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You can use offsetof to measure the location within a structure
type of a particular structure 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 memberelemin astruct s.This macro won't work if member is a bit field; you get an error from the C compiler in that case.