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| 22 | <title>Pointer Arithmetic Drawbacks (GNU C Language Manual)</title>
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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="Pointers.html" rel="up" title="Pointers">
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| 33 | <link href="Pointer_002dInteger-Conversion.html" rel="next" title="Pointer-Integer Conversion">
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| 57 | <body lang="en">
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| 58 | <span id="Pointer-Arithmetic-Drawbacks"></span><div class="header">
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| 59 | <p>
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| 60 | Next: <a href="Pointer_002dInteger-Conversion.html" accesskey="n" rel="next">Pointer-Integer Conversion</a>, Previous: <a href="Pointer-Increment_002fDecrement.html" accesskey="p" rel="prev">Pointer Increment/Decrement</a>, Up: <a href="Pointers.html" accesskey="u" rel="up">Pointers</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="Drawbacks-of-Pointer-Arithmetic"></span><h3 class="section">14.14 Drawbacks of Pointer Arithmetic</h3>
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| 64 | <span id="index-drawbacks-of-pointer-arithmetic"></span>
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| 65 | <span id="index-pointer-arithmetic_002c-drawbacks"></span>
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| 66 |
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| 67 | <p>Pointer arithmetic is clean and elegant, but it is also the cause of a
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| 68 | major security flaw in the C language. Theoretically, it is only
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| 69 | valid to adjust a pointer within one object allocated as a unit in
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| 70 | memory. However, if you unintentionally adjust a pointer across the
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| 71 | bounds of the object and into some other object, the system has no way
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| 72 | to detect this error.
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| 73 | </p>
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| 74 | <p>A bug which does that can easily result in clobbering part of another
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| 75 | object. For example, with <code>array[-1]</code> you can read or write the
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| 76 | nonexistent element before the beginning of an array—probably part
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| 77 | of some other data.
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| 78 | </p>
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| 79 | <p>Combining pointer arithmetic with casts between pointer types, you can
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| 80 | create a pointer that fails to be properly aligned for its type. For
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| 81 | example,
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| 82 | </p>
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| 83 | <div class="example">
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| 84 | <pre class="example">int a[2];
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| 85 | char *pa = (char *)a;
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| 86 | int *p = (int *)(pa + 1);
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| 87 | </pre></div>
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| 88 |
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| 89 | <p>gives <code>p</code> a value pointing to an “integer” that includes part
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| 90 | of <code>a[0]</code> and part of <code>a[1]</code>. Dereferencing that with
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| 91 | <code>*p</code> can cause a fatal <code>SIGSEGV</code> signal or it can return the
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| 92 | contents of that badly aligned <code>int</code> (see <a href="Signals.html">Signals</a>. If it
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| 93 | “works,” it may be quite slow. It can also cause aliasing
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| 94 | confusions (see <a href="Aliasing.html">Aliasing</a>).
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| 95 | </p>
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| 96 | <p><strong>Warning:</strong> Using improperly aligned pointers is risky—don’t do it
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| 97 | unless it is really necessary.
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| 98 | </p>
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| 99 |
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| 100 |
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| 101 |
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| 102 | </body>
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| 103 | </html>
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