how has C bitten you?

[Peter S. Shenkin]

I've had several bugs involving code hidden in macro definitions which have 
been very difficult to find.  One I recall offhand went something like this:

/* OPEN MOUTH *****************************************************************/
#define Coords(I)	(complicated.structure.redirection[I].x, \
			 complicated.structure.redirection[I].y, \
			 complicated.structure.redirection[I].z   )
main()
{
	...
	subr(Coords(i));  /* BITE */
	...
}
/***************************************************************************/
subr(x,y,z)
float x,y,z;
{...}
/* SWALLOW ******************************************************************/

Problem is, when expanded, the call to subr looks like
	subr((exp1,exp2,exp3));
The comma operator is applied, and subr() gets only exp1 !!!  The interesting
thing is that if anyone had asked me, whether (something), ((something)),
and (((something))) mean the same in C, I would have said "Yes," without
thinking.  Obviously, I would have been wrong.

Peter S. Shenkin	philabs!cubsvax!peters		Columbia Univ. Biology