te
Copyright (c) 2006, Sun Microsystems, Inc. All Rights Reserved
The contents of this file are subject to the terms of the Common Development and Distribution License (the "License"). You may not use this file except in compliance with the License.
You can obtain a copy of the license at usr/src/OPENSOLARIS.LICENSE or http://www.opensolaris.org/os/licensing. See the License for the specific language governing permissions and limitations under the License.
When distributing Covered Code, include this CDDL HEADER in each file and include the License file at usr/src/OPENSOLARIS.LICENSE. If applicable, add the following below this CDDL HEADER, with the fields enclosed by brackets "[]" replaced with your own identifying information: Portions Copyright [yyyy] [name of copyright owner]
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);

Solaris DDI specific (Solaris DDI).

sp val name type arg

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.

0 1

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.

kill(2), condvar(9F), mutex(9F)

Writing Device Drivers