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| 22 | <title>Local Variables (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="Variables.html" rel="up" title="Variables">
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| 33 | <link href="File_002dScope-Variables.html" rel="next" title="File-Scope Variables">
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| 57 | <body lang="en">
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| 58 | <span id="Local-Variables"></span><div class="header">
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| 59 | <p>
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| 60 | Next: <a href="File_002dScope-Variables.html" accesskey="n" rel="next">File-Scope Variables</a>, Previous: <a href="Auto-Type.html" accesskey="p" rel="prev">Auto Type</a>, Up: <a href="Variables.html" accesskey="u" rel="up">Variables</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="Local-Variables-1"></span><h3 class="section">20.5 Local Variables</h3>
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| 64 | <span id="index-local-variables"></span>
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| 65 | <span id="index-variables_002c-local"></span>
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| 66 |
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| 67 | <p>Declaring a variable inside a function definition (see <a href="Function-Definitions.html">Function Definitions</a>) makes the variable name <em>local</em> to the containing
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| 68 | block—that is, the containing pair of braces. More precisely, the
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| 69 | variable’s name is visible starting just after where it appears in the
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| 70 | declaration, and its visibility continues until the end of the block.
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| 71 | </p>
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| 72 | <p>Local variables in C are generally <em>automatic</em> variables: each
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| 73 | variable’s storage exists only from the declaration to the end of the
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| 74 | block. Execution of the declaration allocates the storage, computes
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| 75 | the initial value, and stores it in the variable. The end of the
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| 76 | block deallocates the storage.<a id="DOCF6" href="#FOOT6"><sup>6</sup></a>
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| 77 | </p>
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| 78 | <p><strong>Warning:</strong> Two declarations for the same local variable
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| 79 | in the same scope are an error.
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| 80 | </p>
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| 81 | <p><strong>Warning:</strong> Automatic variables are stored in the run-time stack.
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| 82 | The total space for the program’s stack may be limited; therefore,
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| 83 | in using very large arrays, it may be necessary to allocate
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| 84 | them in some other way to stop the program from crashing.
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| 85 | </p>
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| 86 | <p><strong>Warning:</strong> If the declaration of an automatic variable does not
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| 87 | specify an initial value, the variable starts out containing garbage.
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| 88 | In this example, the value printed could be anything at all:
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| 89 | </p>
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| 90 | <div class="example">
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| 91 | <pre class="example">{
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| 92 | int i;
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| 93 |
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| 94 | printf ("Print junk %d\n", i);
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| 95 | }
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| 96 | </pre></div>
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| 97 |
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| 98 | <p>In a simple test program, that statement is likely to print 0, simply
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| 99 | because every process starts with memory zeroed. But don’t rely on it
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| 100 | to be zero—that is erroneous.
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| 101 | </p>
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| 102 | <p><strong>Note:</strong> Make sure to store a value into each local variable (by
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| 103 | assignment, or by initialization) before referring to its value.
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| 104 | </p>
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| 105 | <div class="footnote">
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| 106 | <hr>
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| 107 | <h4 class="footnotes-heading">Footnotes</h4>
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| 108 |
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| 109 | <h5><a id="FOOT6" href="#DOCF6">(6)</a></h3>
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| 110 | <p>Due to compiler optimizations,
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| 111 | allocation and deallocation don’t necessarily really happen at
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| 112 | those times.</p>
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| 113 | </div>
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| 114 | <hr>
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| 115 | <div class="header">
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| 116 | <p>
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| 117 | Next: <a href="File_002dScope-Variables.html" accesskey="n" rel="next">File-Scope Variables</a>, Previous: <a href="Auto-Type.html" accesskey="p" rel="prev">Auto Type</a>, Up: <a href="Variables.html" accesskey="u" rel="up">Variables</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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