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).

[1]There is no linkage between different identifiers.
[2]A function declaration can contain the storage-class specifier static only if it is at file scope; see Storage-class specifiers.
[3]As specified in Scope of Identifiers, the later declaration might hide the prior declaration.

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).

[4]There is only one name space for tags even though three are possible.

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).

[5]The term “constant address” means that two pointers to the object constructed at possibly different times will compare equal. The address may be different during two different executions of the same program.
[6]In the case of a volatile object, the last store need not be explicit in the program.
[7]Leaving the innermost block containing the declaration, or jumping to a point in that block or an embedded block prior to the declaration, leaves the scope of the declaration.
[8]The address of such an object is taken implicitly when an array member is accessed.

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.

[9]A type may be incomplete or complete throughout an entire translation unit, or it may change states at different points within a translation unit.
[10]Implementation-defined keywords shall have the form of an identifier reserved for any use as described in 7.1.3.
[11]Therefore, any statement in this Standard about signed integer types also applies to the extended signed integer types.
[12]Therefore, any statement in this Standard about unsigned integer types also applies to the extended unsigned integer types.
[13]The same representation and alignment requirements are meant to imply interchangeability as arguments to functions, return values from functions, and members of unions.
[14]An implementation may define new keywords that provide alternative ways to designate a basic (or any other) type; this does not violate the requirement that all basic types be different. Implementation-defined keywords shall have the form of an identifier reserved for any use as described in 7.1.3.
[15]CHAR_MIN, defined in <limits.h>, will have one of the values 0 or SCHAR_MIN, and this can be used to distinguish the two options. Irrespective of the choice made, char is a separate type from the other two and is not compatible with either.
[16]Since object types do not include incomplete types, an array of incomplete type cannot be constructed.
[17]Note that aggregate type does not include union type because an object with union type can only contain one member at a time.
[18]See Type qualifiers regarding qualified array and function types.
[19]The same representation and alignment requirements are meant to imply interchangeability as arguments to functions, return values from functions, and members of unions.

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).

[20]A positional representation for integers that uses the binary digits 0 and 1, in which the values represented by successive bits are additive, begin with 1, and are multiplied by successive integral powers of 2, except perhaps the bit with the highest position. (Adapted from the American National Dictionary for Information Processing Systems.) A byte contains CHAR_BIT bits, and the values of type unsigned char range from 0 to \(2^{CHAR\_BIT} - 1\).
[21]Thus, an automatic variable can be initialized to a trap representation without causing undefined behavior, but the value of the variable cannot be used until a proper value is stored in it.
[22]Thus, for example, structure assignment need not copy any padding bits.
[23]It is possible for objects x and y with the same effective type T to have the same value when they are accessed as objects of type T, but to have different values in other contexts. In particular, if == is defined for type T, thenx == ydoes not imply that memcmp(&x, &y, sizeof (T)) == 0. Furthermore, x == y does not necessarily imply that x and y have the same value; other operations on values of type T may distinguish between them.

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.

[24]Some combinations of padding bits might generate trap representations, for example, if one padding bit is a parity bit. Regardless, no arithmetic operation on valid values can generate a trap representation other than as part of an exceptional condition such as an overflow, and this cannot occur with unsigned types. All other combinations of padding bits are alternative object representations of the value specified by the value bits.
[25]Some combinations of padding bits might generate trap representations, for example, if one padding bit is a parity bit. Regardless, no arithmetic operation on valid values can generate a trap representation other than as part of an exceptional condition such as an overflow. All other combinations of padding bits are alternative object representations of the value specified by the value bits.

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.

[26]Two types need not be identical to be compatible.
[27]As specified in Scope of Identifiers, the later declaration might hide the prior declaration.

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.
[28]Every over-aligned type is, or contains, a structure or union type with a member to which an extended alignment has been applied.

