Does the C preprocessor strip comments or expand macros first?

Question

Consider this (horrible, terrible, no good, very bad) code structure:

#define foo(x) // commented out debugging code

// Misformatted to not obscure the point
if (a)
foo(a);
bar(a);

I've seen two compilers' preprocessors generate different results on this code:

if (a)
bar(a);

and

if (a)
;
bar(a);

Obviously, this is a bad thing for a portable code base.

My question: What is the preprocessor supposed to do with this? Elide comments first, or expand macros first?

user_564812 · Answer

#ifdef _TEST_
#define _cerr cerr
#else
#define _cerr / ## / cerr
#endif