cpp.info: Line Control

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Line Control

   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.
   If you write a program which generates source code, such as the
`bison' 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 `bison' are generated from scratch, other parts come from a
standard parser file.  The rest are copied verbatim from `bison''s
input.  You would like compiler error messages and symbolic debuggers
to be able to refer to `bison''s input file.
   `bison' or any such program can arrange this by writing `#line'
directives into the output file.  `#line' is a directive that specifies
the original line number and source file name for subsequent input in
the current preprocessor input file.  `#line' has three variants:
`#line LINENUM'
     LINENUM 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 LINENUM.
     LINENUM is the same as for the first form, and has the same
     effect.  In addition, FILENAME 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.
     FILENAME is interpreted according to the normal rules for a string
     constant: backslash escapes are interpreted.  This is different
     from `#include'.
     Previous versions of GNU CPP did not interpret escapes in `#line';
     we have changed it because the standard requires they be
     interpreted, and most other compilers do.
     ANYTHING ELSE is checked for macro calls, which are expanded.  The
     result should match one of the above two forms.
   `#line' directives alter the results of the `__FILE__' and
`__LINE__' predefined macros from that point on.  *Note Standard
Predefined Macros::.  They do not have any effect on `#include''s idea
of the directory containing the current file.  This is a change from
GCC 2.95.  Previously, a file reading
     #line 1 "../src/gram.y"
     #include "gram.h"
   would search for `gram.h' in `../src', then the `-I' chain; the
directory containing the physical source file would not be searched.
In GCC 3.0 and later, the `#include' is not affected by the presence of
a `#line' referring to a different directory.
   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 `#line' 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.
   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
`-I' 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.