sema_post(9) kernel counting semaphore

Other Alias

sema, sema_init, sema_destroy, sema_wait, sema_timedwait, sema_trywait, sema_value


In sys/types.h In sys/lock.h In 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


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.

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.

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.


The Fn sema_value function returns the current value of the semaphore.

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.

The Fn sema_timedwait function returns 0 if waiting on the semaphore succeeded; otherwise a non-zero error code is returned.


The Fn sema_timedwait function will fail if:

Timeout expired.