|SEMA(9)||Kernel Developer's Manual||SEMA(9)|
— kernel counting semaphore
sema *sema, int
value, const char
sema *sema, int
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
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
A semaphore is posted (incremented) with
A semaphore is waited on (decremented) with
The timo argument to
sema_timedwait() specifies the minimum time in ticks
to wait before returning with failure.
is used to read the current value of the semaphore.
sema_value() function returns the
current value of the semaphore.
If decrementing the semaphore would result in its value being
sema_trywait() returns 0 to indicate
failure. Otherwise, a non-zero value is returned to indicate success.
sema_timedwait() function returns 0 if
waiting on the semaphore succeeded; otherwise a non-zero error code is
sema_timedwait() function will fail
- Timeout expired.
|February 1, 2006||Debian|