stddef —
standard type definitions
The <stddef.h> header defines
  the following types and macros:
  - ptrdiff_t, a signed integer type of the result of
      subtracting two pointers;
- size_t, an unsigned integer type of the result of
      the sizeof() operator;
- 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;
- NULL,
      which expands to an implementation-defined null pointer constant; and
- 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.
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.