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| 22 | <title>Packed Structures (GNU C Language Manual)</title>
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| 23 |
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| 30 | <link href="Symbol-Index.html" rel="index" title="Symbol Index">
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| 31 | <link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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| 32 | <link href="Structures.html" rel="up" title="Structures">
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| 33 | <link href="Bit-Fields.html" rel="next" title="Bit Fields">
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| 55 | </head>
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| 56 |
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| 57 | <body lang="en">
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| 58 | <span id="Packed-Structures"></span><div class="header">
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| 59 | <p>
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| 60 | Next: <a href="Bit-Fields.html" accesskey="n" rel="next">Bit Fields</a>, Previous: <a href="Structure-Layout.html" accesskey="p" rel="prev">Structure Layout</a>, Up: <a href="Structures.html" accesskey="u" rel="up">Structures</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Symbol-Index.html" title="Index" rel="index">Index</a>]</p>
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| 61 | </div>
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| 62 | <hr>
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| 63 | <span id="Packed-Structures-1"></span><h3 class="section">15.5 Packed Structures</h3>
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| 64 | <span id="index-packed-structures"></span>
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| 65 | <span id="index-_005f_005fattribute_005f_005f_0028_0028packed_0029_0029"></span>
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| 66 |
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| 67 | <p>In GNU C you can force a structure to be laid out with no gaps by
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| 68 | adding <code>__attribute__((packed))</code> after <code>struct</code> (or at the
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| 69 | end of the structure type declaration). Here’s an example:
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| 70 | </p>
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| 71 | <div class="example">
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| 72 | <pre class="example">struct __attribute__((packed)) foo
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| 73 | {
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| 74 | char a;
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| 75 | int c;
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| 76 | char b;
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| 77 | };
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| 78 | </pre></div>
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| 79 |
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| 80 | <p>Without <code>__attribute__((packed))</code>, this structure occupies 12
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| 81 | bytes (as described in the previous section), assuming 4-byte
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| 82 | alignment for <code>int</code>. With <code>__attribute__((packed))</code>, it is
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| 83 | only 6 bytes long—the sum of the lengths of its fields.
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| 84 | </p>
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| 85 | <p>Use of <code>__attribute__((packed))</code> often results in fields that
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| 86 | don’t have the normal alignment for their types. Taking the address
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| 87 | of such a field can result in an invalid pointer because of its
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| 88 | improper alignment. Dereferencing such a pointer can cause a
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| 89 | <code>SIGSEGV</code> signal on a machine that doesn’t, in general, allow
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| 90 | unaligned pointers.
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| 91 | </p>
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| 92 | <p>See <a href="Attributes.html">Attributes</a>.
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| 93 | </p>
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| 94 |
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| 95 |
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| 96 |
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| 97 | </body>
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| 98 | </html>
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