Language#
This is going to be meat of the book. This chapter is mapped with chapter 6 of specification. This will almost cover entire language. If you are already proficient in any programming language you must read it. However for it might be too involving without much introduction to language. I would recommend to skip and come back later. I could have put this later but then the flow of specification would have been broken.
Notation#
In the syntax notation used in this clause, syntactic categories (nonterminals) are indicated by italic type, and literal words and character set members (terminals) by bold type. A colon (:) following a nonterminal introduces its definition. Alternative definitions are listed on separate lines, except when prefaced by the words “one of”. An optional symbol is indicated by the subscript “opt”, so that
{ \(expression_{opt}\)}
indicates an optional expression enclosed in braces.
When syntactic categories are referred to in the main text, they are not italicized and words are separated by spaces instead of hyphens.
A summary of the language syntax is given in annex A.
Concepts#
Scope of Identifiers#
An identifier can denote an object; a function; a tag or a member of a structure, union, or enumeration; a typedef name; a label name; a macro name; or a macro parameter. The same identifier can denote different entities at different points in the program. A member of an enumeration is called an enumeration constant. Macro names and macro parameters are not considered further here, because prior to the semantic phase of program translation any occurrences of macro names in the source file are replaced by the preprocessing token sequences that constitute their macro definitions.
The interesting part is “The same identifier can denote different entities at different points in the program.”. This can happen if the identifer is in different scopes.
#include <stdio.h>
void f()
{
int f=0;
}
int main()
{
f();
return 0;
}
For each different entity that an identifier designates, the identifier is visible (i.e., can be used) only within a region of program text called its scope. Different entities designated by the same identifier either have different scopes, or are in different name spaces. There are four kinds of scopes: function, file, block, and function prototype. (A function prototype is a declaration of a function that declares the types of its parameters.)
For example; consider following program:
#include <stdio.h>
int i = 25;
void f(int );
void f(int i)
{
printf("%d\n",i); //uses file scope
i = 24;
printf("%d\n",i); //uses function scope
{
i = 27;
printf("%d\n",i); //uses block scope
}
}
int main()
{
f(i);
return 0;
}
and the output is:
25
24
27
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).
For example, consider following program:
#include <stdio.h>
void f()
{
int i=1;
ONE: if(i==2)
goto TWO;
else {
i=2;
goto ONE;
}
TWO: ;
//TWO: goto MAIN; //will cause error
}
int main()
{
f();
//goto MAIN; //will cause error
//MAIN: ; //will cause error
return 0;
}
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 end of the associated block. If the declarator or type specifier that declares the identifier appears within the list of parameter declarations in a function prototype (not part of a function definition), the identifier has function prototype scope, which terminates at the end of the function declarator. If an identifier designates two different entities in the same name space, the scopes might overlap. If so, the scope of one entity (the inner scope) will be a strict subset of the scope of the other entity (the outer scope). Within the inner scope, the identifier designates the entity declared in the inner scope; the entity declared in the outer scope is hidden (and not visible) within the inner scope.
For example, consider the following program:
#include <stdio.h>
void f(int j);
void f(int i)
{
//identifier j not available in this function
{
int i=5;
printf("%d\n",i);
}
//printf("%d", i); not available outside inner scope will cause error
}
int main()
{
f(7);
return 0;
}
and the output is:
5
Unless explicitly stated otherwise, where International Standard uses the term “identifier” to refer to some entity (as opposed to the syntactic construct), it refers to the entity in the relevant name space whose declaration is visible at the point the identifier occurs.
Two identifiers have the same scope if and only if their scopes terminate at the same point.
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.
For example, consider the following program:
#include <stdio.h>
typedef struct STRUCT {
struct STRUCT *s1; //just a pointer so can be delared
struct STRUCT s2; //objct therefore incomplete type error
}S;
int main()
{
return 0;
}
#include <stdio.h>
typedef struct {
int i;
}S; //type S is has file scope
int main()
{
S s; //s has function scope
return 0;
}
As a special case, a type name (which is not a declaration of an identifier) is considered to have a scope that begins just after the place within the type name where the omitted identifier would appear were it not omitted.
Forward references: declarations (Declarations), function calls (Function calls), function definitions (Function definitions), identifiers (Identifiers), name spaces of identifiers (Name Spaces of the Identifiers), macro replacement (Macro Replacement), source file inclusion (Source File Inclusion), statements (Statements and blocks).
Linkages of identifiers#
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. [1] There are three kinds of linkage: external, internal, and none.
In the set of translation units and libraries that constitutes an entire program, each declaration of a particular identifier with external linkage denotes the same object or function. Within one translation unit, each declaration of an identifier with internal linkage denotes the same object or function. Each declaration of an identifier with no linkage denotes a unique entity.
For example, consider the following program:
#include <stdio.h>
int i; //external scope
void f()
{
// int i=0; same problem
int j=0; //can declare j as j has internal scope
}
int main()
{
// int i=0; will give redeclaration error
int j;
f();
return 0;
}
If the declaration of a file scope identifier for an object or a function
contains the storage-class specifier static, the identifier has internal
linkage. [2]
For example, consider the following program:
#include <stdio.h>
static int i; //internal scope not visible outside file
void f()
{
// int i=0; same problem
int j=0; //can declare j as j has internal scope
}
int main()
{
// int i=0; will give redeclaration error
int j;
f();
return 0;
}
For an identifier declared with the storage-class specifier extern in a
scope in which a prior declaration of that identifier is visible, [3] if the
prior declaration specifies internal or external linkage, the linkage of the
identifier at the later declaration is the same as the linkage specified at the
prior declaration. If no prior declaration is visible, or if the prior
declaration specifies no linkage, then the identifier has external linkage.
For example, consider the following programs:
//test.c
#include <stdio.h>
int i; //external scope not visible outside file
int main()
{
printf("%d\n", i);
f();
return 0;
}
//test1.c
#include <stdio.h>
extern int i;
void f()
{
printf("%d\n", i);
}
compile them like gcc test.c test1.c -o out.
The output is:
0
0
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.
#include <stdio.h>
int i; //external linkage can be called from other files
void f() //external linkage can be called from other files
{
//do something here
}
int main()
{
f();
return 0;
}
The following identifiers have no linkage: an identifier declared to be anything other than an object or a function; an identifier declared to be a function parameter; a block scope identifier for an object declared without the storage-class specifier extern.
#include <stdio.h>
void f(int i) //i has no linkage
{
}
int main()
{
{
int i;//no linkage
f(i);
}
return 0;
}
If, within a translation unit, the same identifier appears with both internal and external linkage, the behavior is undefined.
Forward references: declarations (Declarations), expressions (Expressions), external definitions (External definitions), statements (Statements and blocks).
Name Spaces of the Identifiers#
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 [4] 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).
For example, consider the following
#include <stdio.h>
struct S{
int i;
};
union U {
int i;
};
int main()
{
int i = 0;
int END = 0;
int S = 0;
int U = 0;
struct S s;
union U u;
s.i = 7;
u.i = 9;
goto END;
END: ;
return 0;
}
Forward references: enumeration specifiers (Enumeration specifiers), labeled statements (Labeled statements), structure and union specifiers (Structure and union specifiers), structure and union members (Structure and union members), tags (Tags), the goto statement (The goto statement).
Storage duration of objects#
An object has a storage duration that determines its lifetime. There are three storage durations: static, automatic, and allocated. Allocated storage is described in Memory management functions.
The lifetime of an object is the portion of program execution during which storage is guaranteed to be reserved for it. An object exists, has a constant address, [5] and retains its last-stored value throughout its lifetime. [6] If an object is referred to outside of its lifetime, the behavior is undefined. The value of a pointer becomes indeterminate when the object it points to reaches the end of its lifetime.
#include <stdio.h>
#include <stdlib.h>
int main()
{
int *p;
{
int i=4;
p=&i;
printf("%d %d\n", i, *p);
}
//printf("%d\n", i) //will cause error
printf("%d\n", *p); //dangerous because i is dead
return 0;
}
and the output is:
4 4
4
Note that even though 4 has been printed for *p that is because stack has not
been touched. Make some function calls between brace and printf and most
probably 4 will be overwritten.
An object whose identifier is declared without the storage-class specifier
_Thread_local, and either with external or internal linkage or with the
storage-class specifier static, has static storage duration. Its lifetime
is the entire execution of the program and its stored value is initialized only
once, prior to program startup.
An object whose identifier is declared with the storage-class specifier
_Thread_local has thread storage duration. Its lifetime is the entire
execution of the thread for which it is created, and its stored value is
initialized when the thread is started. There is a distinct object per thread,
and use of the declared name in an expression refers to the object associated
with the thread evaluating the expression. The result of attempting to
indirectly access an object with thread storage duration from a thread other
than the one with which the object is associated is implementation-defined.
Such static values like explicit static variables or global variables go in
data segments and exist in program binary after compilation and linking is done.
#include <stdio.h>
void f()
{
static int i=0;
printf("%d\n", i++);
}
int main()
{
f();
f();
f();
return 0;
}
and the output is:
0
1
2
An object whose identifier is declared with no linkage and without the
storage-class specifier static has automatic storage duration, as do some
compound literals. The result of attempting to indirectly access an object with
automatic storage duration from a thread other than the one with which the
object is associated is implementation-defined.
#include <stdio.h>
void f()
{
{
int i=0;
printf("%d\n", i++);
}
}
int main()
{
f();
f();
f();
return 0;
}
and the output is:
0
0
0
For such an object that does not have a variable length array type, its lifetime extends from entry into the block with which it is associated until execution of that block ends in any way. (Entering an enclosed block or calling a function suspends, but does not end, execution of the current block.) If the block is entered recursively, a new instance of the object is created each time. The initial value of the object is indeterminate. If an initialization is specified for the object, it is performed each time the declaration is reached in the execution of the block; otherwise, the value becomes indeterminate each time the declaration is reached.
#include <stdio.h>
void f()
{
int i=0;
static int n=0;
if(n<4)
printf("%d\n", i++);
n++;
f();
}
int main()
{
f();
return 0;
}
and the output is:
0
0
0
0
For such an object that does have a variable length array type, its lifetime extends from the declaration of the object until execution of the program leaves the scope of the declaration. [7] If the scope is entered recursively, a new instance of the object is created each time. The initial value of the object is indeterminate.
A non-lvalue expression with structure or union type, where the structure or union contains a member with array type (including, recursively, members of all contained structures and unions) refers to an object with automatic storage duration and temporary lifetime. [8] Its lifetime begins when the expression is evaluated and its initial value is the value of the expression. Its lifetime ends when the evaluation of the containing full expression or full declarator ends. Any attempt to modify an object with temporary lifetime results in undefined behavior.
Forward references: statements (Statements and blocks), function calls (Function calls), declarators (Declarators), array declarators (Array declarators), initialization (Initialization).
Types#
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 fully describe objects) and function types (types that describe functions). At various points within a translation unit an object type may be incomplete (lacking sufficient information to determine the size of objects of that type) or complete (having sufficient information). [9]
An object declared as type _Bool is large enough to store the values 0 and
1.
#include <stdio.h>
int main()
{
_Bool b=23;
printf("%d %d\n", b, sizeof(_Bool));
return 0;
}
and the output is:
1 1
Note that size of 1 byte which means whatever nonzero value we assign will be converted to 1 implicitly.
An object declared as type char is large enough to store any member of the
basic execution character set. If a member of the basic execution character set
is stored in a char object, its value is guaranteed to be nonnegative. If
any other character is stored in a char object, the resulting value is
implementation-defined but shall be within the range of values that can be
represented in that type.
First let us see how big is char:
#include <stdio.h>
int main()
{
printf("%d\n", sizeof(char));
return 0;
}
and the output is 1. Now let us what we can store in it.
#include <stdio.h>
#include <conio.h>
int main()
{
for(register int i=0; i<256; i++)
printf("%c\t", i);
return 0;
}
It will show lots of non-printable characters as well. Run it as ./a.out|od -c
and maximum possible characters will be shown.
There are five standard signed integer types, designated as``signed char,
short int, int, long int`` and long long int. (These and other types may be
designated in several additional ways, as described in Type specifiers.) There may also be
implementation-defined extended signed integer types. [10] The standard and
extended signed integer types are collectively called signed integer types. [11]
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>).
#include <stdio.h>
#unclude <limits.h>
int main()
{
printf("%d %d", INT_MIN, INT_MAX);
return 0;
}
and the output is:
-2147483648 2147483647
which is much much greater than -127 and 128 possible for out 1 byte
character.
For each of the signed integer types, there is a corresponding (but different)
unsigned integer type (designated with the keyword unsigned) that uses the
same amount of storage (including sign information) and has the same alignment
requirements. The type _Bool and the unsigned integer types that
correspond to the standard signed integer types are the standard unsigned
integer types. The unsigned integer types that correspond to the extended signed
integer types are the extended unsigned integer types. The standard and
extended unsigned integer types are collectively called unsigned integer types.
[12]
The standard signed integer types and standard unsigned integer types are collectively called the standard integer types, the extended signed integer types and extended unsigned integer types are collectively called the extended integer types.
For any two integer types with the same signedness and different integer conversion rank (see Booleans, Characters and Integers), the range of values of the type with smaller integer conversion rank is a subrange of the values of the other type.
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. [13] 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 type.
There are three real floating types, designated as float, double and
long double. The set of values of the type float is a subset of the
set of values of the type double; the set of values of the type double
is a subset of the set of values of the type long double.
There are three complex types, designated as float _Complex, double
_Complex, and long double _Complex. The real floating and complex
types are collectively called the floating types.
For each floating type there is a corresponding real type, which is always a
real floating type. For real floating types, it is the same type. For complex
types, it is the type given by deleting the keyword _Complex from the type
name.
Each complex type has the same representation and alignment requirements as an array type containing exactly two elements of the corresponding real type; the first element is equal to the real part, and the second element to the imaginary part, of the complex number.
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. [14]
The three types char, signed char and unsigned char are collectively
called the character types. The implementation shall define char to have
the same range, representation, and behavior as either signed char or
unsigned char. [15]
An enumeration comprises a set of named integer constant values. Each distinct enumeration constitutes a different enumerated type.
The type char, the signed and unsigned integer types, and the enumerated types are collectively called integer types. The integer and real floating types are collectively called real types.
Integer and floating types are collectively called arithmetic types. Each arithmetic type belongs to one type domain: the real type domain comprises the real types, the complex type domain comprises the complex types.
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 object, function, and incomplete types, as follows:
An array type describes a contiguously allocated nonempty set of objects with a particular member object type, called the element type. [16] Array types are characterized by their element type and by the number of elements in the array. An array type is said to be derived from its element type, and if its element type is T, the array type is sometimes called “array of T”. The construction of an array type from an element type is called “array type derivation”.
A structure type describes a sequentially allocated nonempty set of member objects (and, in certain circumstances, an incomplete array), each of which has an optionally specified name and possibly distinct type.
A union type describes an overlapping nonempty set of member objects, each of which has an optionally specified name and possibly distinct type.
A function type describes a function with specified return type. A function type is characterized by its return type and the number and types of its parameters. A function type is said to be derived from its return type, and if its return type is T, the function type is sometimes called “function returning T”. The 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”.
An atomic type describes the type designated by the construct
_Atomic( type-name ). (Atomic types are a conditional feature that implementations need not support; see :math:`S(text{6.10.8.3.})
These methods of constructing derived types can be applied recursively. What this mean is you can have structures of structrues of structures and so on. Similarly, you can have array of array of array and so on. And you can have array of array of array of such structures of structures of structures … .
