Unix – Differences between System V and Posix semaphores


What are the trade-offs between using a System V and a Posix semaphore?

Best Answer

From O'Reilly:

  • One marked difference between the System V and POSIX semaphore implementations is that in System V you can control how much the semaphore count can be increased or decreased; whereas in POSIX, the semaphore count is increased and decreased by 1.
  • POSIX semaphores do not allow manipulation of semaphore permissions, whereas System V semaphores allow you to change the permissions of semaphores to a subset of the original permission.
  • Initialization and creation of semaphores is atomic (from the user's perspective) in POSIX semaphores.
  • From a usage perspective, System V semaphores are clumsy, while POSIX semaphores are straight-forward
  • The scalability of POSIX semaphores (using unnamed semaphores) is much higher than System V semaphores. In a user/client scenario, where each user creates her own instances of a server, it would be better to use POSIX semaphores.
  • System V semaphores, when creating a semaphore object, creates an array of semaphores whereas POSIX semaphores create just one. Because of this feature, semaphore creation (memory footprint-wise) is costlier in System V semaphores when compared to POSIX semaphores.
  • It has been said that POSIX semaphore performance is better than System V-based semaphores.
  • POSIX semaphores provide a mechanism for process-wide semaphores rather than system-wide semaphores. So, if a developer forgets to close the semaphore, on process exit the semaphore is cleaned up. In simple terms, POSIX semaphores provide a mechanism for non-persistent semaphores.