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<title>Alignment - Using the GNU Compiler Collection (GCC)</title>
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<h3 class="section">5.31 Inquiring on Alignment of Types or Variables</h3>

<p><a name="index-alignment-1913"></a><a name="index-type-alignment-1914"></a><a name="index-variable-alignment-1915"></a>
The keyword <code>__alignof__</code> allows you to inquire about how an object
is aligned, or the minimum alignment usually required by a type.  Its
syntax is just like <code>sizeof</code>.

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

 <p>Some machines never actually require alignment; they allow reference to any
data type even at an odd address.  For these machines, <code>__alignof__</code>
reports the <em>recommended</em> alignment of a type.

 <p>If the operand of <code>__alignof__</code> 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 <code>__attribute__</code>
extension (see <a href="Variable-Attributes.html#Variable-Attributes">Variable Attributes</a>).  For example, after this
declaration:

<pre class="smallexample">     struct foo { int x; char y; } foo1;
</pre>
 <p class="noindent">the value of <code>__alignof__ (foo1.y)</code> is 1, even though its actual
alignment is probably 2 or 4, the same as <code>__alignof__ (int)</code>.

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

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