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| <title>Using the GNU Compiler Collection (GCC): Alignment</title> |
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| <a name="Inquiring-on-Alignment-of-Types-or-Variables"></a> |
| <h3 class="section">6.38 Inquiring on Alignment of Types or Variables</h3> |
| <a name="index-alignment"></a> |
| <a name="index-type-alignment"></a> |
| <a name="index-variable-alignment"></a> |
| |
| <p>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> |
| <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> |
| <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 smallest alignment that GCC gives the data type, usually as |
| mandated by the target ABI. |
| </p> |
| <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: |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample">struct foo { int x; char y; } foo1; |
| </pre></div> |
| |
| <p>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> |
| <p>It is an error to ask for the alignment of an incomplete type. |
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