47.12. Summary

System V semaphores allow processes to synchronize their actions. This is useful when a process must gain exclusive access to some shared resource, such as a region of shared memory.

Semaphores are created and operated upon in sets containing one or more semaphores. Each semaphore within a set is an integer whose value is always greater than or equal to 0. The semop() system call allows the caller to add an integer to a semaphore, subtract an integer from a semaphore, or wait for a semaphore to equal 0. The last two of these operations may cause the caller to block.