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| 22 | <title>Pointer Arithmetic Low Level (GNU C Language Manual)</title>
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| 57 | <body lang="en">
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| 58 | <span id="Pointer-Arithmetic-Low-Level"></span><div class="header">
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| 59 | <p>
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| 60 | Next: <a href="Pointer-Increment_002fDecrement.html" accesskey="n" rel="next">Pointer Increment/Decrement</a>, Previous: <a href="Pointers-and-Arrays.html" accesskey="p" rel="prev">Pointers and Arrays</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="Pointer-Arithmetic-at-Low-Level"></span><h3 class="section">14.12 Pointer Arithmetic at Low Level</h3>
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| 64 | <span id="index-pointer-arithmetic_002c-low-level"></span>
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| 65 | <span id="index-low-level-pointer-arithmetic"></span>
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| 66 |
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| 67 | <p>The behavior of pointer arithmetic is theoretically defined only when
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| 68 | the pointer values all point within one object allocated in memory.
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| 69 | But the addition and subtraction operators can’t tell whether the
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| 70 | pointer values are all within one object. They don’t know where
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| 71 | objects start and end. So what do they really do?
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| 72 | </p>
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| 73 | <p>Adding pointer <var>p</var> to integer <var>i</var> treats <var>p</var> as a memory
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| 74 | address, which is in fact an integer—call it <var>pint</var>. It treats
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| 75 | <var>i</var> as a number of elements of the type that <var>p</var> points to.
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| 76 | These elements’ sizes add up to <code><var>i</var> * sizeof (*<var>p</var>)</code>.
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| 77 | So the sum, as an integer, is <code><var>pint</var> + <var>i</var> * sizeof
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| 78 | (*<var>p</var>)</code>. This value is reinterpreted as a pointer like <var>p</var>.
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| 79 | </p>
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| 80 | <p>If the starting pointer value <var>p</var> and the result do not point at
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| 81 | parts of the same object, the operation is not officially legitimate,
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| 82 | and C code is not “supposed” to do it. But you can do it anyway,
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| 83 | and it gives precisely the results described by the procedure above.
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| 84 | In some special situations it can do something useful, but non-wizards
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| 85 | should avoid it.
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| 86 | </p>
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| 87 | <p>Here’s a function to offset a pointer value <em>as if</em> it pointed to
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| 88 | an object of any given size, by explicitly performing that calculation:
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| 89 | </p>
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| 90 | <div class="example">
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| 91 | <pre class="example">#include <stdint.h>
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| 92 |
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| 93 | void *
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| 94 | ptr_add (void *p, int i, int objsize)
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| 95 | {
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| 96 | intptr_t p_address = (long) p;
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| 97 | intptr_t totalsize = i * objsize;
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| 98 | intptr_t new_address = p_address + totalsize;
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| 99 | return (void *) new_address;
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| 100 | }
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| 101 | </pre></div>
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| 102 |
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| 103 | <p><span id="index-intptr_005ft"></span>
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| 104 | This does the same job as <code><var>p</var> + <var>i</var></code> with the proper
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| 105 | pointer type for <var>p</var>. It uses the type <code>intptr_t</code>, which is
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| 106 | defined in the header file <samp>stdint.h</samp>. (In practice, <code>long
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| 107 | long</code> would always work, but it is cleaner to use <code>intptr_t</code>.)
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| 108 | </p>
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| 109 | <hr>
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| 110 | <div class="header">
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| 111 | <p>
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| 112 | Next: <a href="Pointer-Increment_002fDecrement.html" accesskey="n" rel="next">Pointer Increment/Decrement</a>, Previous: <a href="Pointers-and-Arrays.html" accesskey="p" rel="prev">Pointers and Arrays</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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