Arithmetic types and pointer types are collectively called scalar types. Array and structure types are collectively called aggregate types. [17]
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 Tags) 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.
#include <stdio.h>
S s[10]; //incomplete type at this moment
typedef struct {
int i;
}S; //type completed here
int main()
{
return 0;
}
A type has known constant size if the type is not incomplete and is not a variable length array type.
Array, function, and pointer types are collectively called derived declarator types. A declarator type derivation from a type T is the construction of a derived declarator type from T by the application of an array-type, a function-type, or a pointer-type derivation to T.
#include <stdio.h>
S s[10];
S f()
{
S s1;
return s1;
}
S (*g)()
{
S s2;
return s2;
}
typedef struct {
int i;
}S;
int main()
{
return 0;
}
A type is characterized by its type category, which is either the outermost derivation of a derived type (as noted above in the construction of derived type), or the type itself if the type consists of no derived types.
Any type so far mentioned is an unqualified type. Each unqualified type has several qualified versions of its type, [18] corresponding to the combinations of one, two, or all three of the const, volatile and restrict qualifiers. The qualified or unqualified versions of a type are distinct types that belong to the same type category and have the same representation and alignment requirements. [19] A derived type is not qualified by the qualifiers (if any) of the type from which it is derived.
Further, there is the _Atomic qualifier. The presence of the _Atomic
qualifier designates an atomic type. The size, representation, and alignment of
an atomic type need not be the same as those of the corresponding unqualified
type. Therefore, this Standard explicitly uses the phrase “atomic, qualified
or unqualified type” whenever the atomic version of a type is permitted along
with the other qualified versions of a type. The phrase “qualified or
unqualified type”, without specific mention of atomic, does not include the
atomic types.
A pointer to void shall have the same representation and alignment
requirements as a pointer to a character type. [17] Similarly, pointers to
qualified or unqualified versions of compatible types shall have the same
representation and alignment requirements. All pointers to structure types shall
have the same representation and alignment requirements as each other. All
pointers to union types shall have the same representation and alignment
requirements as each other. Pointers to other types need not have the same
representation or alignment requirements.
Representation of Types#
General#
The representations of all types are unspecified except as stated in this subclause.
Except for bit-fields, objects are composed of contiguous sequences of one or more bytes, the number, order, and encoding of which are either explicitly specified or implementation-defined.
Values stored in unsigned bit-fields and objects of type unsigned char shall be represented using a pure binary notation. [20]
Values stored in non-bit-field objects of any other object type consist of n
x CHAR_BIT bits, where n is the size of an object of that type, in bytes.
The value may be copied into an object of type unsigned char [n] (e.g., by
memcpy); the resulting set of bytes is called the object representation of the
value. Values stored in bit-fields consist of m bits, where m is the size
specified for the bit-field. The object representation is the set of m bits
the bit-field comprises in the addressable storage unit holding it. Two values
(other than NaNs) with the same object representation compare equal, but values
that compare equal may have different object representations.
#include <stdio.h>
typedef struct {
char c:7;
int i:3;
}S;
int main()
{
S s;
s.c = '0';
s.i = 5;
printf("%c %d\n", s.c, s.i);
return 0;
}
and the output is:
0 -3
Consider the following program:
#include <stdio.h>
typedef struct {
int i:1;
}S;
int main()
{
S s;
s.i = 1;
printf("%d\n", s.i);
return 0;
}
what do you think the output will be? 0 to 1 that is what my gut feeling tells
me but I am wrong. With N bits the range of integer is \(-2^N - 1\) to
\(2^{N-1} - -1\). This evaluates to -1 to 0. So any even no. stored in 1 bits
field is 0 and any odd no. is -1. Useful for finding odd and even numbers eh?
The output is:
-1
Certain object representations need not represent a value of the object type. If the stored value of an object has such a representation and is read by an lvalue expression that does not have character type, the behavior is undefined. If such a representation is produced by a side effect that modifies all or any part of the object by an lvalue expression that does not have character type, the behavior is undefined. [21] Such a representation is called a trap representation.
When a value is stored in an object of structure or union type, including in a member object, the bytes of the object representation that correspond to any padding bytes take unspecified values. [22] The value of a structure or union object is never a trap representation, even though the value of a member of the structure or union object may be a trap representation.
When a value is stored in a member of an object of union type, the bytes of the
object representation that do not correspond to that member but do correspond to
other members take unspecified values. What this means is that say you have
two members where one is of m bytes and another of n bytes and m>n.
Then if you assign value to one having n bytes then m - n bytes will
have unspecified value.
Where an operator is applied to a value that has more than one object representation, which object representation is used shall not affect the value of the result. [23] Where a value is stored in an object using a type that has more than one object representation for that value, it is unspecified which representation is used, but a trap representation shall not be generated.
Loads and stores of objects with atomic types are done with
memory_order_seq_cst semantics.
For example, char is integral type. Therefore all operations of integers
can be done on characters.
Forward references: declarations (Declarations), expressions (Expressions), lvalues, arrays, and function designators (Other Operands).
Integer Types#
For unsigned integer types other than unsigned char, the bits of the object
representation shall be divided into two groups: value bits and padding bits
(there need not be any of the latter). If there are N value bits, each bit
shall represent a different power of 2 between 1 and \(2^{N-1}\), so that
objects of that type shall be capable of representing values from 0 to
\(2^N - 1\) using a pure binary representation; this shall be known as the
value representation. The values of any padding bits are unspecified. [24]
For signed integer types, the bits of the object representation shall be divided into three groups: value bits, padding bits, and the sign bit. There need not be any padding bits; there shall be exactly one sign bit. Each bit that is a value bit shall have the same value as the same bit in the object representation of the corresponding unsigned type (if there are M value bits in the signed type and N in the unsigned type, then M <= N). If the sign bit is zero, it shall not affect the resulting value. If the sign bit is one, the value shall be modified in one of the following ways:
the corresponding value with sign bit 0 is negated (sign and magnitude);
the sign bit has the value -(\(2^N\)) (two’s complement);
the sign bit has the value -(\(2^N - 1\)) (ones’ complement).
Which of these applies is implementation-defined, as is whether the value with sign bit 1 and all value bits zero (for the first two), or with sign bit and all value bits 1 (for ones’ complement), is a trap representation or a normal value. In the case of sign and magnitude and one’’ complement, if this representation is a normal value it is called a negative zero.
If the implementation supports negative zeros, they shall be generated only by:
the &, |, ^, ~, <<, and >> operators with arguments that produce such a value;
the +, -, *, /, and % operators where one argument is a negative zero and the result is zero;
compound assignment operators based on the above cases.
It is unspecified whether these cases actually generate a negative zero or a normal zero, and whether a negative zero becomes a normal zero when stored in an object.
If the implementation does not support negative zeros, the behavior of the &, |, ^, ~, <<, and >> operators with arguments that would produce such a value is undefined.
The values of any padding bits are unspecified. [25] A valid (non-trap) object representation of a signed integer type where the sign bit is zero is a valid object representation of the corresponding unsigned type, and shall represent the same value. For any integer type, the object representation where all the bits are zero shall be a representation of the value zero in that type.
The precision of an integer type is the number of bits it uses to represent values, excluding any sign and padding bits. The width of an integer type is the same but including any sign bit; thus for unsigned integer types the two values are the same, while for signed integer types the width is one greater than the precision.
Compatible and Composite Types#
Two types have compatible type if their types are the same. Additional rules for determining whether two types are compatible are described in Type specifiers for type specifiers, in Type qualifiers for type qualifiers, and in Declarators for declarators. [26] Moreover, two structure, union, or enumerated types declared in separate translation units are compatible if their tags and members satisfy the following requirements: If one is declared with a tag, the other shall be declared with the same tag. If both are complete types, then the following additional requirements apply: there shall be a one-to-one correspondence between their members such that each pair of corresponding members are declared with compatible types, and such that if one member of a corresponding pair is declared with a name, the other member is declared with the same name. For two structures, corresponding members shall be declared in the same order. For two structures or unions, corresponding bit-fields shall have the same widths. For two enumerations, corresponding members shall have the same values.
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 satisfies the following conditions:
If one type is an array of known constant size, the composite type is an array of that size; otherwise, if one type is a variable length array, the composite type is that type.
If only one type is a function type with a parameter type list (a function prototype), the composite type is a function prototype with the parameter type list.
If both types are function types with parameter type lists, the type of each parameter in the composite parameter type list is the composite type of the corresponding parameters.
These rules apply recursively to the types from which the two types are derived.
For an identifier with internal or external linkage declared in a scope in which a prior declaration of that identifier is visible, [27] if the prior declaration specifies internal or external linkage, the type of the identifier at the later declaration becomes the composite type.
Alignment of Objects#
Complete object types have alignment requirements which place restrictions on
the addresses at which objects of that type may be allocated. An alignment is
an implementation-defined integer value representing the number of bytes
between successive addresses at which a given object can be allocated. An
object type imposes an alignment requirement on every object of that type:
stricter alignment can be requested using the _Alignas keyword.
A fundamental alignment is represented by an alignment less than or equal to
the greatest alignment supported by the implementation in all contexts, which
is equal to _Alignof (max_align_t).
An extended alignment is represented by an alignment greater than _Alignof
(max_align_t). It is implementation-defined whether any extended alignments
are supported and the contexts in which they are supported. A type having an
extended alignment requirement is an over-aligned type. [28]
Alignments are represented as values of the type size_t. Valid alignments
include only those values returned by an _Alignof expression for
fundamental types, plus an additional implementation-defined set of values,
which may be empty. Every valid alignment value shall be a nonnegative integral
power of two.
Alignments have an order from weaker to stronger or stricter alignments. Stricter alignments have larger alignment values. An address that satisfies an alignment requirement also satisfies any weaker valid alignment requirement.
The alignment requirement of a complete type can be queried using an
_Alignof expression. The types char, signed char and unsigned char
shall have the weakest alignment requirement.
Comparing alignments is meaningful and provides the obvious results:
Two alignments are equal when their numeric values are equal.
Two alignments are different when their numeric values are not equal.
When an alignment is larger than another it represents a stricter alignment.
Conversions#
Several operators convert operand values from one type to another automatically. This subclause specifies the result required from such an implicit conversion, as well as those that result from a cast operation (an explicit conversion). The list in 4.3.1.8 summarizes the conversions performed by most ordinary operators; it is supplemented as required by the discussion of each operator in 4.5.
Conversion of an operand value to a compatible type causes no change to the value or the representation.
Forward references: cast operators (Cast operators).
Arithmetic Operands#
Booleans, Characters and Integers#
Every integer type has an integer conversion rank defined as follows:
No two signed integer types shall have the same rank, even if they have the same representation.
The rank of a signed integer type shall be greater than the rank of any signed integer type with less precision.
The rank of
long long intshall be greater than the rank oflong int, which shall be greater than the rank ofint, which shall be greater than the rank of short int, which shall be greater than the rank ofsigned char.The rank of any unsigned integer type shall equal the rank of the corresponding signed integer type, if any.
The rank of any standard integer type shall be greater than the rank of any extended integer type with the same width.
The rank of
charshall equal the rank ofsigned charandunsigned char.The rank of
_Boolshall be less than the rank of all other standard integer types.The rank of any enumerated type shall equal the rank of the compatible integer type (see Enumeration specifiers).
The rank of any extended signed integer type relative to another extended signed integer type with the same precision is implementation-defined, but still subject to the other rules for determining the integer conversion rank.
For all integer types
T1, T2,andT3, ifT1has greater rank thanT2andT2has greater rank thanT3, thenT1has greater rank thanT3.
The following may be used in an expression wherever an int or unsigned int may be used:
An object or expression with an integer type whose integer conversion rank is less than or equal to the rank of
intandunsigned int.A bit-field of type
_Bool, int, signed int,orunsigned int.
If an int can represent all values of the original type, the value is
converted to an int; otherwise, it is converted to an unsigned int. These
are called the integer promotions. [29] All other types are unchanged by the
integer promotions.
The integer promotions preserve value including sign. As discussed earlier,
whether a “plain” char is treated as signed is implementation-defined.
Forward references: enumeration specifiers (Enumeration specifiers), structure and union specifiers (Structure and union specifiers).
Boolean Type#
When any scalar value is converted to _Bool, the result is 0 if the value
compares equal to 0; otherwise, the result is 1.
Signed and Unsigned Integers#
When a value with integer type is converted to another integer type other than
_Bool, if the value can be represented by the new type, it is unchanged.
Otherwise, if the new type is unsigned, the value is converted by repeatedly adding or subtracting one more than the maximum value that can be represented in the new type until the value is in the range of the new type. [30]
Otherwise, the new type is signed and the value cannot be represented in it; either the result is implementation-defined or an implementation-defined signal is raised.
Real Floating and Integer#
When a finite value of real floating type is converted to an integer type other
than _Bool, the fractional part is discarded (i.e., the value is truncated
toward zero). If the value of the integral part cannot be represented by the
integer type, the behavior is undefined. [31]
When a value of integer type is converted to a real floating type, if the value being converted can be represented exactly in the new type, it is unchanged. 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 representable value, chosen in an implementation-defined manner. If the value being converted is outside the range of values that can be represented, the behavior is undefined.
Real Floating Types#
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, a long double is demoted to
double or float, or a value being represented in greater precision and
range than required by its semantic type (see Usual Arithmetic Conversions) is explicitly converted to
its semantic type, if the value being converted can be represented exactly in the
new type, it is unchanged. 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 representable value, chosen in an
implementation-defined manner. If the value being converted is outside the range
of values that can be represented, the behavior is undefined.
Complex Types#
When a value of complex type is converted to another complex type, both the real and imaginary parts follow the conversion rules for the corresponding real types.
Real and Complex#
When a value of real type is converted to a complex type, the real part of the complex result value is determined by the rules of conversion to the corresponding real type and the imaginary part of the complex result value is a positive zero or an unsigned zero.
When a value of complex type is converted to a real type, the imaginary part of the complex value is discarded and the value of the real part is converted according to the conversion rules for the corresponding real type.
Usual Arithmetic Conversions#
Many operators that expect operands of arithmetic type cause conversions and yield result types in a similar way. The purpose is to determine a common real type for the operands and result. For the specified operands, each operand is converted, without change of type domain, to a type whose corresponding real type is the common real type. Unless explicitly stated otherwise, the common real type is also the corresponding real type of the result, whose type domain is the type domain of the operands if they are the same, and complex otherwise. This pattern is called the usual arithmetic conversions:
First, if the corresponding real type of either operand is
long double, the other operand is converted, without change of type domain, to a type whose corresponding real type is long double.Otherwise, if the corresponding real type of either operand is double, the other operand is converted, without change of type domain, to a type whose corresponding real type is double.
Otherwise, if the corresponding real type of either operand is float, the other operand is converted, without change of type domain, to a type whose corresponding real type is float. [32]
Otherwise, the integer promotions are performed on both operands. Then the following rules are applied to the promoted operands:
If both operands have the same type, then no further conversion is needed.
Otherwise, if both operands have signed integer types or both have unsigned integer types, the operand with the type of lesser integer conversion rank is converted to the type of the operand with greater rank.
Otherwise, if the operand that has unsigned integer type has rank greater or equal to the rank of the type of the other operand, then the operand with signed integer type is converted to the type of the operand with unsigned integer type.
Otherwise, if the type of the operand with signed integer type can represent all of the values of the type of the operand with unsigned integer type, then the operand with unsigned integer type is converted to the type of the operand with signed integer type.
Otherwise, both operands are converted to the unsigned integer type corresponding to the type of the operand with signed integer type.
