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87 lines
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<title>Alignment - Using the GNU Compiler Collection (GCC)</title>
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<a name="Alignment"></a>
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Next: <a rel="next" accesskey="n" href="Inline.html#Inline">Inline</a>,
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Previous: <a rel="previous" accesskey="p" href="Type-Attributes.html#Type-Attributes">Type Attributes</a>,
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Up: <a rel="up" accesskey="u" href="C-Extensions.html#C-Extensions">C Extensions</a>
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<h3 class="section">6.40 Inquiring on Alignment of Types or Variables</h3>
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<p><a name="index-alignment-3397"></a><a name="index-type-alignment-3398"></a><a name="index-variable-alignment-3399"></a>
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The keyword <code>__alignof__</code> allows you to inquire about how an object
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is aligned, or the minimum alignment usually required by a type. Its
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syntax is just like <code>sizeof</code>.
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<p>For example, if the target machine requires a <code>double</code> value to be
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aligned on an 8-byte boundary, then <code>__alignof__ (double)</code> is 8.
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This is true on many RISC machines. On more traditional machine
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designs, <code>__alignof__ (double)</code> is 4 or even 2.
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<p>Some machines never actually require alignment; they allow reference to any
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data type even at an odd address. For these machines, <code>__alignof__</code>
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reports the smallest alignment that GCC gives the data type, usually as
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mandated by the target ABI.
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<p>If the operand of <code>__alignof__</code> is an lvalue rather than a type,
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its value is the required alignment for its type, taking into account
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any minimum alignment specified with GCC's <code>__attribute__</code>
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extension (see <a href="Variable-Attributes.html#Variable-Attributes">Variable Attributes</a>). For example, after this
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declaration:
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<pre class="smallexample"> struct foo { int x; char y; } foo1;
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</pre>
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<p class="noindent">the value of <code>__alignof__ (foo1.y)</code> is 1, even though its actual
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alignment is probably 2 or 4, the same as <code>__alignof__ (int)</code>.
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<p>It is an error to ask for the alignment of an incomplete type.
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