| <html lang="en"> |
| <head> |
| <title>Line Control - The C Preprocessor</title> |
| <meta http-equiv="Content-Type" content="text/html"> |
| <meta name="description" content="The C Preprocessor"> |
| <meta name="generator" content="makeinfo 4.13"> |
| <link title="Top" rel="start" href="index.html#Top"> |
| <link rel="prev" href="Diagnostics.html#Diagnostics" title="Diagnostics"> |
| <link rel="next" href="Pragmas.html#Pragmas" title="Pragmas"> |
| <link href="http://www.gnu.org/software/texinfo/" rel="generator-home" title="Texinfo Homepage"> |
| <!-- |
| Copyright (C) 1987, 1989, 1991, 1992, 1993, 1994, 1995, 1996, |
| 1997, 1998, 1999, 2000, 2001, 2002, 2003, 2004, 2005, 2006, 2007, |
| 2008, 2009, 2010 |
| Free Software Foundation, Inc. |
| |
| Permission is granted to copy, distribute and/or modify this document |
| under the terms of the GNU Free Documentation License, Version 1.2 or |
| any later version published by the Free Software Foundation. A copy of |
| the license is included in the |
| section entitled ``GNU Free Documentation License''. |
| |
| This manual contains no Invariant Sections. The Front-Cover Texts are |
| (a) (see below), and the Back-Cover Texts are (b) (see below). |
| |
| (a) The FSF's Front-Cover Text is: |
| |
| A GNU Manual |
| |
| (b) The FSF's Back-Cover Text is: |
| |
| You have freedom to copy and modify this GNU Manual, like GNU |
| software. Copies published by the Free Software Foundation raise |
| funds for GNU development. |
| --> |
| <meta http-equiv="Content-Style-Type" content="text/css"> |
| <style type="text/css"><!-- |
| pre.display { font-family:inherit } |
| pre.format { font-family:inherit } |
| pre.smalldisplay { font-family:inherit; font-size:smaller } |
| pre.smallformat { font-family:inherit; font-size:smaller } |
| pre.smallexample { font-size:smaller } |
| pre.smalllisp { font-size:smaller } |
| span.sc { font-variant:small-caps } |
| span.roman { font-family:serif; font-weight:normal; } |
| span.sansserif { font-family:sans-serif; font-weight:normal; } |
| --></style> |
| <link rel="stylesheet" type="text/css" href="../cs.css"> |
| </head> |
| <body> |
| <div class="node"> |
| <a name="Line-Control"></a> |
| <p> |
| Next: <a rel="next" accesskey="n" href="Pragmas.html#Pragmas">Pragmas</a>, |
| Previous: <a rel="previous" accesskey="p" href="Diagnostics.html#Diagnostics">Diagnostics</a>, |
| Up: <a rel="up" accesskey="u" href="index.html#Top">Top</a> |
| <hr> |
| </div> |
| |
| <h2 class="chapter">6 Line Control</h2> |
| |
| <p><a name="index-line-control-98"></a> |
| The C preprocessor informs the C compiler of the location in your source |
| code where each token came from. Presently, this is just the file name |
| and line number. All the tokens resulting from macro expansion are |
| reported as having appeared on the line of the source file where the |
| outermost macro was used. We intend to be more accurate in the future. |
| |
| <p>If you write a program which generates source code, such as the |
| <samp><span class="command">bison</span></samp> parser generator, you may want to adjust the preprocessor's |
| notion of the current file name and line number by hand. Parts of the |
| output from <samp><span class="command">bison</span></samp> are generated from scratch, other parts come |
| from a standard parser file. The rest are copied verbatim from |
| <samp><span class="command">bison</span></samp>'s input. You would like compiler error messages and |
| symbolic debuggers to be able to refer to <code>bison</code>'s input file. |
| |
| <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 |
| ‘<samp><span class="samp">#line</span></samp>’ directives into the output file. ‘<samp><span class="samp">#line</span></samp>’ is a |
| directive that specifies the original line number and source file name |
| for subsequent input in the current preprocessor input file. |
| ‘<samp><span class="samp">#line</span></samp>’ has three variants: |
| |
| <dl> |
| <dt><code>#line </code><var>linenum</var><dd><var>linenum</var> is a non-negative decimal integer constant. It specifies |
| the line number which should be reported for the following line of |
| input. Subsequent lines are counted from <var>linenum</var>. |
| |
| <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 |
| effect. In addition, <var>filename</var> is a string constant. The |
| following line and all subsequent lines are reported to come from the |
| file it specifies, until something else happens to change that. |
| <var>filename</var> is interpreted according to the normal rules for a string |
| constant: backslash escapes are interpreted. This is different from |
| ‘<samp><span class="samp">#include</span></samp>’. |
| |
| <p>Previous versions of CPP did not interpret escapes in ‘<samp><span class="samp">#line</span></samp>’; |
| we have changed it because the standard requires they be interpreted, |
| and most other compilers do. |
| |
| <br><dt><code>#line </code><var>anything else</var><dd><var>anything else</var> is checked for macro calls, which are expanded. |
| The result should match one of the above two forms. |
| </dl> |
| |
| <p>‘<samp><span class="samp">#line</span></samp>’ directives alter the results of the <code>__FILE__</code> and |
| <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 |
| idea of the directory containing the current file. This is a change |
| from GCC 2.95. Previously, a file reading |
| |
| <pre class="smallexample"> #line 1 "../src/gram.y" |
| #include "gram.h" |
| </pre> |
| <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> |
| chain; the directory containing the physical source file would not be |
| searched. In GCC 3.0 and later, the ‘<samp><span class="samp">#include</span></samp>’ is not affected by |
| the presence of a ‘<samp><span class="samp">#line</span></samp>’ referring to a different directory. |
| |
| <p>We made this change because the old behavior caused problems when |
| generated source files were transported between machines. For instance, |
| it is common practice to ship generated parsers with a source release, |
| so that people building the distribution do not need to have yacc or |
| Bison installed. These files frequently have ‘<samp><span class="samp">#line</span></samp>’ directives |
| referring to the directory tree of the system where the distribution was |
| created. If GCC tries to search for headers in those directories, the |
| build is likely to fail. |
| |
| <p>The new behavior can cause failures too, if the generated file is not |
| in the same directory as its source and it attempts to include a header |
| which would be visible searching from the directory containing the |
| source file. However, this problem is easily solved with an additional |
| <samp><span class="option">-I</span></samp> switch on the command line. The failures caused by the old |
| semantics could sometimes be corrected only by editing the generated |
| files, which is difficult and error-prone. |
| |
| </body></html> |
| |
