C preprocessor question

[apn at nonvon.UUCP]

in article <818 at wjvax.wjvax.UUCP>, brett at wjvax.UUCP (Brett Galloway) says:
> 
> 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?
> -- 

	I don't think this should work....

	try running 'cc -E file.c' for some clues.
	this will give you a output essentially a file
	which is what your program looks like to the actual
	c compiler after macros are expanded.

CPP evaluates the macro 'GROUP' with arguments 0,0
to be:

(((0) << 8) | ( 0 ))

which is not a valid c line if it is stand alone.

be carefull.. CPP is for just that , preprocessing.... i.e.
#if's are compile time options and not
run time options. 


-- 
	Alex P Novickis

	UUCP:   {seismo, sun, ihnp4, 'etc' }!ptsfa!nonvon!apn

{* Only those who attempt the absurd   ...   will achieve the impossible   *}
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