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| 22 | <title>Allocating File-Scope (GNU C Language Manual)</title>
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| 58 | <span id="Allocating-File_002dScope"></span><div class="header">
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| 59 | <p>
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| 60 | Next: <a href="auto-and-register.html" accesskey="n" rel="next">auto and register</a>, Previous: <a href="Extern-Declarations.html" accesskey="p" rel="prev">Extern Declarations</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="Allocating-File_002dScope-Variables"></span><h3 class="section">20.9 Allocating File-Scope Variables</h3>
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| 64 | <span id="index-allocation-file_002dscope-variables"></span>
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| 65 | <span id="index-file_002dscope-variables_002c-allocating"></span>
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| 66 |
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| 67 | <p>Some file-scope declarations allocate space for the variable, and some
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| 68 | don’t.
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| 69 | </p>
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| 70 | <p>A file-scope declaration with an initial value <em>must</em> allocate
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| 71 | space for the variable; if there are two of such declarations for the
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| 72 | same variable, even in different compilation modules, they conflict.
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| 73 | </p>
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| 74 | <p>An <code>extern</code> declaration <em>never</em> allocates space for the variable.
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| 75 | If all the top-level declarations of a certain variable are
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| 76 | <code>extern</code>, the variable never gets memory space. If that variable
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| 77 | is used anywhere in the program, the use will be reported as an error,
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| 78 | saying that the variable is not defined.
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| 79 | </p>
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| 80 | <span id="index-tentative-definition"></span>
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| 81 | <p>A file-scope declaration without an initial value is called a
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| 82 | <em>tentative definition</em>. This is a strange hybrid: it <em>can</em>
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| 83 | allocate space for the variable, but does not insist. So it causes no
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| 84 | conflict, no error, if the variable has another declaration that
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| 85 | allocates space for it, perhaps in another compilation module. But if
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| 86 | nothing else allocates space for the variable, the tentative
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| 87 | definition will do it. Any number of compilation modules can declare
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| 88 | the same variable in this way, and that is sufficient for all of them
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| 89 | to use the variable.
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| 90 | </p>
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| 91 | <p>In programs that are very large or have many contributors, it may be
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| 92 | wise to adopt the convention of never using tentative definitions.
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| 93 | You can use the compilation option <samp>-fno-common</samp> to make them
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| 94 | an error, or <samp>--warn-common</samp> to warn about them.
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| 95 | </p>
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| 96 | <p>If a file-scope variable gets its space through a tentative
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| 97 | definition, it starts out containing all zeros.
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| 98 | </p>
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| 99 |
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| 100 |
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| 101 |
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