fsema.c

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

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

int InitSem(sema *s, int count)
{
	double r = drand48();

	if (s == NULL)
	{
		return (-1);
	}

	memset(s->mname, 0, sizeof(s->mname));
	memset(s->wname, 0, sizeof(s->wname));
	sprintf(s->mname, "/tmp/msema%0.10f", r);
	sprintf(s->wname, "/tmp/wsema%0.10f", r);
	s->value = count;
	s->waiters = 0;

	return (1);
}

void FreeSem(sema *s)
{
	unlink(s->wname);
	unlink(s->mname);
}

int GetLock(char *name)
{
	int fd = open(name, O_RDWR | O_CREAT, 0600);
	if (fd < 0)
	{
		fprintf(stderr, "PANIC: %s\n", name);
		exit(1);
	}
	return (fd);
}

void P(sema *s)
{
	int mutex = GetLock(s->mname);
	int wait = GetLock(s->wname);

	flock(mutex, LOCK_EX);

	s->value--;

	while (s->value < 0)
	{
		/*
		 * maintain semaphore invariant
		 */
		if (s->waiters < (-1 * s->value))
		{
			s->waiters++;
			flock(mutex, LOCK_UN);
			flock(wait, LOCK_EX);
			flock(mutex, LOCK_EX);
			s->waiters--;
		}
		else
		{
			if (s->value == 0)
			{
				flock(wait, LOCK_EX);
			}
			break;
		}
	}

	close(mutex);
	close(wait);
	return;
}

void V(sema *s)
{
	int mutex = GetLock(s->mname);
	int wait = GetLock(s->wname);

	flock(mutex, LOCK_EX);

	s->value++;

	if (s->value <= 0)
	{
		while (s->waiters > (-1 * s->value))
		{
			flock(wait, LOCK_UN);
			flock(mutex, LOCK_UN);
			flock(mutex, LOCK_EX);
		}
	}

	if (s->value == 1)
	{
		flock(wait, LOCK_UN);
	}
	flock(mutex, LOCK_UN);

	close(mutex);
	close(wait);

	return;
}