The manual says that al_malloc is

what does that mean? Overriding the implementation with what? Is there a reason I should be using al_malloc? And most importantly, should I be using the al_ memory functions differently from how I use malloc() and friends?

al_malloc() calls malloc() unless you override the memory management functions with al_set_memory_interface().

Going out on a limb (or boxing glove), I'm betting gentix is more confused now than pre-post.

It's just that you can override al_malloc and friends with versions that log memory allocations for debugging purposes. In short, it probably doesn't matter either way which you use

If you are curious you can read memory.c and memory.h in Allegro's source, they are short and easy to understand.

A few functions require you to use al_malloc or al_free. This will be mentioned in the documentation. Apart from being overridable, the version of malloc/free that Allegro sees may not necessarily match the malloc/free that your program sees.