OFFSETOF(3) Library Functions Manual OFFSETOF(3)

offsetof — offset of a structure member

#include <stddef.h>

size_t
offsetof(type, member);

The () macro expands to an integer constant expression of type size_t and yields the offset, in bytes, of the field member from the start of the structure type.

A compiler error will result if member is not aligned to a byte boundary (i.e. it is a bit-field).

The offsetof() macro conforms to ANSI X3.159-1989 (“ANSI C89”).

OpenBSD 7.8 June 5, 2013 OFFSETOF(3)