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)