $3.2.2.1.
The resulting parameter type shall be an object type.
Each parameter has automatic storage duration.
Its identifier is an lvalue./72/
The layout of the storage for parameters is unspecified.
Examples
extern int max(int a, int b)
{
return a > b ? a : b;
}
Here
extern is the storage-class specifier and int is the type specifier (each of
which may be omitted as those are the defaults); 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, which may be omitted
because those are the defaults. 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 may not.
To pass one function to another, one might say
int f(void);
/*...*/
g(f);
Note
that f must be declared explicitly in the calling function, as its appearance in
the expression g(f) was not followed by ( .
Then the definition of g might read
g(int (*funcp)(void))
{
/*...*/ (*funcp)() /* or
funcp() ... */
}
or,
equivalently,
g(int func(void))
{
/*...*/ func() /* or
(*func)() ... */
}
Semantics
If the declaration of an identifier for an object has file scope and an
initializer, the declaration is an external definition for the identifier.
A declaration of an identifier for an object that has file scope without
an initializer, and without a storage-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.
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.
Examples
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; /*
$3.1.2.2 renders undefined, linkage disagreement */
int i3; /*
valid tentative definition, refers to previous */
int i4; /*
valid tentative definition, refers to previous */
int i5; /*
$3.1.2.2 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 */
Syntax
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-expression
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-expression
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
# undef identifier
new-line
# line pp-tokens
new-line
# error pp-tokens<opt>
new-line
# pragma
pp-tokens<opt> new-line
# new-line
lparen:
the left-parenthesis character without preceding white-space
replacement-list:
pp-tokens<opt>
pp-tokens:
preprocessing-token
pp-tokens preprocessing-token
new-line:
the new-line character
Description
A preprocessing directive consists of a sequence of preprocessing tokens
that begins with a # preprocessing token that 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./73/
Constraints
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 in
translation phase 3).
Semantics
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.
The preprocessing tokens within a preprocessing directive are not subject
to macro expansion unless otherwise stated.
Constraints
The expression that controls conditional inclusion shall be an integral
constant expression except that: it shall not contain a cast; identifiers
(including those lexically identical to keywords) are interpreted as described
below;/74/ and it may contain unary operator expressions of the form
defined identifier
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.
Each preprocessing token that remains after all macro replacements have
occurred shall be in the lexical form of a token.
Semantics
Preprocessing directives of the forms
# if constant-expression new-line group<opt>
# elif constant-expression new-line group<opt>
check
whether the controlling constant expression evaluates to nonzero.
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 normal
text. If the token defined is
generated as a result of this replacement process, the behavior is undefined.
After all replacements are finished, the resulting preprocessing tokens
are converted into tokens, and then all remaining identifiers are replaced with
0 . The resulting tokens comprise
the controlling constant expression which is evaluated according to the rules of
$3.4 using arithmetic that has at least the ranges specified in $2.2.4.2, except
that int and unsigned int act as if they have the same representation as,
respectively, long and unsigned long . 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./75/
Also,
whether a single-character character constant may have a negative value is
implementation-defined.
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.
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./76/
Forward
references: macro replacement ($3.8.3), source file inclusion
($3.8.2).
Constraints
A #include directive shall identify a header or source file that can be
processed by the implementation.
Semantics
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.
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.
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 its replacement list of preprocessing tokens.)
The directive resulting after all replacements shall match one of the two
previous forms./77/ 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.
There shall be an implementation-defined mapping between the delimited
sequence and the external source file name.
The implementation shall provide unique mappings for sequences consisting
of one or more letters (as defined in $2.2.1) followed by a period ( . ) and a
single letter. The implementation may ignore the distinctions of
alphabetical case and restrict the mapping to six significant characters before
the period.
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 $2.2.4.1).
Examples
The most common uses of #include preprocessing directives are as in the
following:
#include <stdio.h>
#include "myprog.h"
This example illustrates a macro-replaced #include directive:
#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 ($3.8.3).
Constraints
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.
An identifier currently defined as a macro without use of lparen (an
object-like macro) may be redefined by another #define preprocessing directive
provided that the second definition is an object-like macro definition and the
two replacement lists are identical.
An identifier currently defined as a macro using lparen (a function-like
macro) may be redefined by another #define preprocessing directive provided that
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.
The number of arguments in an invocation of a function-like macro shall
agree with the number of parameters in the macro definition, and there shall
exist a ) preprocessing token that terminates the invocation.
