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| 22 | <title>Aliasing (GNU C Language Manual)</title>
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| 32 | <link href="index.html" rel="up" title="Top">
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| 33 | <link href="Aliasing-Alignment.html" rel="next" title="Aliasing Alignment">
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| 55 | </head>
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| 57 | <body lang="en">
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| 58 | <span id="Aliasing"></span><div class="header">
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| 59 | <p>
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| 60 | Next: <a href="Digraphs.html" accesskey="n" rel="next">Digraphs</a>, Previous: <a href="Type-Alignment.html" accesskey="p" rel="prev">Type Alignment</a>, Up: <a href="index.html" accesskey="u" rel="up">Top</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-1"></span><h2 class="appendix">Appendix B Aliasing</h2>
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| 64 | <span id="index-aliasing-_0028of-storage_0029"></span>
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| 65 | <span id="index-pointer-type-conversion"></span>
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| 66 | <span id="index-type-conversion_002c-pointer"></span>
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| 67 |
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| 68 | <p>We have already presented examples of casting a <code>void *</code> pointer
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| 69 | to another pointer type, and casting another pointer type to
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| 70 | <code>void *</code>.
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| 71 | </p>
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| 72 | <p>One common kind of pointer cast is guaranteed safe: casting the value
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| 73 | returned by <code>malloc</code> and related functions (see <a href="Dynamic-Memory-Allocation.html">Dynamic Memory Allocation</a>). It is safe because these functions do not save the
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| 74 | pointer anywhere else; the only way the program will access the newly
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| 75 | allocated memory is via the pointer just returned.
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| 76 | </p>
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| 77 | <p>In fact, C allows casting any pointer type to any other pointer type.
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| 78 | Using this to access the same place in memory using two
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| 79 | different data types is called <em>aliasing</em>.
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| 80 | </p>
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| 81 | <p>Aliasing is necessary in some programs that do sophisticated memory
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| 82 | management, such as GNU Emacs, but most C programs don’t need to do
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| 83 | aliasing. When it isn’t needed, <strong>stay away from it!</strong> To do
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| 84 | aliasing correctly requires following the rules stated below.
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| 85 | Otherwise, the aliasing may result in malfunctions when the program
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| 86 | runs.
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| 87 | </p>
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| 88 | <p>The rest of this appendix explains the pitfalls and rules of aliasing.
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| 89 | </p>
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| 90 | <table class="menu" border="0" cellspacing="0">
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| 91 | <tr><td align="left" valign="top">• <a href="Aliasing-Alignment.html" accesskey="1">Aliasing Alignment</a></td><td> </td><td align="left" valign="top">Memory alignment considerations for
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| 92 | casting between pointer types.
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| 93 | </td></tr>
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| 94 | <tr><td align="left" valign="top">• <a href="Aliasing-Length.html" accesskey="2">Aliasing Length</a></td><td> </td><td align="left" valign="top">Type size considerations for
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| 95 | casting between pointer types.
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| 96 | </td></tr>
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| 97 | <tr><td align="left" valign="top">• <a href="Aliasing-Type-Rules.html" accesskey="3">Aliasing Type Rules</a></td><td> </td><td align="left" valign="top">Even when type alignment and size matches,
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| 98 | aliasing can still have surprising results.
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| 99 | </td></tr>
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| 100 | <tr><th colspan="3" align="left" valign="top"><pre class="menu-comment">
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| 101 |
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| 102 | </pre></th></tr></table>
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| 103 |
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| 105 |
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| 106 |
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