gcc/doc/gcc/extensions-to-the-c-language-family/determining-the-alignment-of-functions-types-or-variables.rst

   1 ..
   2   Copyright 1988-2022 Free Software Foundation, Inc.
   3   This is part of the GCC manual.
   4   For copying conditions, see the copyright.rst file.
   5
   6 .. index:: alignment, type alignment, variable alignment
   7
   8 .. _alignment:
   9
  10 Determining the Alignment of Functions, Types or Variables
  11 **********************************************************
  12
  13 The keyword ``__alignof__`` determines the alignment requirement of
  14 a function, object, or a type, or the minimum alignment usually required
  15 by a type.  Its syntax is just like ``sizeof`` and C11 ``_Alignof``.
  16
  17 For example, if the target machine requires a ``double`` value to be
  18 aligned on an 8-byte boundary, then ``__alignof__ (double)`` is 8.
  19 This is true on many RISC machines.  On more traditional machine
  20 designs, ``__alignof__ (double)`` is 4 or even 2.
  21
  22 Some machines never actually require alignment; they allow references to any
  23 data type even at an odd address.  For these machines, ``__alignof__``
  24 reports the smallest alignment that GCC gives the data type, usually as
  25 mandated by the target ABI.
  26
  27 If the operand of ``__alignof__`` is an lvalue rather than a type,
  28 its value is the required alignment for its type, taking into account
  29 any minimum alignment specified by attribute :var-attr:`aligned`
  30 (see :ref:`common-variable-attributes`).  For example, after this
  31 declaration:
  32
  33 .. code-block:: c++
  34
  35   struct foo { int x; char y; } foo1;
  36
  37 the value of ``__alignof__ (foo1.y)`` is 1, even though its actual
  38 alignment is probably 2 or 4, the same as ``__alignof__ (int)``.
  39 It is an error to ask for the alignment of an incomplete type other
  40 than ``void``.
  41
  42 If the operand of the ``__alignof__`` expression is a function,
  43 the expression evaluates to the alignment of the function which may
  44 be specified by attribute :fn-attr:`aligned` (see :ref:`common-function-attributes`).