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<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
&lsquo;<samp><span class="samp">#line</span></samp>&rsquo; directives into the output file.  &lsquo;<samp><span class="samp">#line</span></samp>&rsquo; is a
directive that specifies the original line number and source file name
for subsequent input in the current preprocessor input file. 
&lsquo;<samp><span class="samp">#line</span></samp>&rsquo; 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
&lsquo;<samp><span class="samp">#include</span></samp>&rsquo;.

     <p>Previous versions of CPP did not interpret escapes in &lsquo;<samp><span class="samp">#line</span></samp>&rsquo;;
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>&lsquo;<samp><span class="samp">#line</span></samp>&rsquo; 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 &lsquo;<samp><span class="samp">#include</span></samp>&rsquo;'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 &lsquo;<samp><span class="samp">#include</span></samp>&rsquo; is not affected by
the presence of a &lsquo;<samp><span class="samp">#line</span></samp>&rsquo; 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 &lsquo;<samp><span class="samp">#line</span></samp>&rsquo; 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.

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