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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)

This expands to a integer constant expression that is the offset of the structure member named member in a 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.