If you know the code is only making an open call but your getting a SEGV on a open64 then my first thought would be to check the flags on the open call.
The reference lists 'O_LARGEFILE' as an option for open that tells the system that it actually wants a large file not a regular file. If your flags are scrambled and that one is being set it would cause the system to make your open call into an open64 call with the other garbled flags.
As for how to control it, all of the '64' versions of the file APIs are supposed to be exactly like the regular version except they can take a much bigger offset.
The only reason the ones like open that don't take an offset have a 64 version is because it was easier to do it that way in most Unix platforms.
Jay