offsetof(3)

offsetof3

Offset of a structure member

Usage

#include <stddef.h>

size_t

offsetof(type, member);

Overview

The offsetof() macro expands to an integer constant expression of type size_t and yields the offset, in bytes, of the field member from the start of the structure type.

A compiler error will result if member is not aligned to a byte boundary (i.e. it is a bit-field).

Releases

30 releases since FreeBSD 10.0-RELEASE; the text changed 16 times.

  1. 10.0
  2. 10.1
  3. 10.2
  4. 10.3
  5. 10.4
  6. 11.0
  7. 11.1
  8. 11.2
  9. 11.3
  10. 11.4
  11. 12.0
  12. 12.1
  13. 12.2
  14. 12.3
  15. 12.4
  16. 13.0–13.2
  17. 13.3–16.0

Standards

The offsetof() macro conforms to ANSI X3.159-1989 (“ANSI C89”).

From base 16.0-CURRENT-20261027-61c70dc16a79 in FreeBSD 16.0-CURRENT (2026-10-27). Parsed by carta from the page's mdoc source.