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STDDEF(3) Library Functions Manual STDDEF(3)

stddef
standard type definitions

#include <stddef.h>

The <stddef.h> header defines the following types and macros:
  1. ptrdiff_t, a signed integer type of the result of subtracting two pointers;
  2. size_t, an unsigned integer type of the result of the sizeof() operator;
  3. wchar_t, an integer type whose range of values can represent distinct wide-character codes for all members of the largest character set specified among the supported locales: the null character has the code value 0 and each member of the character set has a code value equal to its value when used as the lone character in an integer character constant;
  4. , which expands to an implementation-defined null pointer constant; and
  5. offsetof(), a macro that expands to an integer constant as described in offsetof(3).

Some of the described types and macros may appear also in other headers.

offsetof(3), stdlib(3), unistd(3)

As described here, the <stddef.h> header conforms to ISO/IEC 9899:1999 (“ISO C99”) and IEEE Std 1003.1-2001 (“POSIX.1”). Some of the types and macros conform to earlier standards such as ANSI X3.159-1989 (“ANSI C89”).

In the current form the <stddef.h> header was introduced in NetBSD 0.8, the first official release of NetBSD. Some definitions such as NULL were first introduced already in the <nsys/param.h> header of Version 4 AT&T UNIX.
April 10, 2011 NetBSD-7.0.1