Structure Measurement (The GNU C Library)

#### A.5.4 Structure Field Offset Measurement ¶

You can use `offsetof` to measure the location within a structure type of a particular structure member.

Macro: `size_t` offsetof `(type, member)` ¶

  Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

  This expands to an integer constant expression that is the offset of the structure member named member in the structure type type. For example, `offsetof (struct s, elem)` is the offset, in bytes, of the member `elem` in a `struct s`.

  This macro won’t work if member is a bit field; you get an error from the C compiler in that case.