Allocation and Deallocation

• The calls semget and semctl allocate and deallocate semaphores, which is analogous to shmget and shmctl for shared memory. Invoke semget with:
  • a key specifying a semaphore set,
  • the number of semaphores in the set,
  • permission flags as for shmget
• The return value is a semaphore set identifier. You can obtain the identifier of an existing semaphore set by specifying the right key value.
• Semaphores continue to exist even after all processes using them have terminated. The last process to use a semaphore set must explicitly remove it. To do so, invoke semctl with
  • the semaphore identifier,
  • the number of semaphores in the set,
  • IPC_RMID as the third argument,
  • any union semun value as the fourth argument (which is ignored).
• Unlike shared memory segments, removing a semaphore set causes Linux to deallocate immediately.
• The following http://siber.cankaya.edu.tr/SystemsProgramming/cfiles/sem_all_deall.ccode (see Fig. 6.2.1) presents functions to allocate and deallocate a binary semaphore.

  Figure 6.2: Allocating and Deallocating a Binary Semaphore