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(ipc.info)Semaphores

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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 */

semget

semop

semctl

semlimits

Limits imposed by this implementation.