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Differences from previous versions

   This section details behavior which has changed from previous
versions of GNU CPP.  We do not plan to change it again in the near
future, but we do not promise not to, either.
   The "previous versions" discussed here are 2.95 and before.  The
behavior of GCC 3.0 is mostly the same as the behavior of the widely
used 2.96 and 2.97 development snapshots.  Where there are differences,
they generally represent bugs in the snapshots.
   * Order of evaluation of `#' and `##' operators
     The standard does not specify the order of evaluation of a chain of
     `##' operators, nor whether `#' is evaluated before, after, or at
     the same time as `##'.  You should therefore not write any code
     which depends on any specific ordering.  It is possible to
     guarantee an ordering, if you need one, by suitable use of nested
     macros.
     An example of where this might matter is pasting the arguments `1',
     `e' and `-2'.  This would be fine for left-to-right pasting, but
     right-to-left pasting would produce an invalid token `e-2'.
     GCC 3.0 evaluates `#' and `##' at the same time and strictly left
     to right.  Older versions evaluated all `#' operators first, then
     all `##' operators, in an unreliable order.
   * The form of whitespace betwen tokens in preprocessor output
     *Note Preprocessor Output::, for the current textual format.  This
     is also the format used by stringification.  Normally, the
     preprocessor communicates tokens directly to the compiler's
     parser, and whitespace does not come up at all.
     Older versions of GCC preserved all whitespace provided by the
     user and inserted lots more whitespace of their own, because they
     could not accurately predict when extra spaces were needed to
     prevent accidental token pasting.
   * Optional argument when invoking rest argument macros
     As an extension, GCC permits you to omit the variable arguments
     entirely when you use a variable argument macro.  This is
     forbidden by the 1999 C standard, and will provoke a pedantic
     warning with GCC 3.0.  Previous versions accepted it silently.
   * `##' swallowing preceding text in rest argument macros
     Formerly, in a macro expansion, if `##' appeared before a variable
     arguments parameter, and the set of tokens specified for that
     argument in the macro invocation was empty, previous versions of
     GNU CPP would back up and remove the preceding sequence of
     non-whitespace characters (*not* the preceding token).  This
     extension is in direct conflict with the 1999 C standard and has
     been drastically pared back.
     In the current version of the preprocessor, if `##' appears between
     a comma and a variable arguments parameter, and the variable
     argument is omitted entirely, the comma will be removed from the
     expansion.  If the variable argument is empty, or the token before
     `##' is not a comma, then `##' behaves as a normal token paste.
   * Traditional mode and GNU extensions
     Traditional mode used to be implemented in the same program as
     normal preprocessing.  Therefore, all the GNU extensions to the
     preprocessor were still available in traditional mode.  It is now
     a separate program and does not implement any of the GNU
     extensions, except for a partial implementation of assertions.
     Even those may be removed in a future release.
   * `#line' and `#include'
     The `#line' directive used to change GCC's notion of the
     "directory containing the current file," used by `#include' with a
     double-quoted header file name.  In 3.0 and later, it does not.
     *Note Line Control::, for further explanation.
   * Syntax of `#line'
     In GCC 2.95 and previous, the string constant argument to `#line'
     was treated the same way as the argument to `#include': backslash
     escapes were not honored, and the string ended at the second `"'.
     This is not compliant with the C standard.  In GCC 3.0, an attempt
     was made to correct the behavior, so that the string was treated
     as a real string constant, but it turned out to be buggy.  In 3.1,
     the bugs have been fixed.  (We are not fixing the bugs in 3.0
     because they affect relatively few people and the fix is quite
     invasive.)