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)