..
  Copyright 1988-2022 Free Software Foundation, Inc.
  This is part of the GCC manual.
  For copying conditions, see the copyright.rst file.

.. index:: __builtin_offsetof

.. _offsetof:

Support for offsetof
********************

GCC implements for both C and C++ a syntactic extension to implement
the ``offsetof`` macro.

.. code-block:: c++

  primary:
          "__builtin_offsetof" "(" typename "," offsetof_member_designator ")"

  offsetof_member_designator:
            identifier
          | offsetof_member_designator "." identifier
          | offsetof_member_designator "[" expr "]"

This extension is sufficient such that

.. code-block:: c++

  #define offsetof(type, member)  __builtin_offsetof (type, member)

is a suitable definition of the ``offsetof`` macro.  In C++, :samp:`{type}`
may be dependent.  In either case, :samp:`{member}` may consist of a single
identifier, or a sequence of member accesses and array references.