C preprocessor question

[jtr485 at umich.UUCP]

In article <818 at wjvax.wjvax.UUCP>, brett at wjvax.UUCP writes:
> I have a question about the C preprocessor.  I have the following code
> fragment, which fails to compile on my system (4.2BSD):
> 
> #define	GROUP(group,subgroup)	(((group) << 8) | (subgroup))
> 
> #if	GROUP(0,0)
> #endif
> 
> The #if chokes for some reason.  Can anyone in comp.lang.c see my error?
> If there is no error, is this a known bug of the 4.2BSD cpp?
> Brett Galloway
What do you expect his to do?

The #if is not going to evaluate the expression (((0) << 8) | (0))

The #if takes an expression defined (recursively) by:
an <expression> is
	1) an integer string
     or 2) (<expression> | <expression>)
     or 3) (<expression> & <expression>)
and nothing else.
The only test you could perform on GROUP would be #ifdef.

--j.a.tainter