2010_01_06_rwlock.md

[pthread] 同步机制(2) -- rwlock

rwlock, read-write lock，就是常说的读写锁。

• pthread_rwlock_rdlock / tryrdlock，拿 read lock。对于同一个 rwlock，多个线程可以同时拿到 read lock。
• pthread_rwlock_wrlock / trywrlock，拿 write lock。对于同一个 rwlock，同时只能有一个线程可以拿到 write lock，此时没有任何其他线程可拥有 read / write lock。
• pthread_rwlock_timedrdlock / timedwrlock，一定时间拿不到锁，也返回。

无论 read or write lock，解锁都是 pthread_rwlock_unlock。

rwlock 也有与 mutex 类似的 pshared(process shared) 的属性，一般也是用处不大啦。

#include <stdio.h>
#include <time.h>
#include <unistd.h>     // sleep
#include <pthread.h>

pthread_rwlock_t global_rwl;

void *reader(void *arg)
{
    int n = (int)arg;
    pthread_rwlock_rdlock(&global_rwl);
    printf("reader #%d, time = %d\n", n, time(NULL));
    sleep(2);
    pthread_rwlock_unlock(&global_rwl);
    return (void *)0;
}

void *writer(void *_)
{
    sleep(1);
    if ( pthread_rwlock_rdlock(&global_rwl) != 0 )
        printf("rdlock err!\n");
    printf("writer, time = %d\n", time(NULL));
    pthread_rwlock_unlock(&global_rwl);
    return (void *)0;
}

int main()
{
    pthread_t pid1, pid2, pid3;
    pthread_rwlock_init(&global_rwl, NULL);

    pthread_create(&pid1, NULL, reader, (void*)1);
    pthread_create(&pid2, NULL, reader, (void*)2);
    pthread_create(&pid3, NULL, writer, NULL);

    pthread_join(pid1, NULL);
    pthread_join(pid2, NULL);
    pthread_join(pid3, NULL);
    return 0;
}

运行结果：

$ ./a.out 
reader #1, time = 1262780244
reader #2, time = 1262780244
writer, time = 1262780245