Mastering Linux Kernel Development

System V semaphores

Semaphores in System V are not just a single counter as you might think, but rather a set of counters. This implies that a semaphore set can contain single or multiple counters (0 to n) with an identical semaphore ID. Each counter in the set can protect a shared resource, and a single semaphore set can protect multiple resources. The system call that helps create this kind of semaphore is as follows:

int semget(key_t key, int nsems, int semflg)

• key is used to identify the semaphore. If the key value is IPC_PRIVATE, a new set of semaphores is created.
• nsems indicates the semaphore set with the number of counters needed in the set
• semflg dictates how the semaphore should be created. It can contain two values:
  • IPC_CREATE: ...