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sema_value (9)

>> sema_value (9) ( FreeBSD man: Ядро )

BSD mandoc

NAME



sema

 
sema_init

 
sema_destroy

 
sema_post

 
sema_wait

 
sema_timedwait

 
sema_trywait

 
sema_value

 - kernel counting semaphore

SYNOPSIS

   #include <sys/types.h>
   #include <sys/lock.h>
   #include <sys/sema.h>
void sema_init (struct sema *sema int value const char *description);
void sema_destroy (struct sema *sema);
void sema_post (struct sema *sema);
void sema_wait (struct sema *sema);
int sema_timedwait (struct sema *sema int timo);
int sema_trywait (struct sema *sema);
int sema_value (struct sema *sema);

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.

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 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 sema_destroy (.);
A semaphore is posted (incremented) with sema_post (.);
A semaphore is waited on (decremented) with sema_wait (,);
sema_timedwait (,);
or sema_trywait (.);
The Fa timo argument to sema_timedwait ();
specifies the minimum time in ticks to wait before returning with failure. sema_value ();
is used to read the current value of the semaphore.

RETURN VALUES

The sema_value ();
function returns the current value of the semaphore.

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

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