stddef.h

The stddef.h header file defines the following types:

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.
ssize_t: similar to size_t, but must be a signed type.

The stddef.h header 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).