Re: Pre-Parsers

Hans-Bernhard Broeker <broeker@physik.rwth-aachen.de> writes:


>[someone wrote:
>> #if sizeof(int)==2 ...
>> Here sizeof() must be evaluated by the preprocessor.
>
>'Must' is debatable, here. With equal justification, one could say
>that source that contains such lines 'must' be rejected for being
>incompatible with the definition of the language it's supposed to be
>written in...


and our moderator replies:


>[What definition would that be? ANSI/ISO C certainly permits sizeof in
>preprocessor expressions. -John]


ANSI/ISO C permits the identifier `sizeof' in preprocessor expressions,
but the standard says it must not be treated as a keyword. If it is
a macro, it must be expanded, and otherwise, it must be replaced with `0'.
The same applies to the other identifier `int' in that source code.
If neither `sizeof' nor `int' are defined as macros, the condition must
expand to `0(0)==2', which is a syntax error, and so the
implementation must report a diagnostic.


The main reason for this is to allow the C preprocessor to be
implemented as completely separate pass from the C compiler, since
that is the way it was traditionally done. Some C compilers which
have an integrated preprocessor do support `sizeof' and such like
in `#if' conditions, as an (incompatible) extension to standard C.


--
Fergus Henderson <fjh@cs.mu.oz.au> | "I have always known that the pursuit
WWW: <http://www.cs.mu.oz.au/~fjh> | of excellence is a lethal habit"
PGP: finger fjh@128.250.37.3 | -- the last words of T. S. Garp.
[Oops. Sorry about that. -John]

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