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 number.
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 directive 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 arranges 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 filename
#line anything else
`#line' directives alter the results of the `__FILE__' and `__LINE__' predefined macros from that point on. See section Standard Predefined Macros.
The output of the preprocessor (which is the input for the rest of the compiler) contains directives that look much like `#line' directives. They start with just `#' instead of `#line', but this is followed by a line number and file name as in `#line'. See section C Preprocessor Output.