Posix shared memory and mmap()

[Sad Clouds <cryintothebluesky%googlemail.com@localhost>]

Am I correct in my thinking that on system that do not support Posix shared 
memory, the alternative way to have shared memory between unrelated processes 
is to create a temporary file and then call mmap(... MAP_SHARED).

So why bother implementing Posix shared memory functions (shm_open and 
shm_unlink), if the same can be achieved with mmap()?