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| 22 | <title>Aliasing Alignment (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="Aliasing.html" rel="up" title="Aliasing">
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| 33 | <link href="Aliasing-Length.html" rel="next" title="Aliasing Length">
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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="Aliasing-Alignment"></span><div class="header">
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| 59 | <p>
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| 60 | Next: <a href="Aliasing-Length.html" accesskey="n" rel="next">Aliasing Length</a>, Up: <a href="Aliasing.html" accesskey="u" rel="up">Aliasing</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="Aliasing-and-Alignment"></span><h3 class="appendixsection">B.1 Aliasing and Alignment</h3>
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| 64 |
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| 65 | <p>In order for a type-converted pointer to be valid, it must have the
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| 66 | alignment that the new pointer type requires. For instance, on most
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| 67 | computers, <code>int</code> has alignment 4; the address of an <code>int</code>
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| 68 | must be a multiple of 4. However, <code>char</code> has alignment 1, so the
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| 69 | address of a <code>char</code> is usually not a multiple of 4. Taking the
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| 70 | address of such a <code>char</code> and casting it to <code>int *</code> probably
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| 71 | results in an invalid pointer. Trying to dereference it may cause a
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| 72 | <code>SIGBUS</code> signal, depending on the platform in use (see <a href="Signals.html">Signals</a>).
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| 73 | </p>
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| 74 | <div class="example">
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| 75 | <pre class="example">foo ()
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| 76 | {
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| 77 | char i[4];
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| 78 | int *p = (int *) &i[1]; /* <span class="roman">Misaligned pointer!</span> */
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| 79 | return *p; /* <span class="roman">Crash!</span> */
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| 80 | }
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| 81 | </pre></div>
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| 82 |
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| 83 | <p>This requirement is never a problem when casting the return value
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| 84 | of <code>malloc</code> because that function always returns a pointer
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| 85 | with as much alignment as any type can require.
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| 86 | </p>
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| 87 |
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| 88 |
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| 89 |
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| 90 | </body>
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| 91 | </html>
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