The values of floating operands and of the results of floating expressions may be represented in greater precision and range than that required by the type; the types are not changed thereby. [33]
Other Operands#
Lvalues, Arrays and Function Designators#
An lvalue is an expression with an object type or an incomplete type other
than void; [34] if an lvalue does not designate an object when it is
evaluated,
the behavior is undefined. When an object is said to have a particular type, the
type is specified by the lvalue used to designate the object. A modifiable
lvalue is an lvalue that does not have array type, does not have an incomplete
type, does not have a const-qualified type, and if it is a structure or union,
does not have any member (including, recursively, any member or element of
all contained aggregates or unions) with a const-qualified type.
Except when it is the operand of the sizeof operator, the _Alignof
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), this is
called lvalue conversion. 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.
If the lvalue designates an object of automatic storage duration that could
have been declared with the register storage class (never had its address
taken), and that object is uninitialized (not declared with an initializer and
no assignment to it has been performed prior to use), the behavior is undefined.
Except when it is the operand of the sizeof operator, the _Alignof operator
or the unary & operator, or
is a string literal used to initialize an array, an expression that has type
“array of type” is converted to an expression with type “pointer to type” that
points to the initial element of the array object and is not an lvalue. If the
array object has register storage class, the behavior is undefined.
A function designator is an expression that has function type. Except when it
is the operand of the sizeof operator, the _Alignof operator [35] 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”.
For example, consider following:
#include <stdio.h>
void f()
{
printf("Hello\n");
}
int main()
{
f();
(*f)();
(&f)();
(**f)();
(***f)();
return 0;
}
and the output is:
Hello
Hello
Hello
Hello
Hello
Forward references: address and indirection operators (Address and indirection operators), assignment
operators (Assignment operators), common definitions <stddef.h> (Common definitions <stddef.h>), initialization
(Initialization), postfix increment and decrement operators (Postfix increment and decrement operators), prefix increment
and decrement operators (Prefix increment and decrement operators), the sizeof operator (The sizeof operator), structure and
union members (Structure and union members).
void#
The (nonexistent) value of a void expression (an expression that has type
void) shall not be used in any way, and implicit or explicit conversions
(except to void) shall not be applied to such an expression. If an
expression of any other type is evaluated as a void expression, its value or
designator is discarded. (A void expression is evaluated for its side effects.)
Pointers#
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.
For any qualifier q, a pointer to a non-q-qualified type may be converted to a pointer to the q-qualified version of the type; the values stored in the original and converted pointers shall compare equal.
An integer constant expression with the value 0, or such an expression cast to
type void *, is called a null pointer constant. [36] If a null pointer
constant is converted to a pointer type, the resulting pointer, called a null
pointer, is guaranteed to compare unequal to a pointer to any object or
function.
Conversion of a null pointer to another pointer type yields a null pointer of that type. Any two null pointers shall compare equal.
An integer may be converted to any pointer type. Except as previously specified, the result is implementation-defined, might not be correctly aligned, might not point to an entity of the referenced type, and might be a trap representation. [37]
Any pointer type may be converted to an integer type. Except as previously specified, the result is implementation-defined. If the result cannot be represented in the integer type, the behavior is undefined. The result need not be in the range of values of any integer type.
A pointer to an object or incomplete type may be converted to a pointer to a different object or incomplete type. If the resulting pointer is not correctly aligned [38] for the pointed-to type, the behavior is undefined. Otherwise, when converted back again, the result shall compare equal to the original pointer. When a pointer to an object is converted to a pointer to a character type, the result points to the lowest addressed byte of the object. Successive increments of the result, up to the size of the object, yield pointers to the remaining bytes of the object.
A pointer to a function of one type may be converted to a pointer to a function of another type and back again; the result shall compare equal to the original pointer. If a converted pointer is used to call a function whose type is not compatible with the pointed-to type, the behavior is undefined.
Forward references: cast operators (Cast operators), equality operators (Equality operators), integer types capable of holding object pointers (Integer types capable of holding object pointers), simple assignment (Simple assignment).
Lexical Elements#
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, 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, and punctuators. A preprocessing token is the minimal lexical element of the language in translation phases 3 through 6. The categories of preprocessing tokens are: header names, identifiers, preprocessing numbers, character constants, string literals, punctuators, and single non-white-space characters that do not lexically match the other preprocessing token categories. [39] If a ‘ or a ” character matches the last category, the behavior is undefined. Preprocessing tokens can be separated by white space; this consists of comments (described later), or white-space characters (space, horizontal tab, new-line, vertical tab, and form-feed), or both. As described in Preprocessing Directives, 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. There is one exception to this rule: a header name preprocessing token is only recognized within a #include preprocessing directive, and within such a directive, a sequence of characters that could be either a header name or a string literal is recognized as the former.
EXAMPLE 1 The program fragment 1Ex is parsed as a preprocessing number token
(one that is not a valid floating or integer constant token), even though a
parse as the pair of preprocessing tokens 1 and Ex might produce a
valid expression (for example, if Ex were a macro defined as +1).
Similarly, the program fragment 1E1 is parsed as a preprocessing number
(one that is a valid floating constant token), whether or not E is a
macro name.
EXAMPLE 2 The program fragment x+++++y is parsed as x ++ ++ + y,
which violates a constraint on increment operators, even though the parse
x ++ + ++ y might yield a correct expression.
Forward references: character constants (Character constants), comments (Comments), expressions (Expressions), floating constants (Floating Constants), header names (Header names), macro replacement (Macro Replacement), postfix increment and decrement operators (Postfix increment and decrement operators), prefix increment and decrement operators (Prefix increment and decrement operators), preprocessing directives (Preprocessing Directives), preprocessing numbers (Preprocessing numbers), string literals (String literals).
Keywords#
List of keywords are given in Environmental considerations
The keywords token (case sensitive) are reserved (in translation phases 7 and
8) for use as keywords, and shall not be used otherwise. The keyword
_Imaginary is reserved for specifying imaginary types. [40]
Identifiers#
General#
The characterset for ideantifiers is given in The C Character Set.
Semantics
An identifier is a sequence of nondigit characters (including the underscore
_, the lowercase and uppercase Latin letters, and other characters) and
digits, which designates one or more entities as described in Scope of Identifiers.
Lowercase and uppercase letters are distinct. There is no specific limit
on the maximum length of an identifier
Each universal character name in an identifier shall designate a character whose encoding in ISO/IEC 10646 falls into one of the ranges specified in annex D. [41] The initial character shall not be a universal character name designating a digit. An implementation may allow multibyte characters that are not part of the basic source character set to appear in identifiers; which characters and their correspondence to universal character names is implementation-defined.
When preprocessing tokens are converted to tokens during translation phase 7, if a preprocessing token could be converted to either a keyword or an identifier, it is converted to a keyword.
Implementation limits
As discussed in Translation limits, an implementation may limit the number of significant initial characters in an identifier; the limit for an external name (an identifier that has external linkage) may be more restrictive than that for an internal name (a macro name or an identifier that does not have external linkage). The number of significant characters in an identifier is implementation-defined.
Any identifiers that differ in a significant character are different identifiers. If two identifiers differ only in nonsignificant characters, the behavior is undefined.
Forward references: universal character names (Universal character names), macro replacement (Macro Replacement).
Predefined identifiers#
Semantics
The identifier “__func__” shall be implicitly declared by the translator as if, immediately following the opening brace of each function definition, the declaration
static const char __func__[] = "function-name";
appeared, where function-name is the name of the lexically-enclosing function. [42]
This name is encoded as if the implicit declaration had been written in the source character set and then translated into the execution character set as indicated in translation phase 5.
EXAMPLE Consider the code fragment:
#include <stdio.h>
void myfunc(void)
{
printf("%s\n", __func__);
/* ... */
}
Each time the function is called, it will print to the standard output stream:
myfunc
Forward references: function definitions (Function definitions).
Universal character names#
Constraints
A universal character name shall not specify a character whose short identifier is less than 00A0 other than 0024 ($), 0040 (@), or 0060 (‘), nor one in the range D800 through DFFF inclusive. [43]
Description
Universal character names may be used in identifiers, character constants, and string literals to designate characters that are not in the basic character set.
Semantics
The universal character name \Unnnnnnnn designates the character whose eight-digit short identifier (as specified by ISO/IEC 10646) is nnnnnnnn. [44] Similarly, the universal character name \unnnn designates the character whose four-digit short identifier is nnnn (and whose eight-digit short identifier is 0000nnnn).
Constants#
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.
Integer Constants#
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.
Suffix |
Decimal Constant |
Octal or Hexadecimal Constant |
|---|---|---|
none |
|
|
|
|
|
|
|
|
Both and |
|
|
|
|
|
Both and |
|
|
If an integer constant cannot be represented by any type in its list, it may have an extended integer type, if the extended integer type can represent its value. If all of the types in the list for the constant are signed, the extended integer type shall be signed. If all of the types in the list for the constant are unsigned, the extended integer type shall be unsigned. If the list contains both signed and unsigned types, the extended integer type may be signed or unsigned.
Floating Constants#
Description
A floating constant has a significand part that may be followed by an
exponent part and a suffix that specifies its type. The components of the
significand part may include a digit sequence representing the whole-number
part, followed by a period (.), followed by a digit sequence representing
the fraction part. The components of the exponent part are an e, E, p
or P followed by an exponent consisting of an optionally signed digit
sequence. Either the whole-number part or the fraction part has to be
present; for decimal floating constants, either the period or the exponent
part has to be present.
Semantics
The significand part is interpreted as a (decimal or hexadecimal) rational number; the
digit sequence in the exponent part is interpreted as a decimal integer. For decimal
floating constants, the exponent indicates the power of 10 by which the significand part is
to be scaled. For hexadecimal floating constants, the exponent indicates the power of 2
by which the significand part is to be scaled. For decimal floating constants, and also for
hexadecimal floating constants when FLT_RADIX is not a power of 2, the result is either
the nearest representable value, or the larger or smaller representable value immediately
adjacent to the nearest representable value, chosen in an implementation-defined manner.
For hexadecimal floating constants when FLT_RADIX is a power of 2, the result is
correctly rounded.
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.
Floating constants are converted to internal format as if at translation-time. The conversion of a floating constant shall not raise an exceptional condition or a floating- point exception at execution time.
Recommended practice
The implementation should produce a diagnostic message if a hexadecimal constant cannot be represented exactly in its evaluation format; the implementation should then proceed with the translation of the program.
The translation-time conversion of floating constants should match the execution-time conversion of character strings by library functions, such as strtod, given matching inputs suitable for both conversions, the same result format, and default execution-time rounding. [45]
Enumeration constants#
Semantics
An identifier declared as an enumeration constant has type int.
Forward references: enumeration specifiers (Enumeration specifiers).
Character constants#
Description
An integer character constant is a sequence of one or more multibyte characters enclosed
in single-quotes, as in 'x'. 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 integer values are representable according to the following table of escape sequences:
single quote ' \\'double quote " \\"question mark ? \\?backslash \\ \\\\octal character \\octal digitshexadecimal character \\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 or wide 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 or wide character.
Each octal or hexadecimal escape sequence is the longest sequence of characters that can constitute the escape sequence.
In addition, characters not in the basic character set are representable by universal character names and certain nongraphic characters are representable by escape sequences consisting of the backslash \ followed by a lowercase letter: \a, \b, \f, \n, \r, \t, and \v. [46]
Constraints
The value of an octal or hexadecimal escape sequence shall be in the range of
representable values for the type unsigned char for an integer character constant, or
the unsigned type corresponding to wchar_t for a wide character constant.
Semantics
An integer character constant has type int. The value of an integer character constant
containing a single character that maps to a single-byte execution character 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 (e.g.,
'ab'), or containing a character or escape sequence that does not map to a single-byte
execution character, is implementation-defined. If an integer character constant contains
a single character or escape sequence, its value is the one that results when an object with
type char whose value is that of the single character or escape sequence is converted to
type int.
A wide character constant has type wchar_t, an integer type defined in the
<stddef.h> header. The value of a wide character constant containing a single
multibyte character that maps to a member of the extended execution character set is the
wide character 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.
EXAMPLE 1 The construction '\0' is commonly used to represent the null character.
EXAMPLE 2 Consider implementations that use two’s-complement representation
for integers and eight bits for objects that have type char. In an
implementation in which type char has the same range of values as
signed char, the integer character constant '\xFF' has the value -1;
if type char has the same range of values as unsigned char, the character
constant '\xFF' has the value +255.
EXAMPLE 3 Even if eight bits are used for objects that have type char, the
construction '\x123' specifies an integer character constant containing
only one character, since a hexadecimal escape sequence is terminated only
by a non-hexadecimal character. To specify an integer character constant
containing the two characters whose values are '\x12' and '3', the
construction '\0223' may be used, since an octal escape sequence is
terminated after three octal digits. (The value of this two-character
integer character constant is implementation-defined.)
EXAMPLE 4 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: common definitions <stddef.h> (Common definitions <stddef.h>), the
mbtowc function (The mbtowc function).
String literals#
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
In translation phase 6, the multibyte character sequences specified by any sequence of adjacent character and wide string literal tokens are concatenated into a single multibyte character sequence. If any of the tokens are wide string literal tokens, the resulting multibyte character sequence is treated as a wide string literal; otherwise, it is treated as a character string literal.
In translation phase 7, a byte or code of value zero is appended to each multibyte
character sequence that results from a string literal or literals.66) The multibyte
character sequence is then used to initialize an array of static storage duration
and length just sufficient to contain the sequence. For character string literals,
the array elements have type char, and are initialized with the individual
bytes of the multibyte character sequence; for wide string literals, the array
elements have type wchar_t, and are initialized with the sequence of wide
characters corresponding to the multibyte character sequence, as defined by the
mbstowcs function with an implementation-defined current locale. The value
of a string literal containing a multibyte character or escape sequence
not represented in the execution character set is implementation-defined.
It is unspecified whether these arrays are distinct provided their elements have the appropriate values. If the program attempts to modify such an array, the behavior is undefined.
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> (Common definitions <stddef.h>), the mbstowcs
function (The mbstowcs function).
Punctuators#
These are one of:
[ ] ( ) { } . -> ++ -- & * + - ~ ! / % << >> < > <= > ? : ; ... = *= /=
%= += -= <<= , # ## <: :> <% %> %: %:%: == >>= != &= ^ | ^= && || |=
Semantics
A punctuator is a symbol that has independent syntactic and semantic significance. Depending on context, it may specify an operation to be performed (which in turn may yield a value or a function designator, produce a side effect, or some combination thereof) in which case it is known as an operator (other forms of operator also exist in some contexts). An operand is an entity on which an operator acts.
In all aspects of the language, the six tokens [47]
<: :> <% %> %: %:%:
behave, respectively, the same as the six tokens:
[ ] { } # ##
except for their spelling. [48]
Forward references: expressions (Expressions), declarations (Declarations), preprocessing directives (Preprocessing Directives), statements (Statements and blocks).
Header names#
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 Source File Inclusion.
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. [49] A header name preprocessing token
is recognized only within a #include preprocessing directive.
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 (Source File Inclusion).
Preprocessing numbers#
Description
A preprocessing number begins with a digit optionally preceded by a period (.) and may
be followed by valid identifier characters and the character sequences e+, e-, E+, E-,
p+, p-, P+ or P-.
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.
Expressions#
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 read only to determine the value to be stored. [51]
The grouping of operators and operands is indicated by the syntax. [52] Except as specified
later (for the function-call (), &&, ||, ?: and comma operators), the order of evaluation
of subexpressions and the order in which side effects take place are both unspecified.