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 int shall be greater than the rank of long int, which shall be greater than the rank of int, which shall be greater than the rank of short int, which shall be greater than the rank of signed char.
  • The rank of any unsigned integer type shall equal the rank of the corresponding signed integer type, if any.
  • The rank of any standard integer type shall be greater than the rank of any extended integer type with the same width.
  • The rank of char shall equal the rank of signed char and unsigned char.
  • The rank of _Bool shall be less than the rank of all other standard integer types.
  • The rank of any enumerated type shall equal the rank of the compatible integer type (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, and T3, if T1 has greater rank than T2 and T2 has greater rank than T3, then T1 has greater rank than T3.

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 int and unsigned int.
  • A bit-field of type _Bool, int, signed int, or unsigned int.

If an int can represent all values of the original type, the value is converted to an int; otherwise, it is converted to an unsigned int. These are called the integer promotions. [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).

[29]The integer promotions are applied only: as part of the usual arithmetic conversions, to certain argument expressions, to the operands of the unary +, -, and ~ operators, and to both operands of the shift operators, as specified by their respective subclauses.

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.

[30]The rules describe arithmetic on the mathematical value, not the value of a given type of expression.

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.

[31]The remaindering operation performed when a value of integer type is converted to unsigned type need not be performed when a value of real floating type is converted to unsigned type. Thus, the range of portable real floating values is (-1, U type _ MAX+1 ).

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]

[32]For example, addition of a double _Complex and a float entails just the conversion of the float operand to double (and yields a double _Complex result).
[33]The cast and assignment operators are still required to perform their specified conversions as described in Real Floating and Integer and Real Floating Types.

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).

[34]

The name “lvalue” comes originally from the assignment expression E1 = E2, in which the left operand E1 is required to be a (modifiable) lvalue. It is perhaps better considered as representing an object “locator value”. What is sometimes called “rvalue” is in this International Standard described as the “value of an expression”.

An obvious example of an lvalue is an identifier of an object. As a further example, if E is a unary expression that is a pointer to an object, \*E is an lvalue that designates the object to which E points.

[35]Because this conversion does not occur, the operand of the sizeof or _Alignof operator remains a function designator and violates the constraint in The sizeof operator.

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).

[36]The macro NULL is defined in <stddef.h> (and other headers) as a null pointer constant; see 7.17.
[37]The mapping functions for converting a pointer to an integer or an integer to a pointer are intended to be consistent with the addressing structure of the execution environment.
[38]In general, the concept “correctly aligned” is transitive: if a pointer to type A is correctly aligned for a pointer to type B, which in turn is correctly aligned for a pointer to type C, then a pointer to type A is correctly aligned for a pointer to type C.

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).

[39]An additional category, placemarkers, is used internally in translation phase 4 (see The ## Operator); it cannot occur in source files.

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]

[40]One possible specification for imaginary types appears in annex G.

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).

[41]On systems in which linkers cannot accept extended characters, an encoding of the universal character name may be used in forming valid external identifiers. For example, some otherwise unused character or sequence of characters may be used to encode the \u in a universal character name. Extended characters may produce a long external identifier.

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).

[42]Since the name __func__ is reserved for any use by the implementation (Macro Replacement), if any other identifier is explicitly declared using the name __func__, the behavior is undefined.

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).

[43]The disallowed characters are the characters in the basic character set and the code positions reserved by ISO/IEC 10646 for control characters, the character DELETE, and the S-zone (reserved for use by UTF-16).
[44]Short identifiers for characters were first specified in ISO/IEC 10646-1/AMD9:1997.

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

int

long int

long long int

int

unsigned int

long int

unsigned long int

long long int

unsigned long long int

u or U

unsigned int

unsigned long int

unsgined long long int

unsigned int

unsigned long int

unsigned long long int

l or L

long int

long long int

long int

unsigned long int

long long int

unsigned long long int

Both u or U

and l or L

unsigned long int

unsigned long long int

unsgined long int

unsigned long long int

ll or LL long long int

long long int

unsigned long long int

Both u or U

and ll or LL

unsigned long long int unsigned long long int

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]

[45]The specification for the library functions recommends more accurate conversion than required for floating constants (see The strtod, strtof and strtold functions).

