| 1 | <!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
|
|---|
| 2 | <html>
|
|---|
| 3 | <!-- Copyright (C) 2022 Richard Stallman and Free Software Foundation, Inc.
|
|---|
| 4 |
|
|---|
| 5 | (The work of Trevis Rothwell and Nelson Beebe has been assigned or
|
|---|
| 6 | licensed to the FSF.)
|
|---|
| 7 |
|
|---|
| 8 | Permission is granted to copy, distribute and/or modify this document
|
|---|
| 9 | under the terms of the GNU Free Documentation License, Version 1.3 or
|
|---|
| 10 | any later version published by the Free Software Foundation; with the
|
|---|
| 11 | Invariant Sections being "GNU General Public License," with the
|
|---|
| 12 | Front-Cover Texts being "A GNU Manual," and with the Back-Cover
|
|---|
| 13 | Texts as in (a) below. A copy of the license is included in the
|
|---|
| 14 | section entitled "GNU Free Documentation License."
|
|---|
| 15 |
|
|---|
| 16 | (a) The FSF's Back-Cover Text is: "You have the freedom to copy and
|
|---|
| 17 | modify this GNU manual. Buying copies from the FSF supports it in
|
|---|
| 18 | developing GNU and promoting software freedom." -->
|
|---|
| 19 | <!-- Created by GNU Texinfo 6.7, http://www.gnu.org/software/texinfo/ -->
|
|---|
| 20 | <head>
|
|---|
| 21 | <meta http-equiv="Content-Type" content="text/html; charset=utf-8">
|
|---|
| 22 | <title>Extern Declarations (GNU C Language Manual)</title>
|
|---|
| 23 |
|
|---|
| 24 | <meta name="description" content="Extern Declarations (GNU C Language Manual)">
|
|---|
| 25 | <meta name="keywords" content="Extern Declarations (GNU C Language Manual)">
|
|---|
| 26 | <meta name="resource-type" content="document">
|
|---|
| 27 | <meta name="distribution" content="global">
|
|---|
| 28 | <meta name="Generator" content="makeinfo">
|
|---|
| 29 | <link href="index.html" rel="start" title="Top">
|
|---|
| 30 | <link href="Symbol-Index.html" rel="index" title="Symbol Index">
|
|---|
| 31 | <link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
|
|---|
| 32 | <link href="Variables.html" rel="up" title="Variables">
|
|---|
| 33 | <link href="Allocating-File_002dScope.html" rel="next" title="Allocating File-Scope">
|
|---|
| 34 | <link href="Static-Local-Variables.html" rel="prev" title="Static Local Variables">
|
|---|
| 35 | <style type="text/css">
|
|---|
| 36 | <!--
|
|---|
| 37 | a.summary-letter {text-decoration: none}
|
|---|
| 38 | blockquote.indentedblock {margin-right: 0em}
|
|---|
| 39 | div.display {margin-left: 3.2em}
|
|---|
| 40 | div.example {margin-left: 3.2em}
|
|---|
| 41 | div.lisp {margin-left: 3.2em}
|
|---|
| 42 | kbd {font-style: oblique}
|
|---|
| 43 | pre.display {font-family: inherit}
|
|---|
| 44 | pre.format {font-family: inherit}
|
|---|
| 45 | pre.menu-comment {font-family: serif}
|
|---|
| 46 | pre.menu-preformatted {font-family: serif}
|
|---|
| 47 | span.nolinebreak {white-space: nowrap}
|
|---|
| 48 | span.roman {font-family: initial; font-weight: normal}
|
|---|
| 49 | span.sansserif {font-family: sans-serif; font-weight: normal}
|
|---|
| 50 | ul.no-bullet {list-style: none}
|
|---|
| 51 | -->
|
|---|
| 52 | </style>
|
|---|
| 53 |
|
|---|
| 54 |
|
|---|
| 55 | </head>
|
|---|
| 56 |
|
|---|
| 57 | <body lang="en">
|
|---|
| 58 | <span id="Extern-Declarations"></span><div class="header">
|
|---|
| 59 | <p>
|
|---|
| 60 | Next: <a href="Allocating-File_002dScope.html" accesskey="n" rel="next">Allocating File-Scope</a>, Previous: <a href="Static-Local-Variables.html" accesskey="p" rel="prev">Static Local Variables</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>
|
|---|
| 61 | </div>
|
|---|
| 62 | <hr>
|
|---|
| 63 | <span id="extern-Declarations"></span><h3 class="section">20.8 <code>extern</code> Declarations</h3>
|
|---|
| 64 | <span id="index-extern-declarations"></span>
|
|---|
| 65 | <span id="index-declarations_002c-extern"></span>
|
|---|
| 66 | <span id="index-extern"></span>
|
|---|
| 67 |
|
|---|
| 68 | <p>An <code>extern</code> declaration is used to refer to a global variable
|
|---|
| 69 | whose principal declaration comes elsewhere—in the same module, or in
|
|---|
| 70 | another compilation module. It looks like this:
|
|---|
| 71 | </p>
|
|---|
| 72 | <div class="example">
|
|---|
| 73 | <pre class="example">extern <var>basetype</var> <var>decorated-variable</var>;
|
|---|
| 74 | </pre></div>
|
|---|
| 75 |
|
|---|
| 76 | <p>Its meaning is that, in the current scope, the variable name refers to
|
|---|
| 77 | the file-scope variable of that name—which needs to be declared in a
|
|---|
| 78 | non-<code>extern</code>, non-<code>static</code> way somewhere else.