Some operators (the unary operator ~, and the binary operators <<, >>, &, ^ and |,
collectively described as bitwise operators) are required to have operands that have
integer type. These operators yield values that depend on the internal representations of
integers, and have implementation-defined and undefined aspects for signed types.
If an exceptional condition occurs during the evaluation of an expression (that is, if the result is not mathematically defined or not in the range of representable values for its type), the behavior is undefined.
The effective type of an object for an access to its stored value is the declared type of the
object, if any. [53] If a value is stored into an object having no declared type through an
lvalue having a type that is not a character type, then the type of the lvalue becomes the
effective type of the object for that access and for subsequent accesses that do not modify
the stored value. If a value is copied into an object having no declared type using
memcpy or memmove or is copied as an array of character type, then the effective type
of the modified object for that access and for subsequent accesses that do not modify the
value is the effective type of the object from which the value is copied, if it has one. For
all other accesses to an object having no declared type, the effective type of the object is
simply the type of the lvalue used for the access.
An object shall have its stored value accessed only by an lvalue expression that has one of the following types: [54]
a type compatible with the effective type of the object,
a qualified version of a type compatible with the effective type of the object,
a type that is the signed or unsigned type corresponding to the effective type of the object,
a type that is the signed or unsigned type corresponding to a qualified version of the effective type of the object,
an aggregate or union type that includes one of the aforementioned types among its members (including, recursively, a member of a subaggregate or contained union), or
a character type.
A floating expression may be contracted, that is, evaluated as though it were an atomic
operation, thereby omitting rounding errors implied by the source code and the
expression evaluation method. [55] The FP_CONTRACT pragma in <math.h> provides a
way to disallow contracted expressions. Otherwise, whether and how expressions are
contracted is implementation-defined. [56]
Forward references: the FP_CONTRACT pragma (The FP_CONTRACT pragma), copying functions
(Copying functions).
Primary expressions#
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). [57]
A constant is a primary expression. Its type depends on its form and value, as detailed in Constants.
A string literal is a primary expression. It is an lvalue with type as detailed in String literals.
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 (Declarations).
Postfix operators#
Array subscripting#
Constraints
One of the expressions shall have type “pointer to object type”, the other expression shall have integer type, and the result has type “type”.
Semantics
A postfix expression followed by an expression in square brackets [] is a subscripted
designation of an element of an array object. The definition of the subscript operator []
is that E1[E2] is identical to (*((E1)+(E2))). Because of the conversion rules that
apply to the binary + operator, if E1 is an array object (equivalently, a pointer to the
initial element of an array object) and E2 is an integer, E1[E2] designates the E2-th
element of E1 (counting from zero).
Successive subscript operators designate an element of a multidimensional array object.
If E is an n-dimensional array (\(n~\geq~2\)) with dimensions \(i~*~j~*~.~.~.~*~k\),
then E (used as other than an lvalue) is converted to a pointer to an (n - 1)-dimensional
array with dimensions \(j~*~.~.~.~*~k\). If the unary * operator is applied to this
pointer explicitly, or implicitly as a result of subscripting, the result is the pointed-to
(n - 1)-dimensional array, which itself is converted into a pointer if used as other than
an lvalue. It follows from this that arrays are stored in row-major order (last subscript
varies fastest).
EXAMPLE Consider the array object defined by the declaration
int x[3][5];
Here x is a \(3~*~5\) array of ints; more precisely, x is an array of three
element objects, each of which is an array of five ints. In the expression x[i] which
is equivalent to (*((x)+(i))), x is first converted to a pointer to the initial
array of five ints. Then i is adjusted according to the type of x, which conceptually
entails multiplying i by the size of the object to which the pointer points, namely
an array of five int objects. The results are added and indirection is applied to yield
an array of five ints. When used in the expression x[i][j] that array is in turn
converted to a pointer to the first of the ints, so x[i][j] yields an int.
Forward references: additive operators (Additive operators), address and indirection operators (Address and indirection operators), array declarators (Array declarators).
Function calls#
Constraints
The expression that denotes the called function [58] shall have type pointer to function
returning void or returning an object type other than an array type.
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.
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. [59]
If the expression that denotes the called function has type pointer to function returning an object type, the function call expression has the same type as that object type, and has the value determined as specified in The return statement. Otherwise, the function call has type void. If an attempt is made to modify the result of a function call or to access it after the next sequence point, the behavior is undefined.
If the expression that denotes the called function has a type that does not include a
prototype, the integer promotions are performed on each argument, and arguments that
have type float are promoted to double. These are called the default argument
promotions. If the number of arguments does not equal the number of parameters, the
behavior is undefined. If the function is defined with a type that includes a prototype, and
either the prototype ends with an ellipsis (, ...) or the types of the arguments after
promotion are not compatible with the types of the parameters, the behavior is undefined.
If the function is defined with a type that does not include a prototype, and the types of
the arguments after promotion are not compatible with those of the parameters after
promotion, the behavior is undefined, except for the following cases:
one promoted type is a signed integer type, the other promoted type is the corresponding unsigned integer type, and the value is representable in both types;
both types are pointers to qualified or unqualified versions of a character type or
void.
If the expression that denotes the called function has a type that does include a prototype, the arguments are implicitly converted, as if by assignment, to the types of the corresponding parameters, taking the type of each parameter to be the unqualified version of its declared type. The ellipsis notation in a function prototype declarator causes argument type conversion to stop after the last declared parameter. The default argument promotions are performed on trailing arguments.
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.
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.
The order of evaluation of the function designator, the actual arguments, and subexpressions within the actual arguments is unspecified, but there is a sequence point before the actual call.
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
have to be completed before the function pointed to by pf[f1()] is called.
Forward references: function declarators (including prototypes)
(Function declarators (including prototypes)), function definitions (Function definitions), the return statement
(The return statement), simple assignment (Simple assignment).
Structure and union members#
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 the . operator and an identifier designates a member of
a structure or union object. The value is that of the named member, and is an lvalue if the
first expression is an lvalue. If the first expression has qualified type, the result has the
so-qualified version of the type of the designated member.
A postfix expression followed by the -> operator and an identifier designates a member
of a structure or union object. The value is that of the named member of the object to
which the first expression points, and is an lvalue. [60] 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.
One special guarantee is made in order to simplify the use of unions: if a union contains several structures that share a common initial sequence (see below), and if the union object currently contains one of these structures, it is permitted to inspect the common initial part of any of them anywhere that a declaration of the complete type of the union is visible. Two structures share a common initial sequence if corresponding members have compatible types (and, for bit-fields, the same widths) for a sequence of one or more initial members.
EXAMPLE 1 If f is a function returning a structure or union, and x is a member
of that structure or union, f().x is a valid postfix expression but is not an lvalue.
EXAMPLE 2 In:
struct s { int i; const int ci; };
struct s s;
const struct s cs;
volatile struct s vs;
the various members have the types:
s.i int
s.ci const int
cs.i const int
cs.ci const int
vs.i volatile int
vs.ci volatile const int
EXAMPLE 3 The following is a valid fragment:
union {
struct {
int alltypes;
} n;
struct {
int type;
int intnode;
} ni;
struct {
int type;
double doublenode;
} nf;
} u;
u.nf.type = 1;
u.nf.doublenode = 3.14;
/* ... */
if (u.n.alltypes == 1)
if (sin(u.nf.doublenode) == 0.0)
/* ... */
The following is not a valid fragment (because the union type is not visible within function f):
struct t1 { int m; };
struct t2 { int m; };
int f(struct t1 *p1, struct t2 *p2)
{
if (p1->m < 0)
p2->m = -p2->m;
return p1->m;
}
int g()
{
union {
struct t1 s1;
struct t2 s2;
} u;
/* ... */
return f(&u.s1, &u.s2);
}
Forward references: address and indirection operators (Address and indirection operators), structure and union specifiers (Structure and union specifiers).
Postfix increment and decrement operators#
Constraints
The operand of the postfix increment or decrement operator shall have qualified or unqualified real or pointer type and shall be a modifiable lvalue.
Semantics
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 (Additive operators), compound assignment (Compound assignment).
Compound literals#
Constraints
The type name shall specify an object type or an array of unknown size, but not a variable length array type.
No initializer shall attempt to provide a value for an object not contained within the entire unnamed object specified by the compound literal.
If the compound literal occurs outside the body of a function, the initializer list shall consist of constant expressions.
Semantics
A postfix expression that consists of a parenthesized type name followed by a brace- enclosed list of initializers is a compound literal. It provides an unnamed object whose value is given by the initializer list. [61]
If the type name specifies an array of unknown size, the size is determined by the initializer list as specified in Initialization , and the type of the compound literal is that of the completed array type. Otherwise (when the type name specifies an object type), the type of the compound literal is that specified by the type name. In either case, the result is an lvalue.
The value of the compound literal is that of an unnamed object initialized by the initializer list. If the compound literal occurs outside the body of a function, the object has static storage duration; otherwise, it has automatic storage duration associated with the enclosing block.
All the semantic rules and constraints for initializer lists in Initialization are applicable to compound literals. [62]
String literals, and compound literals with const-qualified types, need not designate distinct objects. [63]
EXAMPLE 1 The file scope definition
int *p = (int []){2, 4};
initializes p to point to the first element of an array of two ints, the first
having the value two and the second, four. The expressions in this compound literal
are required to be constant. The unnamed object has static storage duration.
EXAMPLE 2 In contrast, in
void f(void)
{
int *p;
/*...*/
p = (int [2]){*p};
/*...*/
}
p is assigned the address of the first element of an array of two ints, the
first having the value previously pointed to by p and the second, zero. The
expressions in this compound literal need not be constant. The unnamed object
has automatic storage duration.
EXAMPLE 3 Initializers with designations can be combined with compound literals. Structure objects created using compound literals can be passed to functions without depending on member order:
drawline((struct point){.x=1, .y=1},
(struct point){.x=3, .y=4});
Or, if drawline instead expected pointers to struct point:
drawline(&(struct point){.x=1, .y=1},
&(struct point){.x=3, .y=4});
EXAMPLE 4 A read-only compound literal can be specified through constructions like:
(const float []){1e0, 1e1, 1e2, 1e3, 1e4, 1e5, 1e6}
EXAMPLE 5 The following three expressions have different meanings:
"/tmp/fileXXXXXX"
(char []){"/tmp/fileXXXXXX"}
(const char []){"/tmp/fileXXXXXX"}
The first always has static storage duration and has type array of char, but need not be modifiable; the last two have automatic storage duration when they occur within the body of a function, and the first of these two is modifiable.
EXAMPLE 6 Like string literals, const-qualified compound literals can be placed into read-only memory and can even be shared. For example,
(const char []){"abc"} == "abc"
might yield 1 if the literals’ storage is shared.
EXAMPLE 7 EXAMPLE 7 Since compound literals are unnamed, a single compound literal cannot specify a circularly linked object. For example, there is no way to write a self-referential compound literal that could be used as the function argument in place of the named object endless_zeros below:
struct int_list { int car; struct int_list *cdr; };
struct int_list endless_zeros = {0, &endless_zeros};
eval(endless_zeros);
EXAMPLE 8 Each compound literal creates only a single object in a given scope:
struct s { int i; };
int f (void)
{
struct s *p = 0, *q;
int j = 0;
again:
q = p, p = &((struct s){ j++ });
if (j < 2) goto again;
return p == q && q->i == 1;
}
The function f() always returns the value 1.
Note that if an iteration statement were used instead of an explicit goto and a labeled statement, the
lifetime of the unnamed object would be the body of the loop only, and on entry next time around p would
have an indeterminate value, which would result in undefined behavior.
Forward references: type names (Type names), initialization (Initialization).
Unary operators#
Prefix increment and decrement operators#
Constraints
The operand of the prefix increment or decrement operator shall have qualified or unqualified real or pointer type and shall be a modifiable lvalue.
Semantics
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
onstraints, 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 (Additive operators), compound assignment (Compound assignment).
Address and indirection operators#
Constraints
The operand of the unary & operator shall be either a function designator, the result of a
[] or unary * operator, or an lvalue that designates an object that is not a bit-field
and is not declared with the register storage-class specifier.
The operand of the unary * operator shall have pointer type.
Semantics
The unary & operator yields the address of its operand. If the operand has type “type”,
the result has type “pointer to type”. If the operand is the result of a unary * operator,
neither that operator nor the & operator is evaluated and the result is as if both were
omitted, except that the constraints on the operators still apply and the result is not an
lvalue. Similarly, if the operand is the result of a [] operator, neither the & operator
nor he unary * that is implied by the [] is evaluated and the result is as if the
& operator were removed and the [] operator were changed to a + operator.
Otherwise, the result is a pointer to the object or function designated by its operand.
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. [64]
Forward references: storage-class specifiers (Storage-class specifiers), structure and union specifiers (Structure and union specifiers).
Unary arithmetic operators#
Constraints
The operand of the unary + or - operator shall have arithmetic type; of the -
operator, integer type; of the ! operator, scalar type.
Semantics
The result of the unary + operator is the value of its (promoted) operand. The integer
promotions are performed on the operand, and the result has the promoted type.
The result of the unary - operator is the negative of its (promoted) operand. The integer
promotions are performed on the operand, and the result has the promoted type.
The result of the ~ operator is the bitwise complement of its (promoted) operand (that is,
each bit in the result is set if and only if the corresponding bit in the converted operand is
not set). The integer promotions are performed on the operand, and the result has the
promoted type. If the promoted type is an unsigned type, the expression ~E is equivalent
to the maximum value representable in that type minus E.
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).
The sizeof operator#
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 expression that
designates a bit-field member.
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. The result is an integer. If the type of the operand is a variable length array
type, the operand is evaluated; otherwise, the operand is not evaluated and the result is an
integer constant.
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. [65] 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 integer type)
is size_t, defined in <stddef.h> (and other headers).
EXAMPLE 1 A principal use of the sizeof operator is in communication with routines such as storage allocators and I/O systems. A storage-allocation function might accept a size (in bytes) of an object to allocate and return a pointer to void. For example:
extern void *alloc(size_t);
double *dp = alloc(sizeof *dp);
The implementation of the alloc function should ensure that its return value is aligned suitably for
conversion to a pointer to double.
EXAMPLE 2 Another use of the sizeof operator is to compute the number of elements in an array:
sizeof array / sizeof array[0]
EXAMPLE 3 In this example, the size of a variable length array is computed and returned from a function:
#include <stddef.h>
size_t fsize3(int n)
{
char b[n+3]; // variable length array
return sizeof b; // execution time sizeof
}
int main()
{
size_t size;
size = fsize3(10); // fsize3 returns 13
return 0;
}
Forward references: common definitions <stddef.h> (Common definitions <stddef.h>), declarations (Declarations),
structure and union specifiers (Structure and union specifiers), type names (Type names), array declarators (Array declarators).
Cast operators#
Constraints
Unless the type name specifies a void type, the type name shall specify qualified or
unqualified scalar type and the operand shall have scalar type.
Conversions that involve pointers, other than where permitted by the constraints of Simple assignment, shall be specified by means of an explicit cast.
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. [66] A cast that specifies no conversion has no effect on the type or value of an expression. [67]
Forward references: equality operators (Equality operators), function declarators (including prototypes) (Function declarators (including prototypes)), simple assignment (Simple assignment), type names (Type names).
Multiplicative operators#
Constraints
Each of the operands shall have arithmetic type. The operands of the % operator shall have integer 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, the result of the / operator is the algebraic quotient with any
fractional part discarded. [68] If the quotient a/b is representable, the expression
(a/b)*b + a%b shall equal a.