A parameter identifier in a function-like macro shall be uniquely
declared within its scope.
Semantics
The identifier immediately following the define is called the macro name
.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.
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.
A preprocessing directive of the form
# define identifier replacement-list new-line
defines
an object-like macro that causes each subsequent instance of the macro name/78/
to be replaced by the replacement list of preprocessing tokens that constitute
the remainder of the directive. The
replacement list is then rescanned for more macro names as specified below.
A preprocessing directive of the form
# define identifier lparen identifier-list<opt> )
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.
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 bounded by nested
parentheses do not separate arguments. If
(before argument substitution) any argument consists of no preprocessing tokens,
the behavior is undefined. If there
are sequences of preprocessing tokens within the list of arguments that would
otherwise act as preprocessing directives, the behavior is undefined.
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 source file; no other preprocessing tokens are
available.
Constraints
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
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 comprising 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). If the
replacement that results is not a valid character string literal, the behavior
is undefined. The order of
evaluation of # and ## operators is unspecified.
Constraints
A ## preprocessing token shall not occur at the beginning or at the end
of a replacement list for either form of macro definition.
Semantics
If, in the replacement list, a parameter is immediately preceded or
followed by a ## preprocessing token, the parameter is replaced by the
corresponding argument's preprocessing token sequence.
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. 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.
After all parameters in the replacement list have been substituted, the
resulting preprocessing token sequence is rescanned with the rest of the source
file's preprocessing tokens for more macro names to replace.
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.
The resulting completely macro-replaced preprocessing token sequence is
not processed as a preprocessing directive even if it resembles one.
A macro definition lasts (independent of block structure) until a
corresponding #undef directive is encountered or (if none is encountered) until
the end of the translation unit.
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.
Examples
The simplest use of this facility is to define a ``manifest constant,''
as in
#define TABSIZE 100
int table[TABSIZE];
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 of generating more code than a
function if invoked several times.
#define max(a, b) ((a) > (b) ? (a) : (b))
The
parentheses ensure that the arguments and the resulting expression are bound
properly.
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
f(y+1) + f(f(z)) % t(t(g)(0) + t)(1);
g(x+(3,4)-w) | h 5) & m
(f)^m(m);
results
in
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);
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.
And finally, to demonstrate the redefinition rules, the following
sequence is valid.
#define OBJ_LIKE
(1-1)
#define OBJ_LIKE
/* white space */ (1-1) /* other */
#define FTN_LIKE(a) (
a )
#define FTN_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 FTN_LIKE(b) ( a ) /*
different parameter usage */
#define FTN_LIKE(b) ( b ) /*
different parameter spelling */
Constraints
The string literal of a #line directive, if present, shall be a character
string literal.
Semantics
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 ($2.1.1.2)
while processing the source file to the current token.
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).
A preprocessing directive of the form
# line digit-sequence " s-char-sequence<opt>"
new-line
sets
the line number similarly and changes the presumed name of the source file to be
the contents of the character string literal.
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.
Semantics
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.
Semantics
A preprocessing directive of the form
# pragma pp-tokens<opt> new-line
causes
the implementation to behave in an implementation-defined manner.
Any pragma that is not recognized by the implementation is ignored.
Semantics
A preprocessing directive of the form
# new-line
has
no effect.
The following macro names shall be defined by the implementation: The
line number of the current source line (a decimal constant).
The presumed name of the source file (a character string literal).
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. 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. the
decimal constant 1./79/
The values of the predefined macros (except for __LINE__ and __FILE__ )
remain constant throughout the translation unit.
None of these macro names, nor the identifier defined , shall be the
subject of a #define or a #undef preprocessing directive.
All predefined macro names shall begin with a leading underscore followed
by an upper-case letter or a second underscore.
Forward
references: the asctime function ($4.12.3.1).
Restriction of the significance of an external name to fewer than 31
characters or to only one case is an obsolescent feature that is a concession to
existing implementations.
Lower-case letters as escape sequences are reserved for future
standardization. Other characters
may be used in extensions.
The placement of a storage-class specifier other than at the beginning of
the declaration specifiers in a declaration is an obsolescent feature.
The use of function declarators with empty parentheses (not
prototype-format parameter type declarators) is an obsolescent feature.
The use of function definitions with separate parameter identifier and
declaration lists (not prototype-format parameter type and identifier
declarators) is an obsolescent feature. is
an obsolescent feature.
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