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 digits
hexadecimal 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]

[46]The semantics of these characters were discussed in Character display semantics. If any other character follows a backslash, the result is not a token and a diagnostic is required. See “future language directions” (Character escape sequences).

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).

[47]These tokens are sometimes called “digraphs”.
[48]Thus [ and <: behave differently when “stringized (see The # Operator), but can otherwise be freely interchanged.

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).

[49]Thus, sequences of characters that resemble escape sequences cause undefined behavior.

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.

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

"a//b"                      // four-character string literal
#include "//e"              // undefined behavior
// */                       // comment, not syntax error
f = g/**//h;                // equivalent to f = g / h;
//\
i();                        // part of a two-line comment
/\
/ j();                      // part of a two-line comment
#define glue(x,y) x##y
glue(/,/) k();              // syntax error, not comment
/*//*/ l();                 // equivalent to l();
m = n//**/o
+ p;                        // equivalent to m = n + p;
[50]Thus, /* ... */ comments do not nest.

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).

[51]

This paragraph renders undefined statement expressions such as i = ++i + 1;

a[i++] = i;

while allowing

i = i + 1; a[i] = i;

[52]

The syntax specifies the precedence of operators in the evaluation of an expression, which is the same as the order of the major subclauses of this subclause, highest precedence first. Thus, for example, the expressions allowed as the operands of the binary + operator (Additive operators) are those expressions defined in Primary expressions through Additive operators. The exceptions are cast expressions (Cast operators) as operands of unary operators (Unary operators), and an operand contained between any of the following pairs of operators: grouping parentheses () (Primary expressions), subscripting brackets [] (:ref:`4.5.2.1`), function-call parentheses ``() (Function calls), and the conditional operator ?: (Conditional operator).

Within each major subclause, the operators have the same precedence. Left- or right-associativity is indicated in each subclause by the syntax for the expressions discussed therein.

[53]Allocated objects have no declared type.
[54]The intent of this list is to specify those circumstances in which an object may or may not be aliased.
[55]A contracted expression might also omit the raising of floating-point exceptions.
[56]This license is specifically intended to allow implementations to exploit fast machine instructions that combine multiple C operators. As contractions potentially undermine predictability, and can even decrease accuracy for containing expressions, their use needs to be well-defined and clearly documented.

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).

[57]Thus, an undeclared identifier is a violation of the syntax.

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).

[58]Most often, this is the result of converting an identifier that is a function designator.
[59]A function may change the values of its parameters, but these changes cannot affect the values of the arguments. On the other hand, it is possible to pass a pointer to an object, and the function may change the value of the object pointed to. A parameter declared to have array or function type is adjusted to have a pointer type as described in Function definitions.

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).

[60]If &E is a valid pointer expression (where & is the “address-of” operator, which generates a pointer to its operand), the expression (&E)->MOS is the same as E.MOS.

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).

[61]Note that this differs from a cast expression. For example, a cast specifies a conversion to scalar types or void only, and the result of a cast expression is not an lvalue.
[62]For example, subobjects without explicit initializers are initialized to zero.
[63]This allows implementations to share storage for string literals and constant compound literals with the same or overlapping representations.

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).

[64]Thus, &*E is equivalent to E (even if E is a null pointer), and &(E1[E2]) to ((E1)+(E2)). It is always true that if E is a function designator or an lvalue that is a valid operand of the unary & operator, *&E is a function designator or an lvalue equal to E. If *P is an lvalue and T is the name of an object pointer type, *(T)P is an lvalue that has a type compatible with that to which T points. Among the invalid values for dereferencing a pointer by the unary * operator are a null pointer, an address inappropriately aligned for the type of object pointed to, and the address of an object after the end of its lifetime.

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).

[65]When applied to a parameter declared to have array or function type, the sizeof operator yields the size of the adjusted (pointer) type (see Function definitions).

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).

