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() spec

ifies 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 be

ing 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), mtx_pool(9), mutex(9), sx(9)

BSD June 14, 2004

BSD