NAME

stddef.h — standard type definitions

SYNOPSIS

#include <stddef.h>

DESCRIPTION

^[CX] The functionality described on this reference page is aligned with the ISO C standard. Any conflict between the requirements described here and the ISO C standard is unintentional. This volume of POSIX.1-2024 defers to the ISO C standard.

The <stddef.h> header shall define the following macros:

NULL

Null pointer constant. ^[CX]  The macro shall expand to an integer constant expression with the value 0 cast to type void *. Additionally, any pointer object whose representation has all bits set to zero, perhaps by memset() to 0 or by calloc(), shall be treated as a null pointer.

offsetof(type, member-designator)

Integer constant expression of type size_t, the value of which is the offset in bytes to the structure member (member-designator), from the beginning of its structure (type).

The <stddef.h> header shall define the following types:

max_align_t

Object type whose alignment is the greatest fundamental alignment.

ptrdiff_t

Signed integer type of the result of subtracting two pointers.

wchar_t

Integer type whose range of values can represent distinct codes for all members of the largest extended character set specified among the supported locales; the null character shall have the code value zero. Each member of the basic character set shall have a code value equal to its value when used as the lone character in an integer character constant if an implementation does not define __STDC_MB_MIGHT_NEQ_WC__.

size_t

Unsigned integer type of the result of the sizeof operator.

The implementation shall support one or more programming environments in which the widths of ptrdiff_t, size_t, and wchar_t are no greater than the width of type long. The names of these programming environments can be obtained using the confstr() function or the getconf utility.

APPLICATION USAGE

None.

RATIONALE

The ISO C standard does not require the NULL macro to include the cast to type void * and specifies that the NULL macro be implementation-defined. POSIX.1-2024 requires the cast and therefore need not be implementation-defined.

Likewise, the ISO C standard does not require a pointer object whose representation has all bits set to zero to be treated as a null pointer. While there has been historical hardware where non-zero patterns were more efficient for use as the canonical null pointer, no known POSIX system has tried to target such hardware. However, though unlikely in modern hardware, a compiler is still allowed to treat more than one bit pattern as a representation of the null pointer (all such patterns will compare equal to one another, and unequal to any pointer to any other object). Thus, applications should not assume that a pointer object with non-zero representation is not a null pointer.

FUTURE DIRECTIONS

None.

SEE ALSO

<sys/types.h>, <wchar.h>

XSH confstr

XCU getconf

CHANGE HISTORY

First released in Issue 4. Derived from the ANSI C standard.

Issue 7

This reference page is clarified with respect to macros and symbolic constants.

SD5-XBD-ERN-53 is applied, updating the definition of wchar_t to align with ISO/IEC 9899:1999 standard, Technical Corrigendum 3.

Issue 8

Austin Group Defect 940 is applied, adding a requirement that any pointer object whose representation has all bits set to zero is interpreted as a null pointer.

Austin Group Defect 1302 is applied, adding max_align_t.