<stddef.h>

The <stddef.h> include file declares the commonly used pointers, variables, and types as listed below:

ptrdiff_t

typedef for the type of the difference of two pointers

size_t

typedef for the type of the value that is returned by sizeof

wchar_t

typedef for a wide character constant.

The <stddef.h> include file also defines the macros NULL and offsetof. NULL is a pointer that is guaranteed not to point to a data object. The offsetof macro expands to the number of bytes between a structure member and the start of the structure. The offsetof macro has the form:

   offsetof(structure_type, member)

The <stddef.h> include file also declares the extern variable _EXCP_MSGID, an IBM® i extension to C.