Copyright (C) 2001 Jason Evans <jasone@FreeBSD.org>. All rights reserved.

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$FreeBSD: head/share/man/man9/sema.9 93337 2002-03-28 12:51:06Z dd $

.Dd August 13, 2001 .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 returns the current value of the semaphore.

p .Fn sema_timedwait and .Fn sema_trywait will return 0 if waiting on the semaphore failed; otherwise a non-zero value will be returned to indicate success. .Sh SEE ALSO .Xr condvar 9 , .Xr mtx_pool 9 , .Xr mutex 9 , .Xr sx 9