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.
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
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
The <stddef.h> header shall define the following types:
ptrdiff_t Signed integer type of the result of subtracting two
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
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
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.
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 discrepancy 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
IEEE/The Open Group 2013 stddef.h(0P)