stddef.h(7POSIX) POSIX Programmer's Manual stddef.h(7POSIX)
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
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 Null pointer constant. The macro shall expand to an integer constant expression
with the value 0 cast to type void *.
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
The <stddef.h> header shall define the following types:
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 charac‐
ter set shall have a code value equal to its value when used as the lone charac‐
ter in an integer character constant if an implementation does not define
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 get‐
The following sections are informative.
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.
The System Interfaces volume of POSIX.1‐2008, confstr()
The Shell and Utilities volume of POSIX.1‐2008, getconf
Portions of this text are reprinted and reproduced in electronic form from IEEE Std
1003.1, 2013 Edition, Standard for Information Technology -- Portable Operating System
Interface (POSIX), The Open Group Base Specifications Issue 7, Copyright (C) 2013 by the
Institute of Electrical and Electronics Engineers, Inc and The Open Group. (This is
POSIX.1-2008 with the 2013 Technical Corrigendum 1 applied.) In the event of any discrep‐
ancy between this version and the original IEEE and The Open Group Standard, the original
IEEE and The Open Group Standard is the referee document. The original Standard can be
obtained online at http://www.unix.org/online.html .
Any typographical or formatting errors that appear in this page are most likely to have
been introduced during the conversion of the source files to man page format. To report
such errors, see https://www.kernel.org/doc/man-pages/reporting_bugs.html .
IEEE/The Open Group 2013 stddef.h(7POSIX)