offsetof

From cppreference.com
< c | types
Defined in header <stddef.h>
#define offsetof(type, member) /*implementation-defined*/

The macro offsetof expands to a constant of type size_t, the value of which is the offset, in bytes, from the beginning of an object of specified type to its specified member, including padding if any.

[edit] See also

unsigned integer type returned by the sizeof operator
(typedef)