#ifdef THISOS
# include A
#else
# include B
#endif

gcc -DTHISOS -UFOO
what about a 3rd OS?

CPP: simple textual replacement of tokens!
CPP performs TOKEN replacement
	#define P x
	won't replace 'P' in PORT=1024;



- pros vs. cons: speed code bloat

CPP "function-like" macros, but not same semantics as real functions

------------------------------------------------------------------------------
- why you need to (enclose) variables in ()?
#define check(x)	if (x && y()) printf("something")

call as
	check(a() || b())
becomes
	if (a() || b() && y()) printf("something")

"a || b" is NOT evaluated first! (side effects could change behaviour)
- solution:
#define check(x)	if ((x) && y()) printf("something")
------------------------------------------------------------------------------

pitfalls
a[x,y] vs. a[x][y] (comma operator in C)