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.Dd February 1, 2006 .Dt SEMA 9 .Os .Sh NAME .Nm sema , .Nm sema_init , .Nm sema_destroy , .Nm sema_post , .Nm sema_wait , .Nm sema_timedwait , .Nm sema_trywait , .Nm sema_value .Nd kernel counting semaphore .Sh SYNOPSIS n sys/types.h n sys/lock.h n sys/sema.h .Ft void .Fn sema_init "struct sema *sema" "int value" "const char *description" .Ft void .Fn sema_destroy "struct sema *sema" .Ft void .Fn sema_post "struct sema *sema" .Ft void .Fn sema_wait "struct sema *sema" .Ft int .Fn sema_timedwait "struct sema *sema" "int timo" .Ft int .Fn sema_trywait "struct sema *sema" .Ft int .Fn sema_value "struct sema *sema" .Sh DESCRIPTION Counting semaphores provide a mechanism for synchronizing access to a pool of resources. Unlike mutexes, semaphores do not have the concept of an owner, so they can also be useful in situations where one thread needs to acquire a resource, and another thread needs to release it. Each semaphore has an integer value associated with it. Posting (incrementing) always succeeds, but waiting (decrementing) can only successfully complete if the resulting value of the semaphore is greater than or equal to zero.

p Semaphores should not be used where mutexes and condition variables will suffice. Semaphores are a more complex synchronization mechanism than mutexes and condition variables, and are not as efficient.

p Semaphores are created with .Fn sema_init , where .Fa sema is a pointer to space for a .Vt "struct sema" , .Fa value is the initial value of the semaphore, and .Fa description is a pointer to a null-terminated character string that describes the semaphore. Semaphores are destroyed with .Fn sema_destroy . A semaphore is posted (incremented) with .Fn sema_post . A semaphore is waited on (decremented) with .Fn sema_wait , .Fn sema_timedwait , or .Fn sema_trywait . The .Fa timo argument to .Fn sema_timedwait specifies the minimum time in ticks to wait before returning with failure. .Fn sema_value is used to read the current value of the semaphore. .Sh RETURN VALUES The .Fn sema_value function returns the current value of the semaphore.

p If decrementing the semaphore would result in its value being negative, .Fn sema_trywait returns 0 to indicate failure. Otherwise, a non-zero value is returned to indicate success.

p The .Fn sema_timedwait function returns 0 if waiting on the semaphore succeeded; otherwise a non-zero error code is returned. .Sh ERRORS The .Fn sema_timedwait function will fail if: l -tag -width Er t Bq Er EWOULDBLOCK Timeout expired. .El .Sh SEE ALSO .Xr condvar 9 , .Xr locking 9 , .Xr mtx_pool 9 , .Xr mutex 9 , .Xr rwlock 9 , .Xr sx 9