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| <title>The C Preprocessor: Line Control</title> |
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| <a name="Line-Control-1"></a> |
| <h2 class="chapter">6 Line Control</h2> |
| <a name="index-line-control"></a> |
| |
| <p>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> |
| <p>If you write a program which generates source code, such as the |
| <code>bison</code> 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 <code>bison</code> are generated from scratch, other parts come |
| from a standard parser file. The rest are copied verbatim from |
| <code>bison</code>’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-_0023line"></a> |
| <p><code>bison</code> or any such program can arrange this by writing |
| ‘<samp>#line</samp>’ directives into the output file. ‘<samp>#line</samp>’ is a |
| directive that specifies the original line number and source file name |
| for subsequent input in the current preprocessor input file. |
| ‘<samp>#line</samp>’ has three variants: |
| </p> |
| <dl compact="compact"> |
| <dt><code>#line <var>linenum</var></code></dt> |
| <dd><p><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>. |
| </p> |
| </dd> |
| <dt><code>#line <var>linenum</var> <var>filename</var></code></dt> |
| <dd><p><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>#include</samp>’. |
| </p> |
| <p>Previous versions of CPP did not interpret escapes in ‘<samp>#line</samp>’; |
| we have changed it because the standard requires they be interpreted, |
| and most other compilers do. |
| </p> |
| </dd> |
| <dt><code>#line <var>anything else</var></code></dt> |
| <dd><p><var>anything else</var> is checked for macro calls, which are expanded. |
| The result should match one of the above two forms. |
| </p></dd> |
| </dl> |
| |
| <p>‘<samp>#line</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>#include</samp>’’s |
| idea of the directory containing the current file. This is a change |
| from GCC 2.95. Previously, a file reading |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample">#include "gram.h" |
| </pre></div> |
| |
| <p>would search for <samp>gram.h</samp> in <samp>../src</samp>, then the <samp>-I</samp> |
| chain; the directory containing the physical source file would not be |
| searched. In GCC 3.0 and later, the ‘<samp>#include</samp>’ is not affected by |
| the presence of a ‘<samp>#line</samp>’ referring to a different directory. |
| </p> |
| <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>#line</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> |
| <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>-I</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. |
| </p> |
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