stddef.h
The stddef.h header file contains definitions of the commonly used
pointers, variables, and types, from the typedef statements,
as listed below:
- ptrdiff_t
- The signed long type of the result of subtracting two pointers.
- size_t
- typedef for the type of the value returned by sizeof.
- wchar_t
- typedef for a wide-character constant.
- max_align_t
- An object type whose alignment is as great as is supported in all contexts.
stddef.h defines the macros NULL and offsetof. NULL is a pointer that never points 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)