/*
 * Authors: Martin Nagy <mnagy@redhat.com>
 *
 * Copyright (C) 2008  Red Hat
 * see file 'COPYING' for use and warranty information
 *
 * This program is free software; you can redistribute it and/or
 * modify it under the terms of the GNU General Public License as
 * published by the Free Software Foundation; version 2 or later
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program; if not, write to the Free Software
 * Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
 */

/*
 * Note: This implementation doesn't prevent from starvation. This means that
 * if a thread signals the semaphore and then waits for it, it may catch it's
 * own signal. However, for our purposes, this shouldn't be needed.
 */

#include <isc/condition.h>
#include <isc/result.h>
#include <isc/util.h>

#include "semaphore.h"

/*
 * Initialize a semaphore.
 *
 * sem - allocated semaphore that will be initialized
 * value - number of times we can acquire the semaphore.
 */
isc_result_t
semaphore_init(semaphore_t *sem, int value)
{
	isc_result_t result;

	REQUIRE(sem != NULL);
	REQUIRE(value > 0);

	sem->value = value;
	result = isc_mutex_init(&sem->mutex);
	if (result != ISC_R_SUCCESS)
		return result;

	result = isc_condition_init(&sem->cond);
	if (result != ISC_R_SUCCESS)
		isc_mutex_destroy(&sem->mutex);

	return result;
}

/*
 * Destroy a semaphore.
 *
 * sem - semaphore to be destroyed.
 */
void
semaphore_destroy(semaphore_t *sem)
{
	if (sem == NULL)
		return;

	RUNTIME_CHECK(isc_mutex_destroy(&sem->mutex) == ISC_R_SUCCESS);
	RUNTIME_CHECK(isc_condition_destroy(&sem->cond) == ISC_R_SUCCESS);
}

/*
 * Wait on semaphore. This operation will try to acquire a lock on the
 * semaphore. If the semaphore is already acquired as many times at it allows,
 * the function will block until someone releases the lock.
 */
void
semaphore_wait(semaphore_t *sem)
{
	REQUIRE(sem != NULL);

	LOCK(&sem->mutex);

	sem->value--;
	if (sem->value < 0)
		WAIT(&sem->cond, &sem->mutex);

	UNLOCK(&sem->mutex);
}

/*
 * Release the semaphore. This will make sure that another thread (probably
 * already waiting) will be able to acquire the semaphore.
 */
void
semaphore_signal(semaphore_t *sem)
{
	REQUIRE(sem != NULL);

	LOCK(&sem->mutex);

	sem->value++;
	if (sem->value >= 0)
		SIGNAL(&sem->cond);

	UNLOCK(&sem->mutex);
}