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| 22 | <title>Variables (GNU C Language Manual)</title>
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| 33 | <link href="Variable-Declarations.html" rel="next" title="Variable Declarations">
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| 55 | </head>
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| 57 | <body lang="en">
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| 58 | <span id="Variables"></span><div class="header">
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| 59 | <p>
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| 60 | Next: <a href="Type-Qualifiers.html" accesskey="n" rel="next">Type Qualifiers</a>, Previous: <a href="Statements.html" accesskey="p" rel="prev">Statements</a>, Up: <a href="index.html" accesskey="u" rel="up">Top</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="Variables-1"></span><h2 class="chapter">20 Variables</h2>
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| 64 | <span id="index-variables"></span>
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| 65 |
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| 66 | <p>Every variable used in a C program needs to be made known by a
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| 67 | <em>declaration</em>. It can be used only after it has been declared.
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| 68 | It is an error to declare a variable name more than once in the same
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| 69 | scope; an exception is that <code>extern</code> declarations and tentative
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| 70 | definitions can coexist with another declaration of the same
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| 71 | variable.
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| 72 | </p>
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| 73 | <p>Variables can be declared anywhere within a block or file. (Older
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| 74 | versions of C required that all variable declarations within a block
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| 75 | occur before any statements.)
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| 76 | </p>
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| 77 | <p>Variables declared within a function or block are <em>local</em> to
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| 78 | it. This means that the variable name is visible only until the end
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| 79 | of that function or block, and the memory space is allocated only
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| 80 | while control is within it.
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| 81 | </p>
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| 82 | <p>Variables declared at the top level in a file are called <em>file-scope</em>.
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| 83 | They are assigned fixed, distinct memory locations, so they retain
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| 84 | their values for the whole execution of the program.
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| 85 | </p>
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| 86 | <table class="menu" border="0" cellspacing="0">
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| 87 | <tr><td align="left" valign="top">• <a href="Variable-Declarations.html" accesskey="1">Variable Declarations</a></td><td> </td><td align="left" valign="top">Name a variable and and reserve space for it.
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| 88 | </td></tr>
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| 89 | <tr><td align="left" valign="top">• <a href="Initializers.html" accesskey="2">Initializers</a></td><td> </td><td align="left" valign="top">Assigning inital values to variables.
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| 90 | </td></tr>
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| 91 | <tr><td align="left" valign="top">• <a href="Designated-Inits.html" accesskey="3">Designated Inits</a></td><td> </td><td align="left" valign="top">Assigning initial values to array elements
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| 92 | at particular array indices.
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| 93 | </td></tr>
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| 94 | <tr><td align="left" valign="top">• <a href="Auto-Type.html" accesskey="4">Auto Type</a></td><td> </td><td align="left" valign="top">Obtaining the type of a variable.
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| 95 | </td></tr>
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| 96 | <tr><td align="left" valign="top">• <a href="Local-Variables.html" accesskey="5">Local Variables</a></td><td> </td><td align="left" valign="top">Variables declared in function definitions.
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| 97 | </td></tr>
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| 98 | <tr><td align="left" valign="top">• <a href="File_002dScope-Variables.html" accesskey="6">File-Scope Variables</a></td><td> </td><td align="left" valign="top">Variables declared outside of
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| 99 | function definitions.
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| 100 | </td></tr>
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| 101 | <tr><td align="left" valign="top">• <a href="Static-Local-Variables.html" accesskey="7">Static Local Variables</a></td><td> </td><td align="left" valign="top">Variables declared within functions,
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| 102 | but with permanent storage allocation.
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| 103 | </td></tr>
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| 104 | <tr><td align="left" valign="top">• <a href="Extern-Declarations.html" accesskey="8">Extern Declarations</a></td><td> </td><td align="left" valign="top">Declaring a variable
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| 105 | which is allocated somewhere else.
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| 106 | </td></tr>
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| 107 | <tr><td align="left" valign="top">• <a href="Allocating-File_002dScope.html" accesskey="9">Allocating File-Scope</a></td><td> </td><td align="left" valign="top">When is space allocated
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| 108 | for file-scope variables?
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| 109 | </td></tr>
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| 110 | <tr><td align="left" valign="top">• <a href="auto-and-register.html">auto and register</a></td><td> </td><td align="left" valign="top">Historically used storage directions.
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| 111 | </td></tr>
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| 112 | <tr><td align="left" valign="top">• <a href="Omitting-Types.html">Omitting Types</a></td><td> </td><td align="left" valign="top">The bad practice of declaring variables
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| 113 | with implicit type.
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| 114 | </td></tr>
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| 115 | </table>
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| 116 |
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| 117 | <hr>
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| 118 | <div class="header">
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| 119 | <p>
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| 120 | Next: <a href="Type-Qualifiers.html" accesskey="n" rel="next">Type Qualifiers</a>, Previous: <a href="Statements.html" accesskey="p" rel="prev">Statements</a>, Up: <a href="index.html" accesskey="u" rel="up">Top</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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| 121 | </div>
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