POSIX Semaphores on Mac OS X: sem_timedwait alternative

Question

I am trying to port a project (from linux) that uses Semaphores to Mac OS X however some of the posix semaphores are not implemented on Mac OS X

The one that I hit in this port is sem_timedwait()

I don't know much about semaphores but from the man pages sem_wait() seems to be close to sem_timedwait and it is implemented

From the man pages

sem_timedwait() function shall lock the semaphore referenced by
sem as in the sem_wait() function. However, if the semaphore cannot be
locked without waiting for another process or thread to unlock the
semaphore by performing a sem_post() function, this wait shall be ter-
minated when the specified timeout expires

From my limited understanding of how semphores work I can see that sem_timedwait() is safer, but I still should be able to use sem_wait()

Is this correct? If not what other alternatives do I have...

Thanks

Answer 1

Could you try to mimic the functionality of the sem_timedwait() call by starting a timer in another thread that calls sem_post() after the timer expires if it hasn't been called by the primary thread that is supposed to call sem_post()?

Answer 2

It's likely that the timeout is important to the operation of the algorithm. Therefore just using sem_wait() might not work.

You could use sem_trywait(), which returns right away in all cases. You can then loop, and use a sleep interval that you choose, each time decrementing the total timeout until you either run out of timeout or the semaphore is acquired.

A much better solution is to rewrite the algorithm to use a condition variable, and then you can use pthread_cond_timedwait() to get the appropriate timeout.

Answer 3

Have you considered using the apache portable runtime? It's preinstalled on every Mac OS X Box and many Linux distros and it comes with a platform neutral wrapper around thread concurrency, that works even on MS Windows:

http://apr.apache.org/docs/apr/1.3/group__apr__thread__cond.html

Greets Seb

Answer 4

If you can just use MP API:
- MPCreateSemaphore/MPDeleteSemaphore
- MPSignalSemaphore/MPWaitOnSemaphore

MPWaitOnSemaphore exists with kMPTimeoutErr if specified timeout is exceeded without signaling.