(cpp.info)Combining Sources

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Combining Source Files

   One of the jobs of the C preprocessor is to inform the C compiler of
where each line of C code came from: which source file and which line

   C code can come from multiple source files if you use `#include';
both `#include' and the use of conditionals and macros can cause the
line number of a line in the preprocessor output to be different from
the line's number in the original source file.  You will appreciate the
value of making both the C compiler (in error messages) and symbolic
debuggers such as GDB use the line numbers in your source file.

   The C preprocessor builds on this feature by offering a command by
which you can control the feature explicitly.  This is useful when a
file for input to the C preprocessor is the output from another program
such as the `bison' parser generator, which operates on another file
that is the true source file.  Parts of the output from `bison' are
generated from scratch, other parts come from a standard parser file.
The rest are copied nearly verbatim from the source file, but their
line numbers in the `bison' output are not the same as their original
line numbers.  Naturally you would like compiler error messages and
symbolic debuggers to know the original source file and line number of
each line in the `bison' input.

   `bison' arranges this by writing `#line' commands into the output
file.  `#line' is a command 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'
     Here LINENUM is a decimal integer constant.  This specifies that
     the line number of the following line of input, in its original
     source file, was LINENUM.

     Here LINENUM is a decimal integer constant and FILENAME is a
     string constant.  This specifies that the following line of input
     came originally from source file FILENAME and its line number there
     was LINENUM.  Keep in mind that FILENAME is not just a file name;
     it is surrounded by doublequote characters so that it looks like a
     string constant.

     ANYTHING ELSE is checked for macro calls, which are expanded.  The
     result should be a decimal integer constant followed optionally by
     a string constant, as described above.

   `#line' commands alter the results of the `__FILE__' and `__LINE__'
predefined macros from that point on.  Note: Standard Predefined.

   The output of the preprocessor (which is the input for the rest of
the compiler) contains commands that look much like `#line' commands.
They start with just `#' instead of `#line', but this is followed by a
line number and file name as in `#line'.  Note: Output.

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