sema.c

#include <stdlib.h>
#include <unistd.h>
#include <stdio.h>

#include <pthread.h>
#include "sema.h"

int InitSem(sema *s, int count)
{
	if (s == NULL)
	{
		return (-1);
	}
	s->value = count;
	s->waiters = 0;
	pthread_cond_init(&(s->wait), NULL);
	pthread_mutex_init(&(s->lock), NULL);

	return (1);
}

void FreeSem(sema *s)
{
	free(s);
}

void P(sema *s)
{
	pthread_mutex_lock(&(s->lock));

	s->value--;

	while (s->value < 0)
	{
		/*
		 * maintain semaphore invariant
		 */
		if (s->waiters < (-1 * s->value))
		{
			s->waiters++;
			pthread_cond_wait(&(s->wait), &(s->lock));
			s->waiters--;
		}
		else
		{
			break;
		}
	}

	pthread_mutex_unlock(&(s->lock));

	return;
}

void V(sema *s)
{

	pthread_mutex_lock(&(s->lock));

	s->value++;

	if (s->value <= 0)
	{
		pthread_cond_signal(&(s->wait));
	}

	pthread_mutex_unlock(&(s->lock));
}