/*
 Copyright 2005 Allen B. Downey
 This file contains an example program from The Little Book of
 Semaphores, available from Green Tea Press, greenteapress.com
 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; either version 2 of the License, or
 (at your option) any later version.
 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, see http://www.gnu.org/licenses/gpl.html
 or write to the Free Software Foundation, Inc., 51 Franklin St, 
 Fifth Floor, Boston, MA 02110-1301 USA
*/
#include <stdio.h>
#include <stdlib.h>
#include <pthread.h>
// UTILITY CODE
void perror_exit (char *s)
{
 perror (s); exit (-1);
}
void *check_malloc(int size)
{
 void *p = malloc (size);
 if (p == NULL) perror_exit ("malloc failed");
 return p;
}
// MUTEX WRAPPER
typedef pthread_mutex_t Mutex;
Mutex *make_mutex ()
{
 Mutex *mutex = check_malloc (sizeof(Mutex));
 int n = pthread_mutex_init (mutex, NULL);
 if (n != 0) perror_exit ("make_lock failed"); 
 return mutex;
}
void mutex_lock (Mutex *mutex)
{
 int n = pthread_mutex_lock (mutex);
 if (n != 0) perror_exit ("lock failed");
}
void mutex_unlock (Mutex *mutex)
{
 int n = pthread_mutex_unlock (mutex);
 if (n != 0) perror_exit ("unlock failed");
}
// COND WRAPPER
typedef pthread_cond_t Cond;
Cond *make_cond ()
{
 Cond *cond = check_malloc (sizeof(Cond)); 
 int n = pthread_cond_init (cond, NULL);
 if (n != 0) perror_exit ("make_cond failed");
 
 return cond;
}
void cond_wait (Cond *cond, Mutex *mutex)
{
 int n = pthread_cond_wait (cond, mutex);
 if (n != 0) perror_exit ("cond_wait failed");
}
void cond_signal (Cond *cond)
{
 int n = pthread_cond_signal (cond);
 if (n != 0) perror_exit ("cond_signal failed");
}
typedef struct {
 int value, wakeups;
 Mutex *mutex;
 Cond *cond;
} Semaphore;
// SEMAPHORE
Semaphore *make_semaphore (int value)
{
 Semaphore *semaphore = check_malloc (sizeof(Semaphore));
 semaphore->value = value;
 semaphore->wakeups = 0;
 semaphore->mutex = make_mutex ();
 semaphore->cond = make_cond ();
 return semaphore;
}
void sem_wait (Semaphore *semaphore)
{
 mutex_lock (semaphore->mutex);
 semaphore->value--;
 if (semaphore->value < 0) { do { cond_wait (semaphore->cond, semaphore->mutex);
 } while (semaphore->wakeups < 1); semaphore->wakeups--;
 }
 mutex_unlock (semaphore->mutex);
}
void sem_signal (Semaphore *semaphore)
{
 mutex_lock (semaphore->mutex);
 semaphore->value++;
 if (semaphore->value <= 0) { semaphore->wakeups++;
 cond_signal (semaphore->cond);
 }
 mutex_unlock (semaphore->mutex);
}
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