Another sizeof question

[Stephen Clamage]

Chris Volpe writes:

>I understand now what the rule is regarding sizeof, and why it is. 
>But, could you or someone else point me to a reference in the Standard
>that says that a constant expression in a preprocessor context cannot
>contain sizeof? I'm sure it's in there but I can't find it. 

To the preprocessor, sizeof is just an identifier, not a keyword.  You
will note that the Standard section 3.8 describes what the preprocessor
recognizes, and sizeof is not mentioned.  Neither are "for", "while",
"switch", or other C keywords.  Consider this:
	int x;
	#if goto(x) == 2
or this:
	int x;
	extern int foo(int);
	#if foo(x) == 2
Writing
	int x;
	#if sizeof(x) == 2
has precisely the same effect.  "Goto", "foo", and "sizeof" are treated
by the preprocessor as ordinary identifiers:  if they are not #defined
to the proprocessor, they are treated as constant integers with value 0.
-- 

Steve Clamage, TauMetric Corp, steve at taumet.com