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| 22 | <title>Void Pointers (GNU C Language Manual)</title>
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| 23 |
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| 24 | <meta name="description" content="Void Pointers (GNU C Language Manual)">
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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-Comparison.html" rel="next" title="Pointer Comparison">
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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="Void-Pointers"></span><div class="header">
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| 59 | <p>
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| 60 | Next: <a href="Pointer-Comparison.html" accesskey="n" rel="next">Pointer Comparison</a>, Previous: <a href="Invalid-Dereference.html" accesskey="p" rel="prev">Invalid Dereference</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="Void-Pointers-1"></span><h3 class="section">14.8 Void Pointers</h3>
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| 64 | <span id="index-void-pointers"></span>
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| 65 | <span id="index-pointers_002c-void"></span>
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| 66 |
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| 67 | <p>The peculiar type <code>void *</code>, a pointer whose target type is
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| 68 | <code>void</code>, is used often in C. It represents a pointer to
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| 69 | we-don’t-say-what. Thus,
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| 70 | </p>
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| 71 | <div class="example">
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| 72 | <pre class="example">void *numbered_slot_pointer (int);
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| 73 | </pre></div>
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| 74 |
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| 75 | <p>declares a function <code>numbered_slot_pointer</code> that takes an
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| 76 | integer parameter and returns a pointer, but we don’t say what type of
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| 77 | data it points to.
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| 78 | </p>
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| 79 | <p>With type <code>void *</code>, you can pass the pointer around and test
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| 80 | whether it is null. However, dereferencing it gives a <code>void</code>
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| 81 | value that can’t be used (see <a href="The-Void-Type.html">The Void Type</a>). To dereference the
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| 82 | pointer, first convert it to some other pointer type.
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| 83 | </p>
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| 84 | <p>Assignments convert <code>void *</code> automatically to any other pointer
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| 85 | type, if the left operand has a pointer type; for instance,
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| 86 | </p>
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| 87 | <div class="example">
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| 88 | <pre class="example">{
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| 89 | int *p;
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| 90 | /* <span class="roman">Converts return value to <code>int *</code>.</span> */
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| 91 | p = numbered_slot_pointer (5);
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| 92 | <span class="roman">…</span>
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| 93 | }
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| 94 | </pre></div>
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| 95 |
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| 96 | <p>Passing an argument of type <code>void *</code> for a parameter that has a
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| 97 | pointer type also converts. For example, supposing the function
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| 98 | <code>hack</code> is declared to require type <code>float *</code> for its
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| 99 | argument, this will convert the null pointer to that type.
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| 100 | </p>
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| 101 | <div class="example">
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| 102 | <pre class="example">/* <span class="roman">Declare <code>hack</code> that way.</span>
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| 103 | <span class="roman">We assume it is defined somewhere else.</span> */
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| 104 | void hack (float *);
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| 105 | …
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| 106 | /* <span class="roman">Now call <code>hack</code>.</span> */
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| 107 | {
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| 108 | /* <span class="roman">Converts return value of <code>numbered_slot_pointer</code></span>
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| 109 | <span class="roman">to <code>float *</code> to pass it to <code>hack</code>.</span> */
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| 110 | hack (numbered_slot_pointer (5));
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| 111 | <span class="roman">…</span>
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| 112 | }
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| 113 | </pre></div>
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| 114 |
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| 115 | <p>You can also convert to another pointer type with an explicit cast
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| 116 | (see <a href="Explicit-Type-Conversion.html">Explicit Type Conversion</a>), like this:
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| 117 | </p><div class="example">
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| 118 | <pre class="example">(int *) numbered_slot_pointer (5)
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| 119 | </pre></div>
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| 120 |
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| 121 | <p>Here is an example which decides at run time which pointer
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| 122 | type to convert to:
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| 123 | </p>
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| 124 | <div class="example">
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| 125 | <pre class="example">void
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| 126 | extract_int_or_double (void *ptr, bool its_an_int)
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| 127 | {
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| 128 | if (its_an_int)
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| 129 | handle_an_int (*(int *)ptr);
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| 130 | else
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| 131 | handle_a_double (*(double *)ptr);
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| 132 | }
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| 133 | </pre></div>
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| 134 |
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| 135 | <p>The expression <code>*(int *)ptr</code> means to convert <code>ptr</code>
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| 136 | to type <code>int *</code>, then dereference it.
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| 137 | </p>
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| 138 | <hr>
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| 139 | <div class="header">
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| 140 | <p>
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| 141 | Next: <a href="Pointer-Comparison.html" accesskey="n" rel="next">Pointer Comparison</a>, Previous: <a href="Invalid-Dereference.html" accesskey="p" rel="prev">Invalid Dereference</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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| 142 | </div>
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