Additive operators#
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 integer 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 integer 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.
For the purposes of these operators, a pointer to an object that is not an element of an array behaves the same as a pointer to the first element of an array of length one with the stype of the object as its element type.
When an expression that has integer type is added to or subtracted from a pointer, the
result has the type of the pointer operand. If the pointer operand points to an element of
an array object, and the array is large enough, the result points to an element offset from
the original element such that the difference of the subscripts of the resulting and original
array elements equals the integer expression. In other words, if the expression P points to
the i-th element of an array object, the expressions (P)+N (equivalently, N+(P)) and
(P)-N (where N has the value n) point to, respectively, the i+n-th and i-n-th elements of
the array object, provided they exist. Moreover, if the expression P points to the last
element of an array object, the expression (P)+1 points one past the last element of the
array object, and if the expression Q points one past the last element of an array object,
the expression (Q)-1 points to the last element of the array object. If both the pointer
operand and the result point to elements of the same array object, or one past the last
element of the array object, the evaluation shall not produce an overflow; otherwise, the
behavior is undefined. If the result points one past the last element of the array object, it
shall not be used as the operand of a unary * operator that is evaluated.
When two pointers are subtracted, both shall point to elements of the same array object,
or one past the last element of the array object; the result is the difference of the
subscripts of the two array elements. The size of the result is implementation-defined,
and its type (a signed integer type) is ptrdiff_t defined in the <stddef.h> header.
If the result is not representable in an object of that type, the behavior is undefined. In
other words, if the expressions P and Q point to, respectively, the i-th and j-th elements of
an array object, the expression (P)-(Q) has the value i-j provided the value fits in an
object of type ptrdiff_t. Moreover, if the expression P points either to an element of
an array object or one past the last element of an array object, and the expression Q points
to the last element of the same array object, the expression ((Q)+1)-(P) has the same
value as ((Q)-(P))+1 and as -((P)-((Q)+1)), and has the value zero if the
expression P points one past the last element of the array object, even though the
expression (Q)+1 does not point to an element of the array object. [69]
EXAMPLE Pointer arithmetic is well defined with pointers to variable length array types.
{
int n = 4, m = 3;
int a[n][m];
int (*p)[m] = a; // p == &a[0]
p += 1; // p == &a[1]
(*p)[2] = 99; // a[1][2] == 99
n = p - a; // n == 1
}
If array a in the above example were declared to be an array of known constant size,
and pointer p were declared to be a pointer to an array of the same known constant
size (pointing to a), the results would be the same.
Forward references: array declarators (Array declarators), common definitions <stddef.h>
(Common definitions <stddef.h>).
Bitwise shift operators#
Constraints
Each of the operands shall have integer type.
Semantics
The integer promotions are performed on each of the operands. The type of the result is that of the promoted left operand. If the value of the right operand is negative or is greater than or equal to the width of the promoted left operand, the behavior is undefined.
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 \(E1~*~2^{E2 }\), reduced modulo one more than the maximum value representable in the result type. If \(E1\) has a signed type and nonnegative value, and \(E1~*~2^{E2}\) is representable in the result type, then that is the resulting value; otherwise, the behavior is undefined.
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 \(E1/2^{E2}\). If \(E1\) has a signed type and a negative value, the resulting value is implementation-defined.
Relational operators#
Constraints
One of the following shall hold:
both operands have real type;
both operands are pointers to qualified or unqualified versions of compatible object types; or
both operands are pointers to qualified or unqualified versions of compatible incomplete types.
Semantics
If both of the operands have arithmetic type, the usual arithmetic conversions are performed.
For the purposes of these operators, a pointer to an object that is not an element of an array behaves the same as a pointer to the first element of an array of length one with the type of the object as its element type.
When two pointers are compared, the result depends on the relative locations in the
address space of the objects pointed to. If two pointers to object or incomplete types both
point to the same object, or both point one past the last element of the same array object,
they compare equal. If the objects pointed to are members of the same aggregate object,
pointers to structure members declared later compare greater than pointers to members
declared earlier in the structure, and pointers to array elements with larger subscript
values compare greater than pointers to elements of the same array with lower subscript
values. All pointers to members of the same union object compare equal. If the
expression P points to an element of an array object and the expression Q points to the
last element of the same array object, the pointer expression Q+1 compares greater than
P. In all other cases, the behavior is undefined.
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. [70]
The result has type int.
Equality operators#
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 pointer to 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 != (not equal to) operators are analogous to the relational
operators except for their lower precedence. [71] Each of the operators yields 1 if the
specified relation is true and 0 if it is false. The result has type int. For any pair of
operands, exactly one of the relations is true.
If both of the operands have arithmetic type, the usual arithmetic conversions are performed. Values of complex types are equal if and only if both their real parts are equal and also their imaginary parts are equal. Any two values of arithmetic types from different type domains are equal if and only if the results of their conversions to the (complex) result type determined by the usual arithmetic conversions are equal.
Otherwise, at least one operand is a pointer. If one operand is a pointer and the other is a
null pointer constant, the null pointer constant is converted to the type of the pointer. If
one operand is a pointer to an object or incomplete type and the other is a pointer to a
qualified or unqualified version of void, the former is converted to the type of the latter.
Two pointers compare equal if and only if both are null pointers, both are pointers to the same object (including a pointer to an object and a subobject at its beginning) or function, both are pointers to one past the last element of the same array object, or one is a pointer to one past the end of one array object and the other is a pointer to the start of a different array object that happens to immediately follow the first array object in the address space. [72]
For the purposes of these operators, a pointer to an object that is not an element of an array behaves the same as a pointer to the first element of an array of length one with the type of the object as its element type.
Bitwise AND operator#
Constraints
Each of the operands shall have integer 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).
Bitwise exclusive OR operator#
Constraints
Each of the operands shall have integer 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).
Bitwise inclusive OR operator#
Constraints
Each of the operands shall have integer 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).
Logical AND operator#
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.
Logical OR operator#
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.
Conditional operator#
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 the same structure or union type;
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 result is the value of the second or third operand (whichever is evaluated), converted to the type described below. [73] If an attempt is made to modify the result of a conditional operator or to access it after the next sequence point, the behavior is undefined.
If both the second and third operands have arithmetic type, the result type that would be
determined by the usual arithmetic conversions, were they applied to those two operands,
is the type of the result. If both the operands have structure or union type, the result has
that type. If both operands have void type, the result has void type.
If both the second and third operands are pointers or one is a null pointer constant and the
other is a pointer, the result type is a pointer to a type qualified with all the type qualifiers
of the types pointed-to by both operands. Furthermore, if both operands are pointers to
compatible types or to differently qualified versions of compatible types, the result type is
a pointer to an appropriately qualified version of the composite type; if one operand is a
null pointer constant, the result has the type of the other operand; otherwise, one operand
is a pointer to void or a qualified version of void, in which case the result type is a
pointer to an appropriately qualified version of void.
EXAMPLE The common type that results when the second and third operands are pointers is determined in two independent stages. The appropriate qualifiers, for example, do not depend on whether the two pointers have compatible types.
Given the declarations
const void *c_vp;
void *vp;
const int *c_ip;
volatile int *v_ip;
int *ip;
const char *c_cp;
the third column in the following table is the common type that is the result of a conditional expression in which the first two columns are the second and third operands (in either order):
c_vp c_ip const void *
v_ip 0 volatile int *
c_ip v_ip const volatile int *
vp c_cp const void *
ip c_ip const int *
vp ip void *
Assignment operators#
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. If an attempt is made to modify the result of an assignment operator or to access it after the next sequence point, the behavior is undefined.
Simple assignment#
Constraints
One of the following shall hold: [74]
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;
the left operand is a pointer and the right is a null pointer constant; or
the left operand has type
_Booland the right is a pointer.
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 read 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 1 In the program fragment
int f(void);
char c;
/* ... */
if ((c = f()) == -1)
/* ... */
the int value returned by the function may be truncated when stored in the char, and then converted back
to int width prior to the comparison. In an implementation in which “plain” char has the same range of
values as unsigned char (and char is narrower than int), the result of the conversion cannot be
negative, so the operands of the comparison can never compare equal. Therefore, for full portability, the
variable c should be declared as int.
EXAMPLE 2 In the fragment:
char c;
int i;
long l;
l = (c = i);
the value of i is converted to the type of the assignment expression c = i, that is, char type. The value
of the expression enclosed in parentheses is then converted to the type of the outer assignment expression,
that is, long int type.
EXAMPLE 3 Consider the fragment:
const char **cpp;
char *p;
const char c = 'A';
cpp = &p; // constraint violation
*cpp = &c; // valid
*p = 0; // valid
The first assignment is unsafe because it would allow the following valid code to attempt to change the
value of the const object c.
Compound assignment#
Constraints
For the operators += and -= only, either the left operand shall be a pointer to an object
type and the right shall have integer type, or the left operand shall have qualified or
unqualified arithmetic type and the right shall have arithmetic type.
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.
Comma operator#
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. [75] If an attempt is made to modify the result of a comma operator or to access it after the next sequence point, the behavior is undefined.
EXAMPLE As indicated by the syntax, the comma operator (as described in this subclause) cannot appear in contexts where a comma is used to separate items in a list (such as arguments to functions or lists of initializers). On the other hand, it can be used within a parenthesized expression or within the second expression of a conditional operator in such contexts. In the function call:
f(a, (t=3, t+2), c)
the function has three arguments, the second of which has the value 5.
Forward references: initialization (Initialization).
Constant expressions#
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 a subexpression that is not evaluated. [76]
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. If a floating expression is evaluated in the translation environment, the arithmetic precision and range shall be at least as great as if the expression were being evaluated in the execution environment.
An integer constant expression [77] shall have integer type and shall only have operands
that are integer constants, enumeration constants, character constants, sizeof
expressions whose results are integer constants, and floating constants that are the
immediate operands of casts. Cast operators in an integer constant expression shall only
convert arithmetic types to integer types, except as part of an operand to the sizeof
operator.
More latitude is permitted for constant expressions in initializers. Such a constant expression shall be, or evaluate to, one of the following:
an arithmetic constant expression,
a null pointer constant,
an address constant, or
an address constant for an object type plus or minus an integer constant expression.
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 a
sizeof operator whose result is an integer constant.
An address constant is a null pointer, a pointer to an lvalue designating an object of static
storage duration, or a pointer to a function designator; it shall be created explicitly using
the unary & operator or an integer constant cast to pointer type, or implicitly by the use of
an expression of array or function type. The array-subscript [] and member-access .
and -> operators, the address & and indirection * unary operators, and pointer casts may
be used in the creation of an address constant, but the value of an object shall not be
accessed by use of these operators.
An implementation may accept other forms of constant expressions.
The semantic rules for the evaluation of a constant expression are the same as for nonconstant expressions. [78]
Forward references: array declarators (Array declarators), initialization (Initialization).
Declarations#
Constraints
A declaration shall declare at least a declarator (other than the parameters of a function or the members of a structure or union), a tag, or the members of an enumeration.
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 Tags.
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 definition of an identifier is a declaration for that identifier that:
for an object, causes storage to be reserved for that object;
for a function, includes the function body; [79]
for an enumeration constant or typedef name, is the (only) declaration of the identifier.
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; in the case of function parameters (including in prototypes), it is the adjusted type (see Function declarators (including prototypes)) that is required to be complete.
Forward references: declarators (Declarators), enumeration specifiers (Enumeration specifiers), initialization (Initialization).
Storage-class specifiers#
Constraints
At most, one storage-class specifier may be given in the declaration specifiers in a declaration. [80]
Semantics
The typedef specifier is called a “storage-class specifier” for syntactic convenience
only; it is discussed in Type definitions. The meanings of the various linkages and storage durations
were discussed in Linkages of identifiers and Storage duration of objects.
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. [81]
The declaration of an identifier for a function that has block scope shall have no explicit
storage-class specifier other than extern.
If an aggregate or union object is declared with a storage-class specifier other than
typedef, the properties resulting from the storage-class specifier, except with respect to
linkage, also apply to the members of the object, and so on recursively for any aggregate
or union member objects.
Forward references: type definitions (Type definitions).
Type specifiers#
Constraints
At least one type specifier shall be given in the declaration specifiers in each declaration, and in the specifier-qualifier list in each struct declaration and type name. Each list of type specifiers shall be one of the following sets (delimited by commas, when there is more than one set on a line); the type specifiers may occur in any order, possibly intermixed with the other declaration specifiers.
voidcharsigned charunsigned charshort, signed short, short intorsigned short intunsigned shortorunsigned short intint, signedorsigned intunsignedorunsigned intlong, signed long, long intor signed long int`unsigned longorunsigned long intlong long, signed long long, long long intorsigned long long intunsigned long longorunsigned long long intfloatdoublelong double_Boolfloat _Complexdouble _Complexlong double _Complexstructorunionspecifierenumspecifiertypedefname
The type specifier _Complex shall not be used if the implementation does not provide
complex types. [82]
Semantics
Specifiers for structures, unions, and enumerations are discussed in Structure and union specifiers through
Tags. Declarations of typedef names are discussed in Type definitions. The characteristics of the
other types are discussed in Types.
Each of the comma-separated sets designates the same type, except that for bit-fields, it is
implementation-defined whether the specifier int designates the same type as signed
int or the same type as unsigned int.
Forward references: enumeration specifiers (Enumeration specifiers), structure and union specifiers (Structure and union specifiers), tags (Tags), type definitions (Type definitions).
Structure and union specifiers#
Constraints
A structure or union shall not contain a member with incomplete or function type (hence, a structure shall not contain an instance of itself, but may contain a pointer to an instance of itself), except that the last member of a structure with more than one named member may have incomplete array type; such a structure (and any union containing, possibly recursively, a member that is such a structure) shall not be a member of a structure or an element of an array.
The expression that specifies the width of a bit-field shall be an integer constant expression with a nonnegative value that does not exceed the width of an object of the type that would be specified were the colon and expression omitted. If the value is zero, the declaration shall have no declarator.
A bit-field shall have a type that is a qualified or unqualified version of _Bool, signed
int, unsigned int or some other implementation-defined type.
Semantics
As discussed in Types, a structure is a type consisting of a sequence of members, whose storage is allocated in an ordered sequence, and a union is a type consisting of a sequence of members whose storage overlap.
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. If the struct-declaration-list contains no named
members, the behavior is undefined. The type is incomplete until after the } that
terminates the list.
A member of a structure or union may have any object type other than a variably modified type. [83] 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; [84] its width is preceded by a colon.
A bit-field is interpreted as a signed or unsigned integer type consisting of the specified
number of bits. [85] If the value 0 or 1 is stored into a nonzero-width bit-field of type
_Bool, the value of the bit-field shall compare equal to the value stored.
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. [86] As a special case, a bit-field structure member with a width of 0 indicates that no further bit-field is to be packed into the unit in which the previous bit- field, if any, was placed.
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 converted, points to its initial member (or if that member is a bit-field, then to the unit in which it resides), and vice versa. There may be unnamed padding within a structure object, but not at its beginning.
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 converted, points to each of its members (or if a member is a bit- field, then to the unit in which it resides), and vice versa.
There may be unnamed padding at the end of a structure or union.
