Re: C scanners, was Hand written or tool generated lexical analyzers for FORTRAN

Nick Maclaren <nmm1@cus.cam.ac.uk> wrote:


> But, if you exclude that, and the more contorted use of the # and ##
> preprocessor operations, I believe that most C compilers do work the
> way you say. There are some truly horrible ambiguities with those
> operators, but thankfully nobody seems to use the constructions that
> trigger them.


I don't think that there are "horrible ambiquities" (at least not when
compared to the rest of the language) with the # and ## operators of
the C preprocessor. The semantics of the # and ## operators are
relatively clearly spelled out. However, based on my experience in
writing C preprocessor metaprograms, like


    http://cvs.sourceforge.net/viewcvs.py/chaos-pp/order-pp/example/bottles.c?view=markup


it seems that relatively few preprocessors implement the C
preprocessor correctly.


> #include <stddef.h>
> #define A(x) # x
> A(offsetof)


> is one of the simpler and more unambiguously implementation dependent.


Are you saying that the token sequence resulting from preprocessing
the above macro invocation


    A(offsetof)


is implementation dependent? It isn't:


                      "If, in the replacement list, a parameter is immediately
              preceded by a # preprocessing token, both are replaced by a
              single character string literal preprocessing token that
              contains the spelling of the preprocessing token sequence
              for the corresponding argument. Each occurrence of white
              space between the argument's preprocessing tokens becomes a
              single space character in the character string literal.
              White space before the first preprocessing token and after
              the last preprocessing token composing the argument is
              deleted.")


The result of the above macro invocation must be


    "offsetof"


on all C99 conforming preprocessors.


-Vesa Karvonen

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