I read in sem_init(3) man page that the implementation did not allow
interprocess synchronization using POSIX semaphores (or at least
that's what I understand from it).

Is this correct?  If so, isn't their usefulness greatly reduced,
considering that we already have conditional variables, mutexes and
rwlocks to implement semaphores for threads (other than perhaps for
compatibility)?  Or are this class of semaphores generally only used
between threads?

I noticed that those internally use syscalls, so I am wondering if
the man page is still correct (I didn't write a test program yet).

Thanks,
-- 
Matthew Mondor