[66]A cast does not yield an lvalue. Thus, a cast to a qualified type has the same effect as a cast to the unqualified version of the type.
[67]If the value of the expression is represented with greater precision or range than required by the type named by the cast (Usual Arithmetic Conversions), then the cast specifies a conversion even if the type of the expression is the same as the named type.

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.

[68]This is often called “truncation toward zero”.

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>).

[69]

Another way to approach pointer arithmetic is first to convert the pointer(s) to character pointer(s): In this scheme the integer expression added to or subtracted from the converted pointer is first multiplied by the size of the object originally pointed to, and the resulting pointer is converted back to the original type. For pointer subtraction, the result of the difference between the character pointers is similarly divided by the size of the object originally pointed to.

When viewed in this way, an implementation need only provide one extra byte (which may overlap another object in the program) just after the end of the object in order to satisfy the “one past the last element” requirements.

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.

[70]The expression a<b<c is not interpreted as in ordinary mathematics. As the syntax indicates, it means (a<b)<c; in other words, “if a is less than b, compare 1 to c; otherwise, compare 0 to c”.

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.

[71]Because of the precedences, a<b == c<d is 1 whenever a<b and c<d have the same truth-value.
[72]Two objects may be adjacent in memory because they are adjacent elements of a larger array or adjacent members of a structure with no padding between them, or because the implementation chose to place them so, even though they are unrelated. If prior invalid pointer operations (such as accesses outside array bounds) produced undefined behavior, subsequent comparisons also produce undefined behavior.

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 *
[73]A conditional expression does not yield an lvalue.

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 _Bool and 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.

[74]The asymmetric appearance of these constraints with respect to type qualifiers is due to the conversion (specified in Other Operands) that changes lvalues to “the value of the expression” and thus removes any type qualifiers that were applied to the type category of the expression (for example, it removes const but not volatile from the type int volatile * const).

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).

[75]A comma operator does not yield an lvalue.

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).

[76]The operand of a sizeof operator is usually not evaluated (The sizeof operator).
[77]An integer constant expression is used to specify the size of a bit-field member of a structure, the value of an enumeration constant, the size of an array, or the value of a case constant. Further constraints that apply to the integer constant expressions used in conditional-inclusion preprocessing directives are discussed in Conditional Inclusion.
[78]Thus, in the following initialization, static int i = 2 || 1 / 0; the expression is a valid integer constant expression with value one.

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).

[79]Function definitions have a different syntax, described in Function definitions.

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).

[80]See “future language directions” (Storage-class specifiers).
[81]The implementation may treat any register declaration simply as an auto declaration. However, whether or not addressable storage is actually used, the address of any part of an object declared with storage-class specifier register cannot be computed, either explicitly (by use of the unary & operator as discussed in Address and indirection operators) or implicitly (by converting an array name to a pointer as discussed in :ref:14.3.2.1`). Thus, the only operator that can be applied to an array declared with storage-class specifier register is sizeof.

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.

  • void
  • char
  • signed char
  • unsigned char
  • short, signed short, short int or signed short int
  • unsigned short or unsigned short int
  • int, signed or signed int
  • unsigned or unsigned int
  • long, signed long, long int or signed long int`
  • unsigned long or unsigned long int
  • long long, signed long long, long long int or signed long long int
  • unsigned long long or unsigned long long int
  • float
  • double
  • long double
  • _Bool
  • float _Complex
  • double _Complex
  • long double _Complex
  • struct or union specifier
  • enum specifier
  • typedef name

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).

[82]Freestanding implementations are not required to provide complex types.

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).

[83]A structure or union can not contain a member with a variably modified type because member names are not ordinary identifiers as defined in Name Spaces of the Identifiers.
[84]The unary & (address-of) operator cannot be applied to a bit-field object; thus, there are no pointers to or arrays of bit-field objects.
[85]As specified in Type specifiers above, if the actual type specifier used is int or a typedef-name defined as int, then it is implementation-defined whether the bit-field is signed or unsigned.
[86]An unnamed bit-field structure member is useful for padding to conform to externally imposed layouts.

