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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


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:

     This indicates the start of a new file.

     This indicates returning to a file (after having included another

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

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