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| 22 | <title>Null Pointers (GNU C Language Manual)</title>
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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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| 57 | <body lang="en">
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| 58 | <span id="Null-Pointers"></span><div class="header">
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| 59 | <p>
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| 60 | Next: <a href="Invalid-Dereference.html" accesskey="n" rel="next">Invalid Dereference</a>, Previous: <a href="Pointer-Dereference.html" accesskey="p" rel="prev">Pointer 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="Null-Pointers-1"></span><h3 class="section">14.6 Null Pointers</h3>
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| 64 | <span id="index-null-pointers"></span>
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| 65 | <span id="index-pointers_002c-null"></span>
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| 66 |
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| 67 |
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| 68 | <p>A pointer value can be <em>null</em>, which means it does not point to
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| 69 | any object. The cleanest way to get a null pointer is by writing
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| 70 | <code>NULL</code>, a standard macro defined in <samp>stddef.h</samp>. You can
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| 71 | also do it by casting 0 to the desired pointer type, as in
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| 72 | <code>(char *) 0</code>. (The cast operator performs explicit type conversion;
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| 73 | See <a href="Explicit-Type-Conversion.html">Explicit Type Conversion</a>.)
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| 74 | </p>
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| 75 | <p>You can store a null pointer in any lvalue whose data type
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| 76 | is a pointer type:
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| 77 | </p>
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| 78 | <div class="example">
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| 79 | <pre class="example">char *foo;
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| 80 | foo = NULL;
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| 81 | </pre></div>
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| 82 |
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| 83 | <p>These two, if consecutive, can be combined into a declaration with
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| 84 | initializer,
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| 85 | </p>
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| 86 | <div class="example">
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| 87 | <pre class="example">char *foo = NULL;
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| 88 | </pre></div>
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| 89 |
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| 90 | <p>You can also explicitly cast <code>NULL</code> to the specific pointer type
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| 91 | you want—it makes no difference.
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| 92 | </p>
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| 93 | <div class="example">
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| 94 | <pre class="example">char *foo;
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| 95 | foo = (char *) NULL;
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| 96 | </pre></div>
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| 97 |
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| 98 | <p>To test whether a pointer is null, compare it with zero or
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| 99 | <code>NULL</code>, as shown here:
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| 100 | </p>
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| 101 | <div class="example">
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| 102 | <pre class="example">if (p != NULL)
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| 103 | /* <span class="roman"><code>p</code> is not null.</span> */
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| 104 | operate (p);
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| 105 | </pre></div>
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| 106 |
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| 107 | <p>Since testing a pointer for not being null is basic and frequent, all
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| 108 | but beginners in C will understand the conditional without need for
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| 109 | <code>!= NULL</code>:
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| 110 | </p>
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| 111 | <div class="example">
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| 112 | <pre class="example">if (p)
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| 113 | /* <span class="roman"><code>p</code> is not null.</span> */
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| 114 | operate (p);
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| 115 | </pre></div>
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| 116 |
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| 117 |
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| 118 |
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| 119 |
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| 120 | </body>
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| 121 | </html>
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