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).

[87]Thus, the identifiers of enumeration constants declared in the same scope shall all be distinct from each other and from other identifiers declared in ordinary declarators.
[88]An implementation may delay the choice of which integer type until all enumeration constants have been seen.

Tags

Constraints

A specific type shall have its content defined at most once.

A type specifier of the form:

enum *identifier*

without an enumerator list shall only appear after the type it specifies is complete.

Semantics

All declarations of structure, union, or enumerated types that have the same scope and use the same tag declare the same type. The type is incomplete [89] until the closing brace of the list defining the content, and complete thereafter.

Two declarations of structure, union, or enumerated types which are in different scopes or use different tags declare distinct types. Each declaration of a structure, union, or enumerated type which does not include a tag declares a distinct type.

A type specifier of the form:

struct-or-union identifier(optional) { struct-declaration-list }

or:

enum identifier { enumerator-list }

or:

enum identifier { enumerator-list , }

declares a structure, union, or enumerated type. The list defines the structure content, union content or enumeration content. If an identifier is provided, [90] the type specifier also declares the identifier to be the tag of that type.

A declaration of the form:

struct-or-union identifier ;

specifies a structure or union type and declares the identifier as a tag of that type. [91]

If a type specifier of the form:

struct-or-union identifier

occurs other than as part of one of the above forms, and no other declaration of the identifier as a tag is visible, then it declares an incomplete structure or union type, and declares the identifier as the tag of that type. [88]

If a type specifier of the form:

struct-or-union identifier

or:

enum identifier

occurs other than as part of one of the above forms, and a declaration of the identifier as a tag is visible, then it specifies the same type as that other declaration, and does not redeclare the tag.

EXAMPLE 1 This mechanism allows declaration of a self-referential structure.

struct tnode {
int count;
struct tnode *left, *right;
};

specifies a structure that contains an integer and two pointers to objects of the same type. Once this declaration has been given, the declaration

struct tnode s, *sp;

declares s to be an object of the given type and sp to be a pointer to an object of the given type. With these declarations, the expression sp->left refers to the left struct tnode pointer of the object to which sp points; the expression s.right->count designates the count member of the right struct tnode pointed to from s.

The following alternative formulation uses the typedef mechanism:

typedef struct tnode TNODE;
struct tnode {
  int count;
  TNODE *left, *right;
};
TNODE s, *sp;

EXAMPLE 2 To illustrate the use of prior declaration of a tag to specify a pair of mutually referential structures, the declarations

struct s1 { struct s2 *s2p; /* ... */ }; // D1
struct s2 { struct s1 *s1p; /* ... */ }; // D2

specify a pair of structures that contain pointers to each other. Note, however, that if s2 were already declared as a tag in an enclosing scope, the declaration D1 would refer to it, not to the tag s2 declared in D2. To eliminate this context sensitivity, the declaration

struct s2;

may be inserted ahead of D1. This declares a new tag s2 in the inner scope; the declaration D2 then completes the specification of the new type.

Forward references: declarators (Declarators), array declarators (Array declarators), type definitions (Type definitions).

[89]An incomplete type may only by used when the size of an object of that type is not needed. It is not needed, for example, when a typedef name is declared to be a specifier for a structure or union, or when a pointer to or a function returning a structure or union is being declared. (See incomplete types in Types.) The specification has to be complete before such a function is called or defined.
[90]If there is no identifier, the type can, within the translation unit, only be referred to by the declaration of which it is a part. Of course, when the declaration is of a typedef name, subsequent declarations can make use of that typedef name to declare objects having the specified structure, union, or enumerated type.
[91]A similar construction with enum does not exist.

