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 C Preprocessor Output
 =====================
 
    The output from the C preprocessor looks much like the input, except
 that all preprocessing directive lines have been replaced with blank
 lines and all comments with spaces.  Whitespace within a line is not
 altered; however, unless `-traditional' is used, spaces may be inserted
 into the expansions of macro calls to prevent tokens from being
 concatenated.
 
    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',
 `3', or `4'.  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.
 
 `4'
      This indicates that the following text should be treated as C.
 

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