(ipc.info.gz) Semaphores

 
 1.6 Semaphores
 ==============
 
 Each semaphore has a value >= 0. An id provides access to an array of
 `nsems' semaphores. Operations such as read, increment or decrement
 semaphores in a set are performed by the `semop' call which processes
 `nsops' operations at a time. Each operation is specified in a struct
 `sembuf' described below. The operations are applied only if all of
 them succeed.
 
    If you do not have a need for such arrays, you are probably better
 off using the `test_bit', `set_bit' and  `clear_bit' bit-operations
 defined in <asm/bitops.h>.
 
    Semaphore operations may also be qualified by a SEM_UNDO flag which
 results in the operation being undone when the process exits.
 
    If a decrement cannot go through, a process will be put to sleep on
 a queue waiting for the `semval' to increase unless it specifies
 IPC_NOWAIT. A read operation can similarly result in a sleep on a queue
 waiting for `semval' to become 0. (Actually there are two queues per
 semaphore array).
 
 A semaphore array is described by:
      struct semid_ds
        struct ipc_perm sem_perm;
        time_t          sem_otime;      /* last semop time */
        time_t          sem_ctime;      /* last change time */
        struct wait_queue *eventn;	  /* wait for a semval to increase */
        struct wait_queue *eventz;      /* wait for a semval to become 0 */
        struct sem_undo  *undo;         /* undo entries */
        ushort          sem_nsems;      /* no. of semaphores in array */
 
 Each semaphore is described internally by :
      struct sem
        short   sempid;         /* pid of last semop() */
        ushort  semval;         /* current value */
        ushort  semncnt;        /* num procs awaiting increase in semval */
        ushort  semzcnt;        /* num procs awaiting semval = 0 */
(ipc.info.gz) Semaphores

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