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| 21 | <meta http-equiv="Content-Type" content="text/html; charset=utf-8">
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| 22 | <title>Aliasing Length (GNU C Language Manual)</title>
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| 23 |
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| 24 | <meta name="description" content="Aliasing Length (GNU C Language Manual)">
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| 29 | <link href="index.html" rel="start" title="Top">
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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-Type-Rules.html" rel="next" title="Aliasing Type Rules">
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| 34 | <link href="Aliasing-Alignment.html" rel="prev" title="Aliasing Alignment">
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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-Length"></span><div class="header">
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| 59 | <p>
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| 60 | Next: <a href="Aliasing-Type-Rules.html" accesskey="n" rel="next">Aliasing Type Rules</a>, Previous: <a href="Aliasing-Alignment.html" accesskey="p" rel="prev">Aliasing Alignment</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-Length"></span><h3 class="appendixsection">B.2 Aliasing and Length</h3>
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| 64 |
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| 65 | <p>When converting a pointer to a different pointer type, make sure the
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| 66 | object it really points to is at least as long as the target of the
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| 67 | converted pointer. For instance, suppose <code>p</code> has type <code>int
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| 68 | *</code> and it’s cast as follows:
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| 69 | </p>
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| 70 | <div class="example">
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| 71 | <pre class="example">int *p;
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| 72 |
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| 73 | struct
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| 74 | {
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| 75 | double d, e, f;
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| 76 | } foo;
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| 77 |
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| 78 | struct foo *q = (struct foo *)p;
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| 79 |
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| 80 | q->f = 5.14159;
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| 81 | </pre></div>
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| 82 |
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| 83 | <p>the value <code>q->f</code> will run past the end of the <code>int</code> that
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| 84 | <code>p</code> points to. If <code>p</code> was initialized to the start of an
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| 85 | array of type <code>int[6]</code>, the object is long enough for three
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| 86 | <code>double</code>s. But if <code>p</code> points to something shorter,
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| 87 | <code>q->f</code> will run on beyond the end of that, overlaying some other
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| 88 | data. Storing that will garble that other data. Or it could extend
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| 89 | past the end of memory space and cause a <code>SIGSEGV</code> signal
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| 90 | (see <a href="Signals.html">Signals</a>).
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| 91 | </p>
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| 92 |
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| 93 |
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| 94 |
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| 95 | </body>
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| 96 | </html>
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