As a special case, the last element of a structure with more than one named member may
have an incomplete array type; this is called a flexible array member. In most situations,
the flexible array member is ignored. In particular, the size of the structure is as if the
flexible array member were omitted except that it may have more trailing padding than
the omission would imply. However, when a . (or ->) operator has a left operand that is
(a pointer to) a structure with a flexible array member and the right operand names that
member, it behaves as if that member were replaced with the longest array (with the same
element type) that would not make the structure larger than the object being accessed; the
offset of the array shall remain that of the flexible array member, even if this would differ
from that of the replacement array. If this array would have no elements, it behaves as if
it had one element but the behavior is undefined if any attempt is made to access that
element or to generate a pointer one past it.
EXAMPLE After the declaration:
struct s { int n; double d[]; };
the structure struct s has a flexible array member d. A typical way to use this is:
int m = /* some value */;
struct s *p = malloc(sizeof (struct s) + sizeof (double [m]));
and assuming that the call to malloc succeeds, the object pointed to by p behaves, for most
purposes, as if p had been declared as:
struct { int n; double d[m]; } *p;
(there are circumstances in which this equivalence is broken; in particular, the offsets of
member d might not be the same).
Following the above declaration:
struct s t1 = { 0 }; // valid
struct s t2 = { 1, { 4.2 }}; // valid
t1.n = 4; // valid
t1.d[0] = 4.2; // might be undefined behavior
The initialization of t2 is invalid (and violates a constraint) because
struct s is treated as if it did not contain member d. The assignment
to t1.d[0] is probably undefined behavior, but it is possible that
sizeof (struct s) >= offsetof(struct s, d) + sizeof (double)
in which case the assignment would be legitimate. Nevertheless, it cannot appear in strictly conforming code.
After the further declaration:
struct ss { int n; };
the expressions:
sizeof (struct s) >= sizeof (struct ss)
sizeof (struct s) >= offsetof(struct s, d)
are always equal to 1.
If sizeof (double) is 8, then after the following code is executed:
struct s *s1;
struct s *s2;
s1 = malloc(sizeof (struct s) + 64);
s2 = malloc(sizeof (struct s) + 46);
and assuming that the calls to malloc succeed, the objects pointed to by
s1 and s2 behave, for most purposes, as if the identifiers had been
declared as:
struct { int n; double d[8]; } *s1;
struct { int n; double d[5]; } *s2;
Following the further successful assignments:
s1 = malloc(sizeof (struct s) + 10);
s2 = malloc(sizeof (struct s) + 6);
they then behave as if the declarations were:
struct { int n; double d[1]; } *s1, *s2;
and:
double *dp;
dp = &(s1->d[0]); // valid
*dp = 42; // valid
dp = &(s2->d[0]); // valid
*dp = 42; // undefined behavior
The assignment:
*s1 = *s2;
only copies the member n; if any of the array elements are within the first
sizeof (struct s) bytes of the structure, they might be copied or simply
overwritten with indeterminate values.
Forward references: tags (Tags).
Enumeration specifiers#
Constraints
The expression that defines the value of an enumeration constant shall be an integer
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. [87] An enumerator with = defines its enumeration constant as the value of the constant expression. If the first enumerator has no =, the value of its enumeration constant is 0. Each subsequent enumerator with no = defines its enumeration constant as the value of the constant expression obtained by adding 1 to the value of the previous enumeration constant. (The use of enumerators with = may produce enumeration constants with values that duplicate other values in the same enumeration.) The enumerators of an enumeration are also known as its members.
Each enumerated type shall be compatible with char, a signed integer type, or an unsigned integer type. The choice of type is implementation-defined, [88] but shall be capable of representing the values of all the members of the enumeration. The enumerated type is incomplete until after the } that terminates the list of enumerator declarations.
The following fragment:
enum hue { chartreuse, burgundy, claret=20, winedark };
enum hue col, *cp;
col = claret;
cp = &col;
if (*cp != burgundy)
/* ... */
makes hue the tag of an enumeration, and then declares col as an object that has that type and cp as a
pointer to an object that has that type. The enumerated values are in the set { 0, 1, 20, 21 }.
Forward references: tags (Tags).
Type qualifiers#
Constraints
Types other than pointer types derived from object or incomplete types shall not be restrict-qualified.
Semantics
The properties associated with qualified types are meaningful only for expressions that are lvalues. [92]
If the same qualifier appears more than once in the same specifier-qualifier-list, either directly or via one or more typedefs, the behavior is the same as if it appeared only once.
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. [93]
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 Program Execution. 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. [94] What constitutes an access to an object that has volatile-qualified type is implementation-defined.
An object that is accessed through a restrict-qualified pointer has a special association
with that pointer. This association, defined in Formal definition of restrict below, requires that all accesses to
that object use, directly or indirectly, the value of that particular pointer. [95] The intended
use of the restrict qualifier (like the register storage class) is to promote
optimization, and deleting all instances of the qualifier from all preprocessing translation
units composing a conforming program does not change its meaning (i.e., observable
behavior).
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. [96]
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.
EXAMPLE 1 An object declared
extern const volatile int real_time_clock;
may be modifiable by hardware, but cannot be assigned to, incremented, or decremented.
EXAMPLE 2 The following declarations and expressions illustrate the behavior when type qualifiers modify an aggregate type:
const struct s { int mem; } cs = { 1 };
struct s ncs; // the object ncs is modifiable
typedef int A[2][3];
const A a = {{4, 5, 6}, {7, 8, 9}}; // array of array of const int
int *pi;
const int *pci;
ncs = cs; // valid
cs = ncs; // violates modifiable lvalue constraint for =
pi = &ncs.mem; // valid
pi = &cs.mem; // violates type constraints for =
pci = &cs.mem; // valid
pi = a[0]; // nvalid: a[0] has type ‘‘const int *’’
Formal definition of restrict#
Let D be a declaration of an ordinary identifier that provides a means of designating an
object P as a restrict-qualified pointer to type T.
If D appears inside a block and does not have storage class extern, let B denote the
block. If D appears in the list of parameter declarations of a function definition, let B
denote the associated block. Otherwise, let B denote the block of main (or the block of
whatever function is called at program startup in a freestanding environment).
In what follows, a pointer expression E is said to be based on object P if (at some
sequence point in the execution of B prior to the evaluation of E) modifying P to point to
a copy of the array object into which it formerly pointed would change the value of E. [97]
Note that “based” is defined only for expressions with pointer types.
During each execution of B, let L be any lvalue that has &L based on P`. If L is used to
access the value of the object X that it designates, and X is also modified (by any means),
then the following requirements apply: T shall not be const-qualified. Every other lvalue
used to access the value of X shall also have its address based on P. Every access that
modifies X shall be considered also to modify P, for the purposes of this subclause. If P
is assigned the value of a pointer expression E that is based on another restricted pointer
object P2, associated with block B2, then either the execution of B2 shall begin before
the execution of B, or the execution of B2 shall end prior to the assignment. If these
requirements are not met, then the behavior is undefined.
Here an execution of B means that portion of the execution of the program that would
correspond to the lifetime of an object with scalar type and automatic storage duration
ssociated with B.
A translator is free to ignore any or all aliasing implications of uses of restrict.
EXAMPLE 1 The file scope declarations
int * restrict a;
int * restrict b;
extern int c[];
assert that if an object is accessed using one of a, b or c, and that object is modified anywhere in the
program, then it is never accessed using either of the other two.
EXAMPLE 2 The function parameter declarations in the following example
void f(int n, int * restrict p, int * restrict q)
{
while (n-- > 0)
*p++ = *q++;
}
assert that, during each execution of the function, if an object is accessed through one of the pointer parameters, then it is not also accessed through the other.
The benefit of the restrict qualifiers is that they enable a translator to make an effective dependence
analysis of function f without examining any of the calls of f in the program. The cost is that the
programmer has to examine all of those calls to ensure that none give undefined behavior. For example, the
second call of f in g has undefined behavior because each of d[1] through d[49] is accessed through
both p and q.
void g(void)
{
extern int d[100];
f(50, d + 50, d); // valid
f(50, d + 1, d); // undefined behavior
}
EXAMPLE 3 The function parameter declarations
void h(int n, int * restrict p, int * restrict q, int * restrict r)
{
int i;
for (i = 0; i < n; i++)
p[i] = q[i] + r[i];
}
illustrate how an unmodified object can be aliased through two restricted pointers. In particular, if a and b
are disjoint arrays, a call of the form h(100, a, b, b) has defined behavior, because array b is not
modified within function h.
EXAMPLE 4 The rule limiting assignments between restricted pointers does not distinguish between a function call and an equivalent nested block. With one exception, only “outer-to-inner” assignments between restricted pointers declared in nested blocks have defined behavior.
{
int * restrict p1;
int * restrict q1;
p1 = q1; // undefined behavior
{
int * restrict p2 = p1; // valid
int * restrict q2 = q1; // valid
p1 = q2; // undefined behavior
p2 = q2; // undefined behavior
}
}
The one exception allows the value of a restricted pointer to be carried out of the block in which it (or, more precisely, the ordinary identifier used to designate it) is declared when that block finishes execution. For example, this permits new_vector to return a vector.
typedef struct { int n; float * restrict v; } vector;
vector new_vector(int n)
{
vector t;
t.n = n;
t.v = malloc(n * sizeof (float));
return t;
}
Function specifiers#
Constraints
Function specifiers shall be used only in the declaration of an identifier for a function.
An inline definition of a function with external linkage shall not contain a definition of a modifiable object with static storage duration, and shall not contain a reference to an identifier with internal linkage.
In a hosted environment, the inline function specifier shall not appear in a declaration
of main.
Semantics
A function declared with an inline function specifier is an inline function. The
function specifier may appear more than once; the behavior is the same as if it appeared
only once. Making a function an inline function suggests that calls to the function be as
fast as possible. [98] The extent to which such suggestions are effective is
implementation-defined. [99]
Any function with internal linkage can be an inline function. For a function with external
linkage, the following restrictions apply: If a function is declared with an inline
function specifier, then it shall also be defined in the same translation unit. If all of the
file scope declarations for a function in a translation unit include the inline` function
specifier without ``extern, then the definition in that translation unit is an inline
definition. An inline definition does not provide an external definition for the function,
and does not forbid an external definition in another translation unit. An inline definition
provides an alternative to an external definition, which a translator may use to implement
any call to the function in the same translation unit. It is unspecified whether a call to the
function uses the inline definition or the external definition. [100]
EXAMPLE The declaration of an inline function with external linkage can result in either an external definition, or a definition available for use only within the translation unit. A file scope declaration with extern creates an external definition. The following example shows an entire translation unit.
inline double fahr(double t)
{
return (9.0 * t) / 5.0 + 32.0;
}
inline double cels(double t)
{
return (5.0 * (t - 32.0)) / 9.0;
}
extern double fahr(double);
// creates an external definition
double convert(int is_fahr, double temp)
{
/* A translator may perform inline substitutions */
return is_fahr ? cels(temp) : fahr(temp);
}
Note that the definition of fahr is an external definition because fahr is also declared with extern, but
the definition of cels is an inline definition. Because cels has external linkage and is referenced, an
external definition has to appear in another translation unit (see External definitions); the inline definition and the external
definition are distinct and either may be used for the call.
Forward references: function definitions (Function definitions).
Declarators#
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.
A full declarator is a declarator that is not part of another declarator. The end of a full declarator is a sequence point. If the nested sequence of declarators in a full declarator contains a variable length array type, the type specified by the full declarator is said to be variably modified.
In the following subclauses, consider a declaration:
T D1
where T contains the declaration specifiers that specify a type T (such as int) and D1 is
a declarator that contains an identifier ident. The type specified for the identifier ident in
the various forms of declarator is described inductively using this notation.
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 complicated declarators may be altered by parentheses.
Implementation limits
As discussed in Translation limits, an implementation may limit the number of pointer, array, and function declarators that modify an arithmetic, structure, union, or incomplete type, either directly or via one or more typedefs.
Forward references: array declarators (Array declarators), type definitions (Type definitions).
Pointer declarators#
Semantics
If, in the declaration "T D1", D1 has the form:
* type-qualifier-listopt 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.
EXAMPLE The following pair of declarations demonstrates the difference between a “variable pointer to a constant value” and a “constant pointer to a variable value”.
const int *ptr_to_constant;
int *const constant_ptr;
The contents of any object pointed to by ptr_to_constant shall not be modified through that pointer,
but ptr_to_constant itself may be changed to point to another object. Similarly, the contents of the
int pointed to by constant_ptr may be modified, but constant_ptr itself shall always point to the
same location.
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”.
Array declarators#
Constraints
- 1 In addition to optional type qualifiers and the keyword static, the [ and ] may delimit
an expression or
*. If they delimit an expression (which specifies the size of an array), the expression shall have an integer type. If the expression is a constant expression, it shall have a value greater than zero. The element type shall not be an incomplete or function type. The optional type qualifiers and the keyword static shall appear only in a declaration of a function parameter with an array type, and then only in the outermost array type derivation.- 2 Only an ordinary identifier (as defined in 6.2.3) with both block scope or function
prototype scope and no linkage shall have a variably modified type. If an identifier is declared to be an object with static storage duration, it shall not have a variable length array type.
Semantics
If, in the declaration ‘‘T D1’’, D1 has one of the forms:
D[ type-qualifier-listopt assignment-expressionopt ]
D[ static type-qualifier-list(opt) assignment-expression ]
D[ type-qualifier-list static assignment-expression ]
D[ type-qualifier-listopt * ]
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”. [101]
(See Function declarators (including prototypes) for the meaning of the optional type qualifiers and the keyword static.)
If the size is not present, the array type is an incomplete type. If the size is * instead of
being an expression, the array type is a variable length array type of unspecified size,
which can only be used in declarations with function prototype scope; [102] such arrays are
nonetheless complete types. If the size is an integer constant expression and the element
type has a known constant size, the array type is not a variable length array type;
otherwise, the array type is a variable length array type.
If the size is an expression that is not an integer constant expression: if it occurs in a
declaration at function prototype scope, it is treated as if it were replaced by *; otherwise,
each time it is evaluated it shall have a value greater than zero. The size of each instance
of a variable length array type does not change during its lifetime. Where a size
expression is part of the operand of a sizeof operator and changing the value of the
size expression would not affect the result of the operator, it is unspecified whether or not
the size expression is evaluated.
For two array types to be compatible, both shall have compatible element types, and if both size specifiers are present, and are integer constant expressions, then both size specifiers shall have the same constant value. If the two array types are used in a context which requires them to be compatible, it is undefined behavior if the two size specifiers evaluate to unequal values.
EXAMPLE 1
float fa[11], *afp[17];
declares an array of float numbers and an array of pointers to float numbers.
EXAMPLE 2 Note the distinction between the declarations
extern int *x;
extern int y[];
The first declares x to be a pointer to int; the second declares y to be an array of int of unspecified size
(an incomplete type), the storage for which is defined elsewhere.
EXAMPLE 3 The following declarations demonstrate the compatibility rules for variably modified types.
extern int n;
extern int m;
void fcompat(void)
{
int a[n][6][m];
int (*p)[4][n+1];
int c[n][n][6][m];
int (*r)[n][n][n+1];
p = a; // invalid: not compatible because 4 != 6
r = c; // compatible, but defined behavior only if
// n == 6 and m == n+1
}
EXAMPLE 4 All declarations of variably modified (VM) types have to be at either block scope or
function prototype scope. Array objects declared with the static or extern storage-class specifier
cannot have a variable length array (VLA) type. However, an object declared with the static storage-
class specifier can have a VM type (that is, a pointer to a VLA type). Finally, all identifiers declared with a
VM type have to be ordinary identifiers and cannot, therefore, be members of structures or unions.
extern int n; // invalid: file scope VLA
int A[n]; // invalid: file scope VM
extern int (*p2)[n]; // valid: file scope but not VM
int B[100];
void fvla(int m, int C[m][m]); // valid: VLA with prototype scope
void fvla(int m, int C[m][m]) // valid: adjusted to auto pointer to VLA
{
typedef int VLA[m][m]; // valid: block scope typedef VLA
struct tag {
int (*y)[n]; // invalid: y not ordinary identifier
int z[n]; // invalid: z not ordinary identifier
};
int D[m]; // valid: auto VLA
static int E[m]; // invalid: static block scope VLA
extern int F[m]; // invalid: F has linkage and is VLA
int (*s)[m]; // valid: auto pointer to VLA
extern int (*r)[m]; // invalid: r has linkage and points to VLA
static int (*q)[m] = &B; // valid: q is a static block pointer to VLA
}
Forward references: function declarators (Function declarators (including prototypes)), function definitions (Function definitions), initialization (Initialization).
