semaphore,
sema_init,
sema_destroy,
sema_p,
sema_p_sig,
sema_v,
sema_tryp
—
semaphore functions
#include
<sys/ksynch.h>
void
sema_init(
ksema_t
*sp,
uint_t val,
char *name,
ksema_type_t type,
void *arg);
void
sema_destroy(
ksema_t
*sp);
void
sema_p(
ksema_t
*sp);
void
sema_v(
ksema_t
*sp);
int
sema_p_sig(
ksema_t
*sp);
int
sema_tryp(
ksema_t
*sp);
illumos DDI specific (illumos DDI).
-
-
- sp
- A pointer to a semaphore, type
ksema_t.
-
-
- val
- Initial value for semaphore.
-
-
- name
- Descriptive string. This is obsolete and should be
NULL.
(Non-NULL strings are legal, but they
are a waste of kernel memory.)
-
-
- type
- Variant type of the semaphore. Currently, only
SEMA_DRIVER is supported.
-
-
- arg
- Type-specific argument; should be
NULL.
These functions implement counting semaphores as described by Dijkstra. A
semaphore has a value which is atomically decremented by
sema_p() and atomically incremented by
sema_v(). The value must always be greater than
or equal to zero. If
sema_p() is called and the
value is zero, the calling thread is blocked until another thread performs a
sema_v() operation on the semaphore.
Semaphores are initialized by calling
sema_init().
The argument,
val, gives the initial value
for the semaphore. The semaphore storage is provided by the caller but more
may be dynamically allocated, if necessary, by
sema_init(). For this reason,
sema_destroy() should be called before
deallocating the storage containing the semaphore.
The
sema_p_sig() function decrements the semaphore,
as does
sema_p(). However, if the semaphore value
is zero,
sema_p_sig() will return without
decrementing the value if a signal (that is, from
kill(2)) is pending for the thread.
The
sema_tryp() function will decrement the
semaphore value only if it is greater than zero, and will not block.
These functions can be called from user, interrupt, or kernel context, except
for
sema_init() and
sema_destroy(), which can be called from user or
kernel context only. None of these functions can be called from a high-level
interrupt context. In most cases,
sema_v() and
sema_p() should not be called from any interrupt
context.
If
sema_p() is used from interrupt context,
lower-priority interrupts will not be serviced during the wait. This means
that if the thread that will eventually perform the
sema_v() becomes blocked on anything that
requires the lower-priority interrupt, the system will hang.
For example, the thread that will perform the
sema_v() may need to first allocate memory. This
memory allocation may require waiting for paging I/O to complete, which may
require a lower-priority disk or network interrupt to be serviced. In general,
situations like this are hard to predict, so it is advisable to avoid waiting
on semaphores or condition variables in an interrupt context.
Similar to many other synchronization mechanisms, semaphores should not be used
in any code path that requires synchronization while handling system panic, at
which time many of the semaphore operations become no-ops.
-
-
0- sema_tryp() could not
decrement the semaphore value because it was zero.
-
-
1- sema_p_sig() was not able to
decrement the semaphore value and detected a pending signal.
kill(2),
condvar(9F),
mutex(9F)
Writing Device Drivers