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 *’’
[92]The implementation may place a const object that is not volatile in a read-only region of storage. Moreover, the implementation need not allocate storage for such an object if its address is never used.
[93]This applies to those objects that behave as if they were defined with qualified types, even if they are never actually defined as objects in the program (such as an object at a memory-mapped input/output address).
[94]A volatile declaration may be used to describe an object corresponding to a memory-mapped input/output port or an object accessed by an asynchronously interrupting function. Actions on objects so declared shall not be “optimized out” by an implementation or reordered except as permitted by the rules for evaluating expressions.
[95]For example, a statement that assigns a value returned by malloc to a single pointer establishes this association between the allocated object and the pointer.
[96]Both of these can occur through the use of typedefs.

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;
}
[97]In other words, E depends on the value of P itself rather than on the value of an object referenced indirectly through P. For example, if identifier p has type (int **restrict`), then the pointer expressions ``p and p+1 are based on the restricted pointer object designated by p, but the pointer expressions *p and p[1] are not.

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).

[98]By using, for example, an alternative to the usual function call mechanism, such as “inline substitution”. Inline substitution is not textual substitution, nor does it create a new function. Therefore, for example, the expansion of a macro used within the body of the function uses the definition it had at the point the function body appears, and not where the function is called; and identifiers refer to the declarations in scope where the body occurs. Likewise, the function has a single address, regardless of the number of inline definitions that occur in addition to the external definition.
[99]For example, an implementation might never perform inline substitution, or might only perform inline substitutions to calls in the scope of an inline declaration.
[100]Since an inline definition is distinct from the corresponding external definition and from any other corresponding inline definitions in other translation units, all corresponding objects with static storage duration are also distinct in each of the 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).

[101]When several “array of” specifications are adjacent, a multidimensional array is declared.
[102]Thus, * can be used only in function declarations that are not definitions (see Function declarators (including prototypes)).

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).

[103]The macros defined in the <stdarg.h> header (Variable arguments <stdarg.h>) may be used to access arguments that correspond to the ellipsis.
[104]See “future language directions” (Function declarators).
[105]If both function types are “old style”, parameter types are not compared.

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.

[106]As indicated by the syntax, empty parentheses in a type name are interpreted as “function with no parameter specification”, rather than redundant parentheses around the omitted identifier.

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>).

[107]If the initializer list for a subaggregate or contained union does not begin with a left brace, its subobjects are initialized as usual, but the subaggregate or contained union does not become the current object: current objects are associated only with brace-enclosed initializer lists.
[108]After a union member is initialized, the next object is not the next member of the union; instead, it is the next subobject of an object containing the union.
[109]Thus, a designator can only specify a strict subobject of the aggregate or union that is associated with the surrounding brace pair. Note, too, that each separate designator list is independent.
[110]Any initializer for the subobject which is overridden and so not used to initialize that subobject might not be evaluated at all.
[111]In particular, the evaluation order need not be the same as the order of subobject initialization.

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).

[112]Such as assignments, and function calls which have side effects.

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.

[113]That is, the declaration either precedes the switch statement, or it follows the last case or default label associated with the switch that is in the block containing the declaration.

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.

[114]Thus, clause-1 specifies initialization for the loop, possibly declaring one or more variables for use in the loop; the controlling expression, expression-2, specifies an evaluation made before each iteration, such that execution of the loop continues until the expression compares equal to 0; and expression-3 specifies an operation (such as incrementing) that is performed after each iteration.

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:

  1. The general initialization code accesses objects only visible to the current function.
  2. The general initialization code is too large to warrant duplication.
  3. The code to determine the next operation is at the head of the loop. (To allow it to be reached by continue statements, for example.)
/* ... */
goto first_time;
for (;;) {
  // determine next operation
  /* ... */
  if (need to reinitialize) {
    // reinitialize-only code
    /* ... */
    first_time:
    // general initialization code
    /* ... */
    continue;
  }
  // handle other operations
  /* ... */
}

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]

[115]Following the contin: label is a null statement.

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).

[116]The return statement is not an assignment. The overlap restriction of subclause 6.5.16.1 does not apply to the case of function return.

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]

[117]Thus, if an identifier declared with external linkage is not used in an expression, there need be no external definition for it.

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)() ... */
}
[118]See “future language directions” (Function definitions).

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.