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)
- 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.
