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| 22 | <title>Dynamic Memory Allocation (GNU C Language Manual)</title>
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| 23 |
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| 24 | <meta name="description" content="Dynamic Memory Allocation (GNU C Language Manual)">
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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="Dynamic-Memory-Allocation"></span><div class="header">
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| 59 | <p>
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| 60 | Next: <a href="Field-Offset.html" accesskey="n" rel="next">Field Offset</a>, Previous: <a href="Referencing-Fields.html" accesskey="p" rel="prev">Referencing Fields</a>, Up: <a href="Structures.html" accesskey="u" rel="up">Structures</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="Dynamic-Memory-Allocation-1"></span><h3 class="section">15.2 Dynamic Memory Allocation</h3>
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| 64 | <span id="index-dynamic-memory-allocation"></span>
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| 65 | <span id="index-memory-allocation_002c-dynamic"></span>
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| 66 | <span id="index-allocating-memory-dynamically"></span>
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| 67 |
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| 68 | <p>To allocate an object dynamically, call the library function
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| 69 | <code>malloc</code> (see <a href="https://www.gnu.org/software/libc/manual/html_node/Basic-Allocation.html#Basic-Allocation">The GNU C Library</a> in <cite>The GNU C Library
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| 70 | Reference Manual</cite>). Here is how to allocate an object of type
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| 71 | <code>struct intlistlink</code>. To make this code work, include the file
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| 72 | <samp>stdlib.h</samp>, like this:
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| 73 | </p>
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| 74 | <div class="example">
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| 75 | <pre class="example">#include <stddef.h> /* <span class="roman">Defines <code>NULL</code>.</span> */
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| 76 | #include <stdlib.h> /* <span class="roman">Declares <code>malloc</code>.</span> */
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| 77 |
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| 78 | …
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| 79 |
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| 80 | struct intlistlink *
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| 81 | alloc_intlistlink ()
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| 82 | {
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| 83 | struct intlistlink *p;
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| 84 |
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| 85 | p = malloc (sizeof (struct intlistlink));
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| 86 |
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| 87 | if (p == NULL)
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| 88 | fatal ("Ran out of storage");
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| 89 |
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| 90 | /* <span class="roman">Initialize the contents.</span> */
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| 91 | p->datum = 0;
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| 92 | p->next = NULL;
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| 93 |
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| 94 | return p;
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| 95 | }
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| 96 | </pre></div>
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| 97 |
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| 98 | <p><code>malloc</code> returns <code>void *</code>, so the assignment to <code>p</code>
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| 99 | will automatically convert it to type <code>struct intlistlink *</code>.
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| 100 | The return value of <code>malloc</code> is always sufficiently aligned
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| 101 | (see <a href="Type-Alignment.html">Type Alignment</a>) that it is valid for any data type.
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| 102 | </p>
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| 103 | <p>The test for <code>p == NULL</code> is necessary because <code>malloc</code>
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| 104 | returns a null pointer if it cannot get any storage. We assume that
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| 105 | the program defines the function <code>fatal</code> to report a fatal error
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| 106 | to the user.
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| 107 | </p>
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| 108 | <p>Here’s how to add one more integer to the front of such a list:
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| 109 | </p>
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| 110 | <div class="example">
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| 111 | <pre class="example">struct intlistlink *my_list = NULL;
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| 112 |
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| 113 | void
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| 114 | add_to_mylist (int my_int)
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| 115 | {
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| 116 | struct intlistlink *p = alloc_intlistlink ();
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| 117 |
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| 118 | p->datum = my_int;
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| 119 | p->next = mylist;
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| 120 | mylist = p;
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| 121 | }
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| 122 | </pre></div>
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| 123 |
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| 124 | <p>The way to free the objects is by calling <code>free</code>. Here’s
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| 125 | a function to free all the links in one of these lists:
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| 126 | </p>
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| 127 | <div class="example">
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| 128 | <pre class="example">void
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| 129 | free_intlist (struct intlistlink *p)
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| 130 | {
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| 131 | while (p)
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| 132 | {
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| 133 | struct intlistlink *q = p;
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| 134 | p = p->next;
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| 135 | free (q);
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| 136 | }
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| 137 | }
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| 138 | </pre></div>
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| 139 |
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| 140 | <p>We must extract the <code>next</code> pointer from the object before freeing
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| 141 | it, because <code>free</code> can clobber the data that was in the object.
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| 142 | For the same reason, the program must not use the list any more after
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| 143 | freeing its elements. To make sure it won’t, it is best to clear out
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| 144 | the variable where the list was stored, like this:
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| 145 | </p>
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| 146 | <div class="example">
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| 147 | <pre class="example">free_intlist (mylist);
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| 148 |
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| 149 | mylist = NULL;
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| 150 | </pre></div>
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| 151 |
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| 152 | <hr>
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| 153 | <div class="header">
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| 154 | <p>
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| 155 | Next: <a href="Field-Offset.html" accesskey="n" rel="next">Field Offset</a>, Previous: <a href="Referencing-Fields.html" accesskey="p" rel="prev">Referencing Fields</a>, Up: <a href="Structures.html" accesskey="u" rel="up">Structures</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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