SEMA(9)

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 sema is a pointer to space for a struct sema, value is the initial value of the semaphore, and 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 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.

ERRORS
The sema_timedwait() function will fail if:

[EWOULDBLOCK]
    Timeout expired.

SEE ALSO
condvar(9), locking(9), mtx_pool(9), mutex(9), rwlock(9), sx(9)