Differences between System V and Posix semaphores

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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.

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