cpp.info: Preprocessor Output

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Preprocessor Output

   When the C preprocessor is used with the C, C++, or Objective-C
compilers, it is integrated into the compiler and communicates a stream
of binary tokens directly to the compiler's parser.  However, it can
also be used in the more conventional standalone mode, where it produces
textual 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.  Long runs of blank lines are
discarded.
   The ISO standard specifies that it is implementation defined whether
a preprocessor preserves whitespace between tokens, or replaces it with
e.g. a single space.  In GNU CPP, whitespace between tokens is collapsed
to become a single space, with the exception that the first token on a
non-directive line is preceded with sufficient spaces that it appears in
the same column in the preprocessed output that it appeared in the
original source file.  This is so the output is easy to read.  *Note
Differences from previous versions::.  CPP does not insert any
whitespace where there was none in the original source, except where
necessary to prevent an accidental token paste.
   Source file name and line number information is conveyed by lines of
the form
     # LINENUM FILENAME FLAGS
These are called "linemarkers".  They are inserted as needed into the
output (but never within a string or character constant).  They mean
that the following line originated in file FILENAME at line LINENUM.
FILENAME will never contain any non-printing characters; they are
replaced with octal escape sequences.
   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 being
     wrapped in an implicit `extern "C"' block.
   As an extension, the preprocessor accepts linemarkers in
non-assembler input files.  They are treated like the corresponding
`#line' directive, (*note Line Control::), except that trailing flags
are permitted, and are interpreted with the meanings described above.
If multiple flags are given, they must be in ascending order.
   Some directives may be duplicated in the output of the preprocessor.
These are `#ident' (always), `#pragma' (only if the preprocessor does
not handle the pragma itself), and `#define' and `#undef' (with certain
debugging options).  If this happens, the `#' of the directive will
always be in the first column, and there will be no space between the
`#' and the directive name.  If macro expansion happens to generate
tokens which might be mistaken for a duplicated directive, a space will
be inserted between the `#' and the directive name.