Re: Preprocessing directive in the middle of macro arguments

John Bode wrote:
On Jan 16, 8:57 am, Francois Grieu <fgr...@xxxxxxxxx> wrote:
One of the C compiler that I use <OT>(Keil's CX51)</OT> barks at

#define a(b) b
int main(void){return a(
#if 0
#endif
0);}

More generally, this compiler seems confused by any preprocessing
directive in the middle of macro arguments, which comes handy e.g. to
conditionaly select one of several arguments passed to a macro.

Is it a compiler bug? Where does C89 define if this is valid or not?

TIA.
Francois Grieu

It's not a bug; you cannot embed preprocessor commands in a macro.
All preprocessor directives are processed before any macro expansion
takes place, so any preprocessor directives embedded in a macro will
not be recognized as such, but will instead be translated as regular C
code.
The OP's code has no embedded directives; it's equivalent to
#define a(b) b
int main(void){return a(
0);}
Which is good C, IMHO. So I think it's a compiler bug.
<OT>
Getting the C front end right is surprisingly hard; there are enough C vendors to make a market for C front end parsers, and it exists.
Still, some people do it on their own, and sometimes incorrectly. I had trouble with Keil for ARM (see http://www.macroexpressions.com/dl/compileme.c), but I never tried their latest version.
I wonder if this fixes the OP problem :)
#de\
fine \
a(b\
b)=bb
</OT>
--
Ark
.

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