<stddef.h> header file defines several types and
macros, some of which are also defined in other header files.
size_tand a value that is the offset, in bytes, to the structure member (specified by member-designator) from the beginning of its structure (specified by type). The member-designator is such that the expression
&(t.member-designator)evaluates to an address constant given the following:
static type t;
If the specified member is a bit field, the behavior is undefined.