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 */