mythread_rwlock.c

/* $Id: mythread_rwlock.c,v 1.3 2001/12/20 03:17:10 acs Exp $
   Written by Adam Siepel, Spring 2001 
   Copyright 2001, Adam Siepel */

/* Simple implementation of reader-writer locks that uses pthread
   mutexes and POSIX semaphores.  Interface is modeled on that of the
   "pthread_rwlock" functions, which seem not to be widely supported
   (otherwise I would have used them directly).  Note that the
   implementation here gives priority to readers (i.e., there is a
   possibility of starvation of writers).  */


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

int
mythread_rwlock_init ( mythread_rwlock_t * rwlock )
{
    rwlock->readcount = 0;
    pthread_mutex_init ( &rwlock->countermutex, NULL );
    sem_init ( &rwlock->writesem, 0, 1 );

    return 0;
}

int
mythread_rwlock_wrlock ( mythread_rwlock_t * rwlock )
{
    sem_wait ( &rwlock->writesem );
    return 0;
}

int
mythread_rwlock_rdlock ( mythread_rwlock_t * rwlock )
{
    pthread_mutex_lock ( &rwlock->countermutex );
    rwlock->readcount++;
    if ( rwlock->readcount == 1 )
        sem_wait ( &rwlock->writesem );
    pthread_mutex_unlock ( &rwlock->countermutex );

    return 0;
}

int
mythread_rwlock_rdunlock ( mythread_rwlock_t * rwlock )
{
    pthread_mutex_lock ( &rwlock->countermutex );
    rwlock->readcount--;
    if ( rwlock->readcount == 0 )
        sem_post ( &rwlock->writesem );
    pthread_mutex_unlock ( &rwlock->countermutex );

    return 0;
}

int
mythread_rwlock_wrunlock ( mythread_rwlock_t * rwlock )
{
    sem_post ( &rwlock->writesem );
    return 0;
}