Another sizeof question

[Stephen Clamage]

volpe at camelback.crd.ge.com (Christopher R Volpe) writes:

>Can someone tell me whether "sizeof" expressions are allowed as 
>operands of "#if"? I'm trying to do some conditional compilation
>based on the size of some data structure, and I get parse errors
>before sizeof. The Bible says "sizeof" yields a constant integer,
>and that #if requires an integral constant expression, so I can't
>tell what's wrong. Can someone clue me in? Thanks.

In ANSI C, sizeof may not usefully appear in an #if because it is part
of a different phase of translation.  The preprocessor knows nothing
about types or variables, and sizeof is not a preprocessor keyword.
The preprocessor must treat "sizeof" as it does any undefined macro name.

Some C compilers support sizeof in the preprocessor.  The results are
then kind of weird.  Such preprocessors sometimes recognize the size of
the built-in types (int, long, etc), but not user types.  Recognizing
the size of user types is part of the C compilation process, not just
preprocessing, so a stand-alone preprocessor must either act as a
complete C parser, or act differently than when it is part of the
complete compilation.
-- 

Steve Clamage, TauMetric Corp, steve at taumet.com