I'm not from that generation so that's a bit hard for me to understand. Even if you used a closed-source C compiler, wouldn't you still have been able to look at the header file, which would have been pretty self-explanatory?
E.g.
void qsort(void* base, size_t nmemb, size_t size,
int (compar)(const void , const void* ));
And surely if you bought a C compiler, you would have gotten a manual or two with it? Documentation from the pre-Internet age tended to be much better than today.