I found a lot of bugs went away when I switched to STL (Standard Template Library) arrays and ditched managing my own memory. That's C++, I guess it's not available in straight C?
(ptr) = malloc((n) * sizeof (ptr)[0]);
(ptr) = GC_MALLOC((n) * sizeof (ptr)[0]);
No, because C doesn't have templates. The best you can do for a "vector" in C is macros like above, that also realloc, or write an API around structs for each type.