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132 lines
6.9 KiB
HTML
132 lines
6.9 KiB
HTML
4 years ago
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<html lang="en">
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<head>
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<title>Line Control - The C Preprocessor</title>
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<meta http-equiv="Content-Type" content="text/html">
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<meta name="description" content="The C Preprocessor">
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<meta name="generator" content="makeinfo 4.13">
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<link title="Top" rel="start" href="index.html#Top">
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<link rel="prev" href="Diagnostics.html#Diagnostics" title="Diagnostics">
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<link rel="next" href="Pragmas.html#Pragmas" title="Pragmas">
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<link href="http://www.gnu.org/software/texinfo/" rel="generator-home" title="Texinfo Homepage">
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<!--
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Copyright (C) 1987-2015 Free Software Foundation, Inc.
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Permission is granted to copy, distribute and/or modify this document
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under the terms of the GNU Free Documentation License, Version 1.3 or
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any later version published by the Free Software Foundation. A copy of
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the license is included in the
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section entitled ``GNU Free Documentation License''.
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This manual contains no Invariant Sections. The Front-Cover Texts are
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(a) (see below), and the Back-Cover Texts are (b) (see below).
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(a) The FSF's Front-Cover Text is:
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A GNU Manual
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(b) The FSF's Back-Cover Text is:
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You have freedom to copy and modify this GNU Manual, like GNU
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software. Copies published by the Free Software Foundation raise
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funds for GNU development.
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<style type="text/css"><!--
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--></style>
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</head>
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<body>
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<div class="node">
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<a name="Line-Control"></a>
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<p>
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Next: <a rel="next" accesskey="n" href="Pragmas.html#Pragmas">Pragmas</a>,
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Previous: <a rel="previous" accesskey="p" href="Diagnostics.html#Diagnostics">Diagnostics</a>,
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Up: <a rel="up" accesskey="u" href="index.html#Top">Top</a>
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<hr>
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</div>
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<h2 class="chapter">6 Line Control</h2>
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<p><a name="index-line-control-98"></a>
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The C preprocessor informs the C compiler of the location in your source
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code where each token came from. Presently, this is just the file name
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and line number. All the tokens resulting from macro expansion are
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reported as having appeared on the line of the source file where the
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outermost macro was used. We intend to be more accurate in the future.
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<p>If you write a program which generates source code, such as the
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<samp><span class="command">bison</span></samp> parser generator, you may want to adjust the preprocessor's
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notion of the current file name and line number by hand. Parts of the
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output from <samp><span class="command">bison</span></samp> are generated from scratch, other parts come
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from a standard parser file. The rest are copied verbatim from
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<samp><span class="command">bison</span></samp>'s input. You would like compiler error messages and
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symbolic debuggers to be able to refer to <code>bison</code>'s input file.
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<p><a name="index-g_t_0023line-99"></a><samp><span class="command">bison</span></samp> or any such program can arrange this by writing
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‘<samp><span class="samp">#line</span></samp>’ directives into the output file. ‘<samp><span class="samp">#line</span></samp>’ is a
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directive that specifies the original line number and source file name
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for subsequent input in the current preprocessor input file.
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‘<samp><span class="samp">#line</span></samp>’ has three variants:
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<dl>
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<dt><code>#line </code><var>linenum</var><dd><var>linenum</var> is a non-negative decimal integer constant. It specifies
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the line number which should be reported for the following line of
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input. Subsequent lines are counted from <var>linenum</var>.
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<br><dt><code>#line </code><var>linenum</var> <var>filename</var><dd><var>linenum</var> is the same as for the first form, and has the same
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effect. In addition, <var>filename</var> is a string constant. The
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following line and all subsequent lines are reported to come from the
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file it specifies, until something else happens to change that.
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<var>filename</var> is interpreted according to the normal rules for a string
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constant: backslash escapes are interpreted. This is different from
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‘<samp><span class="samp">#include</span></samp>’.
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<p>Previous versions of CPP did not interpret escapes in ‘<samp><span class="samp">#line</span></samp>’;
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we have changed it because the standard requires they be interpreted,
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and most other compilers do.
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<br><dt><code>#line </code><var>anything else</var><dd><var>anything else</var> is checked for macro calls, which are expanded.
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The result should match one of the above two forms.
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</dl>
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<p>‘<samp><span class="samp">#line</span></samp>’ directives alter the results of the <code>__FILE__</code> and
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<code>__LINE__</code> predefined macros from that point on. See <a href="Standard-Predefined-Macros.html#Standard-Predefined-Macros">Standard Predefined Macros</a>. They do not have any effect on ‘<samp><span class="samp">#include</span></samp>’'s
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idea of the directory containing the current file. This is a change
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from GCC 2.95. Previously, a file reading
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<pre class="smallexample"> #line 1 "../src/gram.y"
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#include "gram.h"
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</pre>
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<p>would search for <samp><span class="file">gram.h</span></samp> in <samp><span class="file">../src</span></samp>, then the <samp><span class="option">-I</span></samp>
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chain; the directory containing the physical source file would not be
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searched. In GCC 3.0 and later, the ‘<samp><span class="samp">#include</span></samp>’ is not affected by
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the presence of a ‘<samp><span class="samp">#line</span></samp>’ referring to a different directory.
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<p>We made this change because the old behavior caused problems when
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generated source files were transported between machines. For instance,
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it is common practice to ship generated parsers with a source release,
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so that people building the distribution do not need to have yacc or
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Bison installed. These files frequently have ‘<samp><span class="samp">#line</span></samp>’ directives
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referring to the directory tree of the system where the distribution was
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created. If GCC tries to search for headers in those directories, the
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build is likely to fail.
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<p>The new behavior can cause failures too, if the generated file is not
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in the same directory as its source and it attempts to include a header
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which would be visible searching from the directory containing the
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source file. However, this problem is easily solved with an additional
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<samp><span class="option">-I</span></samp> switch on the command line. The failures caused by the old
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semantics could sometimes be corrected only by editing the generated
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files, which is difficult and error-prone.
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</body></html>
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