stddef.h man page


This manual page is part of the POSIX Programmer's Manual. The Linux implementation of this interface may differ (consult the corresponding Linux manual page for details of Linux behavior), or the interface may not be implemented on Linux.

stddef.h — standard type definitions


#include <stddef.h>


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.1‐2008 defers to the ISO C standard.

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


Null pointer constant. The macro shall expand to an integer constant expression with the value 0 cast to type void *.

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:


Signed integer type of the result of subtracting two pointers.


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__.


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.

The following sections are informative.

Application Usage



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.1‐2008 requires the cast and therefore need not be implementation-defined.

Future Directions


See Also

<sys_types.h>, <wchar.h>

The System Interfaces volume of POSIX.1‐2008, confstr()

The Shell and Utilities volume of POSIX.1‐2008, getconf


2013 IEEE/The Open Group POSIX Programmer's Manual