线程同步方式---3 读写锁

　　读写锁对比互斥锁好处是：

　　1）高并行性，读写锁根据线程对资源的操作类型分为 读 和 写，分别上锁，能够提高访问效率。而不是像互斥锁一样，不分读还是写操作，上锁之后其他进程都不能访问。如果当前临界区上了读锁，之后的写锁会阻塞。知道获得临界区，那么之后的读锁也会阻塞，可以防止写锁饥饿。

　　2）如果写锁占领了临界区，那么之后不管写锁还是读锁都会阻塞。

 

　　一个上了读写锁的例子，4个进程，2读2写。1个临界资源 global_num

/*
* 4 threads 2 threads read lock 2 threads write lock
* Observe sequence order 
*/

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

pthread_rwlock_t rwlock = PTHREAD_RWLOCK_INITIALIZER;

int global_num = 10;

void err_exit(const char* err_msg)
{
    printf("error: %s\n", err_msg);
    exit(1);
}

void *thread_read_lock( void *arg)
{
    char *pthr_name = (char *) arg;
    while(1)
    {
        // read lock
        pthread_rwlock_rdlock(&rwlock);
        printf("thread: %s entered in critical area, global_num=%d\n", pthr_name, global_num);
        sleep(1);
        printf("thread: %s leaving from critical area...\n", pthr_name);
        // read unlock
        pthread_rwlock_unlock(&rwlock);
        sleep(1);
    }
    return NULL;
}

void *thread_write_lock( void *arg)
{
    char *pthr_name = (char *)arg;
    while(1){
        // write lock
        pthread_rwlock_wrlock(&rwlock);

        // write op
        global_num ++;
        printf("thread: %s entered in critical area, global_num = %d\n", pthr_name, global_num);
        sleep(1);
        printf("thread: %s leaving from area\n", pthr_name);

        // write unlock
        pthread_rwlock_unlock(&rwlock);

        sleep(2);
    }
    return NULL;
}

int main(void)
{
    pthread_t tid_read_1, tid_read_2, tid_write_1, tid_write_2;

    if(pthread_create(&tid_read_1, NULL, thread_read_lock, (void *)"read_1") )
        err_exit("create tid_read_1");

    if(pthread_create(&tid_read_2, NULL, thread_read_lock, (void *)"read_1") )
        err_exit("create tid_read_2");

    if(pthread_create(&tid_write_1, NULL, thread_write_lock, (void *)"write_1") )
        err_exit("create tid_write_1");

    if(pthread_create(&tid_write_2, NULL, thread_write_lock, (void *)"write_2") )
        err_exit("create tid_write_2");

    // wait any thread in case of process terminating early
    if(pthread_join(tid_read_1, NULL))
        err_exit("pthread_join()");

    return 0;
}

　上面的都是阻塞锁：

#include <pthread.h> 
 int pthread_rwlock_rdlock(pthread_rwlock_t *rwlock);
 int pthread_rwlock_wrlock(pthread_rwlock_t *rwlock);
 int pthread_rwlock_unlock(pthread_rwlock_t *rwlock);

　　
非阻塞锁

#include <pthread.h>
#include <time.h>


int pthread_rwlock_tryrdlock(pthread_rwlock_t *rwlock);
int pthread_rwlock_trywrlock(pthread_rwlock_t *rwlock);

int pthread_rwlock_timedrdlock(pthread_rwlock_t *restrict rwlock, const struct timespec *restrict abs_timeout);
int pthread_rwlock_timedwrlock(pthread_rwlock_t *restrict rwlock, const struct timespec *restrict abs_timeout);