• 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.