|
|---|
| 79 | </p>
|
|---|
| 80 | <p>For instance, if one compilation module has this global variable
|
|---|
| 81 | declaration
|
|---|
| 82 | </p>
|
|---|
| 83 | <div class="example">
|
|---|
| 84 | <pre class="example">int error_count = 0;
|
|---|
| 85 | </pre></div>
|
|---|
| 86 |
|
|---|
| 87 | <p>then other compilation modules can specify this
|
|---|
| 88 | </p>
|
|---|
| 89 | <div class="example">
|
|---|
| 90 | <pre class="example">extern int error_count;
|
|---|
| 91 | </pre></div>
|
|---|
| 92 |
|
|---|
| 93 | <p>to allow reference to the same variable.
|
|---|
| 94 | </p>
|
|---|
| 95 | <p>The usual place to write an <code>extern</code> declaration is at top level
|
|---|
| 96 | in a source file, but you can write an <code>extern</code> declaration
|
|---|
| 97 | inside a block to make a global or static file-scope variable
|
|---|
| 98 | accessible in that block.
|
|---|
| 99 | </p>
|
|---|
| 100 | <p>Since an <code>extern</code> declaration does not allocate space for the
|
|---|
| 101 | variable, it can omit the size of an array:
|
|---|
| 102 | </p>
|
|---|
| 103 | <div class="example">
|
|---|
| 104 | <pre class="example">extern int array[];
|
|---|
| 105 | </pre></div>
|
|---|
| 106 |
|
|---|
| 107 | <p>You can use <code>array</code> normally in all contexts where it is
|
|---|
| 108 | converted automatically to a pointer. However, to use it as the
|
|---|
| 109 | operand of <code>sizeof</code> is an error, since the size is unknown.
|
|---|
| 110 | </p>
|
|---|
| 111 | <p>It is valid to have multiple <code>extern</code> declarations for the same
|
|---|
| 112 | variable, even in the same scope, if they give the same type. They do
|
|---|
| 113 | not conflict—they agree. For an array, it is legitimate for some
|
|---|
| 114 | <code>extern</code> declarations can specify the size while others omit it.
|
|---|
| 115 | However, if two declarations give different sizes, that is an error.
|
|---|
| 116 | </p>
|
|---|
| 117 | <p>Likewise, you can use <code>extern</code> declarations at file scope
|
|---|
| 118 | (see <a href="File_002dScope-Variables.html">File-Scope Variables</a>) followed by an ordinary global
|
|---|
| 119 | (non-static) declaration of the same variable. They do not conflict,
|
|---|
| 120 | because they say compatible things about the same meaning of the variable.
|
|---|
| 121 | </p>
|
|---|
| 122 | <hr>
|
|---|
| 123 | <div class="header">
|
|---|
| 124 | <p>
|
|---|
| 125 | Next: <a href="Allocating-File_002dScope.html" accesskey="n" rel="next">Allocating File-Scope</a>, Previous: <a href="Static-Local-Variables.html" accesskey="p" rel="prev">Static Local Variables</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>
|
|---|
| 126 | </div>
|
|---|
| 127 |
|
|---|
| 128 |
|
|---|
| 129 |
|
|---|
| 130 | </body>
|
|---|
| 131 | </html>
|
|---|
