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| 8 | Permission is granted to copy, distribute and/or modify this document
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| 20 | <head>
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| 21 | <meta http-equiv="Content-Type" content="text/html; charset=utf-8">
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| 22 | <title>Line Control (GNU C Language Manual)</title>
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| 23 |
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| 24 | <meta name="description" content="Line Control (GNU C Language Manual)">
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| 25 | <meta name="keywords" content="Line Control (GNU C Language Manual)">
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| 29 | <link href="index.html" rel="start" title="Top">
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| 30 | <link href="Symbol-Index.html" rel="index" title="Symbol Index">
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| 31 | <link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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| 32 | <link href="Preprocessing.html" rel="up" title="Preprocessing">
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| 33 | <link href="Null-Directive.html" rel="next" title="Null Directive">
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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="Line-Control"></span><div class="header">
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| 59 | <p>
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| 60 | Next: <a href="Null-Directive.html" accesskey="n" rel="next">Null Directive</a>, Previous: <a href="Diagnostics.html" accesskey="p" rel="prev">Diagnostics</a>, Up: <a href="Preprocessing.html" accesskey="u" rel="up">Preprocessing</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="Line-Control-1"></span><h3 class="section">26.8 Line Control</h3>
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| 64 | <span id="index-line-control"></span>
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| 65 |
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| 66 | <p>Due to C’s widespread availability and low-level nature, it is often
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| 67 | used as the target language for translation of other languages, or for
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| 68 | the output of lexical analyzers and parsers (e.g., lex/flex and
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| 69 | yacc/bison). Line control enables the user to track diagnostics back
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| 70 | to the location in the original language.
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| 71 | </p>
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| 72 | <p>The C compiler knows the location in the source file where each token
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| 73 | came from: file name, starting line and column, and final line and column.
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| 74 | (Column numbers are used only for error messages.)
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| 75 | </p>
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| 76 | <p>When a program generates C source code, as the Bison parser generator
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| 77 | does, often it copies some of that C code from another file. For
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| 78 | instance parts of the output from Bison are generated from scratch or
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| 79 | come from a standard parser file, but Bison copies the rest from
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| 80 | Bison’s input file. Errors in that code, at compile time or run time,
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| 81 | should refer to that file, which is the real source code. To make that happen,
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| 82 | Bison generates line-control directives that the C compiler understands.
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| 83 | </p>
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| 84 | <span id="index-_0023line"></span>
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| 85 | <p><code>#line</code> is a directive that specifies the original line number
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| 86 | and source file name for subsequent code. <code>#line</code> has three
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| 87 | variants:
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| 88 | </p>
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| 89 | <dl compact="compact">
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| 90 | <dt><code>#line <var>linenum</var></code></dt>
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| 91 | <dd><p><var>linenum</var> is a non-negative decimal integer constant. It specifies
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| 92 | the line number that should be reported for the following line of
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| 93 | input. Subsequent lines are counted from <var>linenum</var>.
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| 94 | </p>
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| 95 | </dd>
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| 96 | <dt><code>#line <var>linenum</var> <var>filename</var></code></dt>
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| 97 | <dd><p><var>linenum</var> is the same as for the first form, and has the same
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| 98 | effect. In addition, <var>filename</var> is a string constant that
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| 99 | specifies the source file name. Subsequent source lines are recorded
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| 100 | as coming from that file, until something else happens to change that.
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| 101 | <var>filename</var> is interpreted according to the normal rules for a
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| 102 | string constant. Backslash escapes are interpreted, in contrast to
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| 103 | <code>#include</code>.
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| 104 | </p>
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| 105 | </dd>
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| 106 | <dt><code>#line <var>anything else</var></code></dt>
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| 107 | <dd><p><var>anything else</var> is checked for macro calls, which are expanded.
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| 108 | The result should match one of the above two forms.
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| 109 | </p></dd>
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| 110 | </dl>
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| 111 |
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| 112 | <p><code>#line</code> directives alter the results of the <code>__FILE__</code> and
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| 113 | <code>__LINE__</code> symbols from that point on. See <a href="Predefined-Macros.html">Predefined Macros</a>.
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| 114 | </p>
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| 115 | <hr>
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| 116 | <div class="header">
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| 117 | <p>
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| 118 | Next: <a href="Null-Directive.html" accesskey="n" rel="next">Null Directive</a>, Previous: <a href="Diagnostics.html" accesskey="p" rel="prev">Diagnostics</a>, Up: <a href="Preprocessing.html" accesskey="u" rel="up">Preprocessing</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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| 119 | </div>
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| 120 |
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| 121 |
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| 122 |
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