stddef.h — typedef statements

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)