Function declarators (including prototypes)#
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 definition of that function shall be empty.
After adjustment, the parameters in a parameter type list in a function declarator that is part of a definition of that function shall not have incomplete type. Semantics
If, in the declaration "T D1", D1 has the form:
D( parameter-type-list )
or:
D( identifier-listopt )
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.
A declaration of a parameter as “array of type” shall be adjusted to “qualified pointer to
type”, where the type qualifiers (if any) are those specified within the [ and ] of the
array type derivation. If the keyword static also appears within the [ and ] of the
array type derivation, then for each call to the function, the value of the corresponding
actual argument shall provide access to the first element of an array with at least as many
elements as specified by the size expression.
A declaration of a parameter as “function returning type” shall be adjusted to “pointer to function returning type”, as in Other Operands.
If the list terminates with an ellipsis (, ...), no information about the number or types
of the parameters after the comma is supplied. [103]
The special case of an unnamed parameter of type void as the only item in the list
specifies that the function has no parameters.
If, in a parameter declaration, an identifier can be treated either as a typedef name or as a parameter name, it shall be taken as a typedef name.
If the function declarator is not part of a definition of that function, parameters may have
incomplete type and may use the [*] notation in their sequences of declarator specifiers
to specify variable length array types.
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 definition of that function specifies that the function has no parameters. The empty list in a function declarator that is not part of a definition of that function specifies that no information about the number or types of the parameters is supplied. [104]
For two function types to be compatible, both shall specify compatible return types. [105] 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. (In the determination of type compatibility and of a composite type, each parameter declared with function or array type is taken as having the adjusted type and each parameter declared with qualified type is taken as having the unqualified version of its declared type.)
EXAMPLE 1 The declaration
int f(void), *fip(), (*pfi)();
declares a function f with no parameters returning an int, a function fip with no parameter specification
returning a pointer to an int, and a pointer pfi to a function with no parameter specification returning an
int. It is especially useful to compare the last two. The binding of *fip() is *(fip()), so that the
declaration suggests, and the same construction in an expression requires, the calling of a function fip,
and then using indirection through the pointer result to yield an int. In the declarator (*pfi)(), the
extra parentheses are necessary to indicate that indirection through a pointer to a function yields a function
designator, which is then used to call the function; it returns an int.
If the declaration occurs outside of any function, the identifiers have file scope and external linkage. If the
declaration occurs inside a function, the identifiers of the functions f and fip have block scope and either
internal or external linkage (depending on what file scope declarations for these identifiers are visible), and
the identifier of the pointer pfi has block scope and no linkage.
EXAMPLE 2 The declaration
int (*apfi[3])(int *x, int *y);
declares an array apfi of three pointers to functions returning int. Each of these functions has two
parameters that are pointers to int. The identifiers x and y are declared for descriptive purposes only and
go out of scope at the end of the declaration of apfi.
EXAMPLE 3 The declaration
int (*fpfi(int (*)(long), int))(int, ...);
declares a function fpfi that returns a pointer to a function returning an int. The function fpfi has two
parameters: a pointer to a function returning an int (with one parameter of type long int), and an int.
The pointer returned by fpfi points to a function that has one int parameter and accepts zero or more
additional arguments of any type.
EXAMPEL 4 The following prototype has a variably modified parameter.
void addscalar(int n, int m,
double a[n][n*m+300], double x);
int main()
{
double b[4][308];
addscalar(4, 2, b, 2.17);
return 0;
}
void addscalar(int n, int m, double a[n][n*m+300], double x)
{
for (int i = 0; i < n; i++)
for (int j = 0, k = n*m+300; j < k; j++)
// a is a pointer to a VLA with n*m+300 elements
a[i][j] += x;
}
EXAMPLE 5 The following are all compatible function prototype declarators.
double maximum(int n, int m, double a[n][m]);
double maximum(int n, int m, double a[*][*]);
double maximum(int n, int m, double a[ ][*]);
double maximum(int n, int m, double a[*][m]);
as are:
void f(double (*restrict a)[5]);
void f(double a[restrict][5]);
void f(double a[restrict 3][5]);
void f(double a[restrict static 3][5]);
(Note that the last declaration also specifies that the argument corresponding to a in any call to f must be a
non-null pointer to the first of at least three arrays of 5 doubles, which the others do not.)
Forward references: function definitions (Function definitions), type names (Type names).
Type names#
Semantics
In several contexts, it is necessary to specify a type. This is accomplished using a type name, which is syntactically a declaration for a function or an object of that type that omits the identifier. [106]
EXAMPLE The constructions:
(a) int
(b) int *
(c) int *[3]
(d) int (*)[3]
(e) int (*)[*]
(f) int *()
(g) int(*)(void)
(h) int (*const [])(unsigned int, ...)
name respectively the types (a) int, (b) pointer to int, (c) array of three pointers to int, (d) pointer to an
array of three ints, (e) pointer to a variable length array of an unspecified number of ints, (f) function
with no parameter specification returning a pointer to int, (g) pointer to function with no parameters
returning an int, and (h) array of an unspecified number of constant pointers to functions, each with one
parameter that has type unsigned int and an unspecified number of other parameters, returning an
int.
Type definitions#
Constraints
If a typedef name specifies a variably modified type then it shall have block scope.
Semantics
In a declaration whose storage-class specifier is typedef, each declarator defines an
identifier to be a typedef name that denotes the type specified for the identifier in the way
described in Declarators. Any array size expressions associated with variable length array
declarators are evaluated each time the declaration of the typedef name is reached in the
order of execution. A typedef declaration does not introduce a new type, only a
synonym for the type so specified. That is, in the following declarations:
typedef T type_ident;
type_ident D;
type_ident is defined as a typedef name with the type specified by the declaration
specifiers in T (known as T ), and the identifier in D has the type “derived-declarator-
type-list T” where the derived-declarator-type-list is specified by the declarators of D. A
typedef name shares the same name space as other identifiers declared in ordinary
declarators.
EXAMPLE 1 After
typedef int MILES, KLICKSP();
typedef struct { double hi, lo; } range;
the constructions
MILES distance;
extern KLICKSP *metricp;
range x;
range z, *zp;
are all valid declarations. The type of distance is int, that of metricp is “pointer to function with no
parameter specification returning int”, and that of x and z is the specified structure; zp is a pointer to
such a structure. The object distance has a type compatible with any other int object.
EXAMPLE 2 After the declarations
typedef struct s1 { int x; } t1, *tp1;
typedef struct s2 { int x; } t2, *tp2;
type t1 and the type pointed to by tp1 are compatible. Type t1 is also compatible with type struct
s1, but not compatible with the types struct s2, t2 the type pointed to by tp2 or int.
The following obscure constructions
typedef signed int t;
typedef int plain;
struct tag {
unsigned t:4;
const t:5;
plain r:5;
};
declare a typedef name t with type signed int, a typedef name plain with type int and a structure
with three bit-field members, one named t` that contains values in the range [0, 15], an unnamed const-
qualified bit-field which (if it could be accessed) would contain values in either the range [-15, +15] or
[-16, +15], and one named r that contains values in one of the ranges [0, 31], [-15, +15] or [-16, +15].
(The choice of range is implementation-defined.) The first two bit-field declarations differ in that
unsigned is a type specifier (which forces t to be the name of a structure member), while const is a
type qualifier (which modifies t which is still visible as a typedef name). If these declarations are followed
in an inner scope by
t f(t (t));
long t;
then a function f is declared with type “function returning signed int with one unnamed parameter
with type pointer to function returning signed int with one unnamed parameter with type signed
int”, and an identifier t with type long int.
EXAMPLE 4 On the other hand, typedef names can be used to improve code readability. All three of the
following declarations of the signal function specify exactly the same type, the first without making use
of any typedef names.
typedef void fv(int), (*pfv)(int);
void (*signal(int, void (*)(int)))(int);
fv *signal(int, fv *);
pfv signal(int, pfv);
EXAMPLE 5 If a typedef name denotes a variable length array type, the length of the array is fixed at the time the typedef name is defined, not each time it is used:
void copyt(int n)
{
typedef int B[n]; // B is n ints, n evaluated now
n += 1;
B a; // a is n ints, n without += 1
int b[n]; // a and b are different sizes
for (int i = 1; i < n;
a[i-1] = b[i];
}
Initialization#
Constraints
No initializer shall attempt to provide a value for an object not contained within the entity being initialized.
The type of the entity to be initialized shall be an array of unknown size or an object type that is not a variable length array type.
All the expressions in an initializer for an object that has static storage duration shall be constant expressions or string literals.
If the declaration of an identifier has block scope, and the identifier has external or internal linkage, the declaration shall have no initializer for the identifier.
If a designator has the form
\([ constant-expression ]\)
then the current object (defined below) shall have array type and the expression shall be an integer constant expression. If the array is of unknown size, any nonnegative value is valid.
If a designator has the form
\(. identifier\)
then the current object (defined below) shall have structure or union type and the identifier shall be the name of a member of that type.
Semantics
An initializer specifies the initial value stored in an object.
Except where explicitly stated otherwise, for the purposes of this subclause unnamed members of objects of structure and union type do not participate in initialization. Unnamed members of structure objects have indeterminate value even after initialization.
If an object that has automatic storage duration is not initialized explicitly, its value is indeterminate. If an object that has static storage duration is not initialized explicitly, then:
if it has pointer type, it is initialized to a null pointer;
if it has arithmetic type, it is initialized to (positive or unsigned) zero;
if it is an aggregate, every member is initialized (recursively) according to these rules;
if it is a union, the first named member is initialized (recursively) according to these rules.
The initializer for a scalar shall be a single expression, optionally enclosed in braces. The initial value of the object is that of the expression (after conversion); the same type constraints and conversions as for simple assignment apply, taking the type of the scalar to be the unqualified version of its declared type.
The rest of this subclause deals with initializers for objects that have aggregate or union type.
The initializer for a structure or union object that has automatic storage duration shall be either an initializer list as described below, or a single expression that has compatible structure or union type. In the latter case, the initial value of the object, including unnamed members, is that of the expression.
An array of character type may be initialized by a character string literal, optionally enclosed in braces. Successive characters of the character string literal (including the terminating null character if there is room or if the array is of unknown size) initialize the elements of the array.
An array with element type compatible with wchar_t may be initialized by a wide
string literal, optionally enclosed in braces. Successive wide characters of the wide string
literal (including the terminating null wide character if there is room or if the array is of
unknown size) initialize the elements of the array.
Otherwise, the initializer for an object that has aggregate or union type shall be a brace- enclosed list of initializers for the elements or named members.
Each brace-enclosed initializer list has an associated current object. When no designations are present, subobjects of the current object are initialized in order according to the type of the current object: array elements in increasing subscript order, structure members in declaration order, and the first named member of a union. [107] In contrast, a designation causes the following initializer to begin initialization of the subobject described by the designator. Initialization then continues forward in order, beginning with the next subobject after that described by the designator. [108]
Each designator list begins its description with the current object associated with the closest surrounding brace pair. Each item in the designator list (in order) specifies a particular member of its current object and changes the current object for the next designator (if any) to be that member. [109] The current object that results at the end of the designator list is the subobject to be initialized by the following initializer.
The initialization shall occur in initializer list order, each initializer provided for a particular subobject overriding any previously listed initializer for the same subobject; [110] all subobjects that are not initialized explicitly shall be initialized implicitly the same as objects that have static storage duration.
If the aggregate or union contains elements or members that are aggregates or unions, these rules apply recursively to the subaggregates or contained unions. If the initializer of a subaggregate or contained union begins with a left brace, the initializers enclosed by that brace and its matching right brace initialize the elements or members of the subaggregate or the contained union. Otherwise, only enough initializers from the list are taken to account for the elements or members of the subaggregate or the first member of the contained union; any remaining initializers are left to initialize the next element or member of the aggregate of which the current subaggregate or contained union is a part.
If there are fewer initializers in a brace-enclosed list than there are elements or members of an aggregate, or fewer characters in a string literal used to initialize an array of known size than there are elements in the array, the remainder of the aggregate shall be initialized implicitly the same as objects that have static storage duration. 22 If an array of unknown size is initialized, its size is determined by the largest indexed element with an explicit initializer. At the end of its initializer list, the array no longer has incomplete type.
The order in which any side effects occur among the initialization list expressions is unspecified. [111]
EXAMPLE 1 Provided that <complex.h> has been #included, the declarations
int i = 3.5;
double complex c = 5 + 3 * I;
define and initialize i with the value 3 and c with the value 5. 0 + i3. 0.
EXAMPLE 2 The declaration
int x[] = { 1, 3, 5 };
defines and initializes x as a one-dimensional array object that has three elements, as no size was specified
and there are three initializers.
EXAMPLE 3 The declaration
int y[4][3] = {
{ 1, 3, 5 },
{ 2, 4, 6 },
{ 3, 5, 7 },
};
is a definition with a fully bracketed initialization: 1, 3, and 5 initialize the first row of y (the array object
y[0]), namely y[0][0], y[0][1] and y[0][2]. Likewise the next two lines initialize y[1] and
y[2]. The initializer ends early, so y[3] is initialized with zeros. Precisely the same effect could have
been achieved by
int y[4][3] = {
1, 3, 5, 2, 4, 6, 3, 5, 7
};
The initializer for y[0] does not begin with a left brace, so three items from the list are used. Likewise the
next three are taken successively for y[1] and y[2].
EXAMPLE 4 The declaration
int z[4][3] = {
{ 1 }, { 2 }, { 3 }, { 4 }
};
initializes the first column of z as specified and initializes the rest with zeros.
EXAMPLE 5 The declaration
struct { int a[3], b; } w[] = { { 1 }, 2 };
is a definition with an inconsistently bracketed initialization. It defines an array with two element
structures: w[0].a[0] is 1 and w[1].a[0] is 2; all the other elements are zero.
EXAMPLE 6 The declaration
short q[4][3][2] = {
{ 1 },
{ 2, 3 },
{ 4, 5, 6 }
};
contains an incompletely but consistently bracketed initialization. It defines a three-dimensional array
object: q[0][0][0] is 1, q[1][0][0] is 2, q[1][0][1] is 3, and 4, 5, and 6 initialize
q[2][0][0], q[2][0][1] and q[2][1][0], respectively; all the rest are zero. The initializer for
q[0][0] does not begin with a left brace, so up to six items from the current list may be used. There is
only one, so the values for the remaining five elements are initialized with zero. Likewise, the initializers
for q[1][0] and q[2][0] do not begin with a left brace, so each uses up to six items, initializing their
respective two-dimensional subaggregates. If there had been more than six items in any of the lists, a
diagnostic message would have been issued. The same initialization result could have been achieved by:
short q[4][3][2] = {
1, 0, 0, 0, 0, 0,
2, 3, 0, 0, 0, 0,
4, 5, 6
};
or by:
short q[4][3][2] = {
{
{ 1 },
},
{
{ 2, 3 },
},
{
{ 4, 5 },
{ 6 },
}
};
in a fully bracketed form.
