(cpp.info)Output

C Preprocessor Output
=====================

   The output from the C preprocessor looks much like the input, except
that all preprocessor command lines have been replaced with blank lines
and all comments with spaces.  Whitespace within a line is not altered;
however, a space is inserted after the expansions of most macro calls.

   Source file name and line number information is conveyed by lines of
the form

     # LINENUM FILENAME FLAGS

which are inserted as needed into the middle of the input (but never
within a string or character constant).  Such a line means that the
following line originated in file FILENAME at line LINENUM.

   After the file name comes zero or more flags, which are `1', `2' or
`3'.  If there are multiple flags, spaces separate them.  Here is what
the flags mean:

`1'
     This indicates the start of a new file.

`2'
     This indicates returning to a file (after having included another
     file).

`3'
     This indicates that the following text comes from a system header
     file, so certain warnings should be suppressed.