bytes malloc-ed less bytes free-ed

as kind of a followup to my last question...

is there a way to determine this quantity: the amount of memory
malloc-ed minus the amount of memory free-ed? It seems that this would
be easy for malloc()/free() to keep track of.

I would like to compare that value with the process size to get an
idea of how much memory my program needs vs. how much it is actually
taking up.

Thanks,

ER