.. _alignment:

Inquiring on Alignment of Types or Variables
********************************************

.. index:: alignment

.. index:: type alignment

.. index:: variable alignment

The keyword ``__alignof__`` allows you to inquire about how an object
is aligned, or the minimum alignment usually required by a type.  Its
syntax is just like ``sizeof``.

For example, if the target machine requires a ``double`` value to be
aligned on an 8-byte boundary, then ``__alignof__ (double)`` is 8.
This is true on many RISC machines.  On more traditional machine
designs, ``__alignof__ (double)`` is 4 or even 2.

Some machines never actually require alignment; they allow reference to any
data type even at an odd address.  For these machines, ``__alignof__``
reports the smallest alignment that GCC gives the data type, usually as
mandated by the target ABI.

If the operand of ``__alignof__`` is an lvalue rather than a type,
its value is the required alignment for its type, taking into account
any minimum alignment specified with GCC's ``__attribute__``
extension (see :ref:`variable-attributes`).  For example, after this
declaration:

.. code-block:: c++

  struct foo { int x; char y; } foo1;

the value of ``__alignof__ (foo1.y)`` is 1, even though its actual
alignment is probably 2 or 4, the same as ``__alignof__ (int)``.

It is an error to ask for the alignment of an incomplete type.