Note that the fully bracketed and minimally bracketed forms of initialization are, in general, less likely to cause confusion.
EXAMPLE 7 One form of initialization that completes array types involves typedef names. Given the declaration
typedef int A[]; // OK - declared with block scope
the declaration
A a = { 1, 2 }, b = { 3, 4, 5 };
is identical to
int a[] = { 1, 2 }, b[] = { 3, 4, 5 };
due to the rules for incomplete types.
EXAMPLE 8 The declaration
char s[] = "abc", t[3] = "abc";
defines “plain” char array objects s and t whose elements are initialized with character string literals.
This declaration is identical to
char s[] = { 'a', 'b', 'c', '\0' },
t[] = { 'a', 'b', 'c' };
The contents of the arrays are modifiable. On the other hand, the declaration
char *p = "abc";
defines p with type “pointer to char” and initializes it to point to an object with type “array of char”
with length 4 whose elements are initialized with a character string literal. If an attempt is made to use p to
modify the contents of the array, the behavior is undefined.
EXAMPLE 9 Arrays can be initialized to correspond to the elements of an enumeration by using designators:
enum { member_one, member_two };
const char *nm[] = {
[member_two] = "member two",
[ember_one] = "member one",
};
EXAMPLE 10 Structure members can be initialized to nonzero values without depending on their order:
div_t answer = { .quot = 2, .rem = -1 };
EXAMPLE 11 Designators can be used to provide explicit initialization when unadorned initializer lists might be misunderstood:
struct { int a[3], b; } w[] =
{ [0].a = {1}, [1].a[0] = 2 };
EXAMPLE 12 Space can be “allocated” from both ends of an array by using a single designator:
int a[MAX] = {
1, 3, 5, 7, 9, [MAX-5] = 8, 6, 4, 2, 0
};
In the above, if MAX is greater than ten, there will be some zero-valued elements in the middle; if it is less
than ten, some of the values provided by the first five initializers will be overridden by the second five.
EXAMPLE 13 Any member of a union can be initialized:
union { /* ... */ } u = { .any_member = 42 };
Forward references: common definitions <stddef.h> (Common definitions <stddef.h>).
Statements and blocks#
Semantics
A statement specifies an action to be performed. Except as indicated, statements are executed in sequence.
A block allows a set of declarations and statements to be grouped into one syntactic unit. The initializers of objects that have automatic storage duration, and the variable length array declarators of ordinary identifiers with block scope, are evaluated and the values are stored in the objects (including storing an indeterminate value in objects without an initializer) each time the declaration is reached in the order of execution, as if it were a statement, and within each declaration in the order that declarators appear.
A full expression is an expression that is not part of another expression or of a declarator.
Each of the following is a full expression: an initializer; the expression in an expression
statement; the controlling expression of a selection statement (if or switch); the
controlling expression of a while or do statement; each of the (optional) expressions of
a for statement; the (optional) expression in a return statement. The end of a full
expression is a sequence point.
Forward references: expression and null statements (Expression and null statements), selection statements
(Selection statements), iteration statements (Iteration statements), the return statement (The return statement).
Labeled statements#
Constraints
A case or default label shall appear only in a switch statement. Further
constraints on such labels are discussed under the switch statement.
Label names shall be unique within a function.
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 (The goto statement), the switch statement (The switch statement).
Compound statement#
Semantics
A compound statement is a block.
Expression and null statements#
Semantics
The expression in an expression statement is evaluated as a void expression for its side effects. [112]
A null statement (consisting of just a semicolon) performs no operations.
EXAMPLE 1 If a function call is evaluated as an expression statement for its side effects only, the discarding of its value may be made explicit by converting the expression to a void expression by means of a cast:
int p(int);
/* ... */
(void)p(0);
EXAMPLE 2 In the program fragment
char *s;
/* ... */
while (*s++ != '\0')
;
a null statement is used to supply an empty loop body to the iteration statement.
EXAMPLE 3 A null statement may also be used to carry a label just before the closing } of a compound
statement.
while (loop1) {
/* ... */
while (loop2) {
/* ... */
if (want_out)
goto end_loop1;
/* ... */
}
/* ... */
end_loop1: ;
}
Forward references: iteration statements (Iteration statements).
Selection statements#
Semantics
A selection statement selects among a set of statements depending on the value of a controlling expression.
A selection statement is a block whose scope is a strict subset of the scope of its enclosing block. Each associated substatement is also a block whose scope is a strict subset of the scope of the selection statement.
The if statement#
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 nearest preceding if that is allowed by the
syntax.
The switch statement#
Constraints
The controlling expression of a switch statement shall have integer type.
If a switch statement has an associated case or default label within the scope of an
identifier with a variably modified type, the entire switch statement shall be within the
scope of that identifier. [113]
The expression of each case label shall be an integer constant expression and no two of
the case constant expressions in the same switch statement shall have the same value
after conversion. There may be at most one default label in a switch statement.
(Any enclosed switch statement may have a default label or case constant
expressions with values that duplicate case constant expressions in the enclosing
switch statement.)
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 integer promotions are performed on the controlling expression. The constant
expression in each case label is converted to the promoted type of the controlling
expression. If a converted value matches that of the promoted controlling expression,
control jumps to the statement following the matched case label. Otherwise, if there is
a default label, control jumps to the labeled statement. If no converted case constant
expression matches and there is no default label, no part of the switch body is
executed.
Implementation limits
As discussed in Translation limits, the implementation may limit the number of case values in a
switch statement.
EXAMPLE In the artificial program fragment
switch (expr)
{
int i = 4;
f(i);
case 0:
i = 17;
/* falls through into default code */
default:
printf("%d\n", i);
}
the object whose identifier is i exists with automatic storage duration (within the block) but is never
initialized, and thus if the controlling expression has a nonzero value, the call to the printf function will
access an indeterminate value. Similarly, the call to the function f cannot be reached.
Iteration statements#
Constraints
The controlling expression of an iteration statement shall have scalar type.
The declaration part of a for statement shall only declare identifiers for objects having
storage class auto or register.
Semantics
An iteration statement causes a statement called the loop body to be executed repeatedly until the controlling expression compares equal to 0.
An iteration statement is a block whose scope is a strict subset of the scope of its enclosing block. The loop body is also a block whose scope is a strict subset of the scope of the iteration statement.
The while statement#
The evaluation of the controlling expression takes place before each execution of the loop body.
The do statement#
The evaluation of the controlling expression takes place after each execution of the loop body.
The for statement#
The statement:
for ( clause-1 ; expression-2 ; expression-3 ) statement
behaves as follows: The expression expression-2 is the controlling expression that is evaluated before each execution of the loop body. The expression expression-3 is evaluated as a void expression after each execution of the loop body. If clause-1 is a declaration, the scope of any variables it declares is the remainder of the declaration and the entire loop, including the other two expressions; it is reached in the order of execution before the first evaluation of the controlling expression. If clause-1 is an expression, it is evaluated as a void expression before the first evaluation of the controlling expression. [114]
Both clause-1 and expression-3 can be omitted. An omitted expression-2 is replaced by a nonzero constant.
Jump statements#
Semantics
A jump statement causes an unconditional jump to another place.
The goto statement#
Constraints
The identifier in a goto statement shall name a label located somewhere in the enclosing
function. A goto statement shall not jump from outside the scope of an identifier having
a variably modified type to inside the scope of that identifier.
Semantics
A goto statement causes an unconditional jump to the statement prefixed by the named
label in the enclosing function.
EXAMPLE 1 It is sometimes convenient to jump into the middle of a complicated set of statements. The following outline presents one possible approach to a problem based on these three assumptions:
The general initialization code accesses objects only visible to the current function.
The general initialization code is too large to warrant duplication.
The code to determine the next operation is at the head of the loop. (To allow it to be reached by continue statements, for example.)
/* ... */
goto first_time;
for (;;) {
// determine next operation
/* ... */
if (need to reinitialize) {
// reinitialize-only code
/* ... */
first_time:
// general initialization code
/* ... */
continue;
}
// handle other operations
/* ... */
}
EXAMPLE 2 A goto statement is not allowed to jump past any declarations of objects with variably
modified types. A jump within the scope, however, is permitted.
goto lab3; // invalid: going INTO scope of VLA.
{
double a[n];
a[j] = 4.4;
lab3:
a[j] = 3.3;
goto lab4; // valid: going WITHIN scope of VLA.
a[j] = 5.5;
lab4:
a[j] = 6.6;
}
goto lab4; // invalid: going INTO scope of VLA.
The continue statement#
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 (/* ... */) {
/* ... */
continue;
/* ... */
contin: ;
}
do {
/* ... */
continue;
/* ... */
contin: ;
} while (/* ... */);
for (/* ... */) {
/* ... */
continue;
/* ... */
contin: ;
}
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;. [115]
The break statement#
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.
The return statement#
Constraints
A return statement with an expression shall not appear in a function whose return type
is void. A return statement without an expression shall only appear in a function
whose return type is void.
Semantics
A return statement terminates execution of the current function and returns control to
its caller. A function may have any number of return statements.
If a return statement with an expression is executed, the value of the expression is
returned to the caller as the value of the function call expression. If the expression has a
type different from the return type of the function in which it appears, the value is
converted as if by assignment to an object having the return type of the function. [116]
EXAMPLE In:
struct s { double i; } f(void);
union {
struct {
int f1;
struct s f2;
} u1;
struct {
struct s f3;
int f4;
} u2;
} g;
struct s f(void)
{
return g.u1.f2;
}
/* ... */
g.u2.f3 = f();
there is no undefined behavior, although there would be if the assignment were done directly (without using a function call to fetch the value).
External definitions#
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 whose result is an integer constant), there shall be exactly one external definition
for the identifier in the translation unit.
Semantics
As discussed in Program Structure, 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
Declarations, 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
(other than an inline definition) 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
whose result is an integer constant), somewhere in the entire program there shall be
exactly one external definition for the identifier; otherwise, there shall be no more than
one. [117]
Function definitions#
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.
The intent is that the type category in a function definition cannot be inherited from a typedef:
typedef int F(void); // type F is ‘‘function with no parameters
// returning int’’
F f, g; // fand g both have type compatible with F
F f { /* ... */ } // WRONG: syntax/constraint error
F g() { /* ... */ } // WRONG: declares that g returns a function
int f(void) { /* ... */ } // RIGHT: f has type compatible with F
int g() { /* ... */ } // RIGHT: g has type compatible with F
F *e(void) { /* ... */ } // e returns a pointer to a function
F *((e))(void) { /* ... */ } // same: parentheses irrelevant
int (*fp)(void); // fp points to a function that has type F
F*Fp; //Fp points to a function that has type F
The return type of a function shall be void or an object type other than array type.
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 case there shall not be an identifier. No declaration list
shall follow.
If the declarator includes an identifier list, each declaration in the declaration list shall
have at least one declarator, those declarators shall declare only identifiers from the
identifier list, and every identifier in the identifier list shall be declared. An identifier
declared as a typedef name shall not be redeclared as a parameter. The declarations in the
declaration list shall contain no 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, [118] the types of the parameters shall be declared in a following declaration list. In either case, the type of each parameter is adjusted as described in Function declarators (including prototypes) for a parameter type list; the resulting type shall be an object type.
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.
Each parameter has automatic storage duration. Its identifier is an lvalue, which is in effect declared at the head of the compound statement that constitutes the function body (and therefore cannot be redeclared in the function body except in an enclosed block). The layout of the storage for parameters is unspecified. 10 On entry to the function, the size expressions of each variably modified parameter are evaluated and the value of each argument expression is converted to the type of the corresponding parameter as if by assignment. (Array expressions and function designators as arguments were converted to pointers before the call.)
After all parameters have been assigned, the compound statement that constitutes the body of the function definition is executed.
If the } that terminates a function is reached, and the value of the function call is used by
the caller, the behavior is undefined.
EXAMPLE 1 In the following:
extern int max(int a, int b)
{
return a > b ? a : b;
}
extern is the storage-class specifier and int is the type specifier;`` max(int a, int b)`` is the
function declarator; and
{ return a > b ? a : b; }
is the function body. The following similar definition uses the identifier-list form for the parameter declarations:
extern int max(a, b)
int a, b;
{
return a > b ? a : b;
}
Here int a, b; is the declaration list for the parameters. The difference between these two definitions is
that the first form acts as a prototype declaration that forces conversion of the arguments of subsequent calls
to the function, whereas the second form does not.
EXAMPLE 2 To pass one function to another, one might say
int f(void);
/* ... */
g(f);
Then the definition of g might read
void g(int (*funcp)(void))
{
/* ... */
(*funcp)() /* or funcp() ... */
}
or, equivalently,
void g(int func(void))
{
/* ... */
func() /* or (*func)() ... */
}
External object definitions#
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.
EXAMPLE 1
int i1 = 1; // definition, external linkage
static int i2 = 2; // definition, internal linkage
extern int i3 = 3; // definition, external linkage
int i4; // tentative definition, external linkage
static int i5; // tentative definition, internal linkage
int i1; // valid tentative definition, refers to pre vious
int i2; // 4.2.2 renders undefined, linkage disagreement
int i3; // valid tentative definition, refers to pre vious
int i4; // valid tentative definition, refers to pre vious
int i5; // 4.2.2 renders undefined, linkage disagreement
extern int i1; // refers to pre vious, whose linkage is external
extern int i2; // refers to pre vious, whose linkage is internal
extern int i3; // refers to pre vious, whose linkage is external
extern int i4; // refers to pre vious, whose linkage is external
extern int i5; // refers to pre vious, whose linkage is internal
5 EXAMPLE 2 If at the end of the translation unit containing
int i[];
the array i still has incomplete type, the implicit initializer causes it to have one element, which is set to
zero on program startup.
Preprocessing directives#
This is dicussed in chapter Preprocessing Directives.
Future language directions#
Floating types#
Future standardization may include additional floating-point types, including those with
greater range, precision, or both than long double.
Linkages of identifiers#
Declaring an identifier with internal linkage at file scope without the static storage-class
specifier is an obsolescent feature.
External names#
Restriction of the significance of an external name to fewer than 255 characters (considering each universal character name or extended source character as a single character) is an obsolescent feature that is a concession to existing implementations.
Character escape sequences#
Lowercase letters as escape sequences are reserved for future standardization. Other characters may be used in extensions.
Storage-class specifiers#
The placement of a storage-class specifier other than at the beginning of the declaration specifiers in a declaration is an obsolescent feature.
Function declarators#
The use of function declarators with empty parentheses (not prototype-format parameter type declarators) is an obsolescent feature.
Function definitions#
The use of function definitions with separate parameter identifier and declaration lists (not prototype-format parameter type and identifier declarators) is an obsolescent feature.
Pragma directives#
Pragmas whose first preprocessing token is STDC are reserved for future standardization.
Predefined macro names#
Macro names beginning with __STDC_ are reserved for future standardization.
Comments#
Except within a character constant, a string literal, or a comment, the characters
/*introduce a comment. The contents of such a comment are examined only to identify multibyte characters and to find the characters*/that terminate it. [50]Except within a character constant, a string literal, or a comment, the characters
//ntroduce a comment that includes all multibyte characters up to, but not including, the next new-line character. The contents of such a comment are examined only to identify multibyte characters and to find the terminating new-line character.EXAMPLE