Benchmarking issues apart, not only opendir and open don't do the same thing: more precisely the OP gives two lists of items presenting them, for some strange reason, as "examples system operators" and "built in functions" respectively. But no item from the first one is equivalent to any of the second one for any reasonable acceptation of "equivalent"!

Also, all of those items represent a built in functions with that name, that actually do system calls. So I really don't know what he may have in mind, but I wonder how he could even cmpthese() stuff that bears no resemblance to each other. Maybe he's not (yet) prematurely optimizing, but he seems definitely to be concerned about premature optimization, which we all know, is Bad(TM).