读写锁

  读写锁是多线程同步中的一个很有用的东东，区分开读者和写者，有助于效率的提升。

    就好像是一个签名簿，写者相当于签名的人，读者相当于看签名簿的人，写签名簿的一群人需要排队，一个一个地签名，而读者不需要排队，他们可以一起看。当一群围观的正在看时，匆匆忙忙跑来一个签名的，大喊“让开，我要迟到了，让我签到”，于是大家让开让他签了名子，这说明了什么？

    1.写者比读者优先级高，同时竞争读写锁的话，写者将获得锁。

    2.写者必须一个一个地获得锁

    3.而一群读者可以同时获得读写锁。

 

下面一段程序说明了这个道理。

                                     编译：   gcc -o a rwlock.c -lpthread

                                     运行：   ./a

 

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

#define THREADCNT 10

pthread_rwlock_t rwlock;                                                  //建立读写锁
int global_var =  1;

void* reader(void* pvoid)
{
        pthread_rwlock_rdlock(&rwlock);                               //读方式获取锁
        sleep(1);
        printf("reader %d worked. global_var = %d\n", (int)pvoid, global_var);
        if(pthread_rwlock_unlock(&rwlock)) {
                printf("reader %d unlock error!\n", (int)pvoid);
        }
        return NULL;
}
void* writer(void* pvoid)
{
        pthread_rwlock_wrlock(&rwlock);                               //写方式获得锁
        sleep(1);
        printf("writer %d worked. global_var = %d\n", (int)pvoid, global_var++);
        if(pthread_rwlock_unlock(&rwlock)) {
                printf("writer %d unlock error!\n", (int)pvoid);
        }
        return NULL;
}
int main()
{
        pthread_t reader_id, writer_id;
        pthread_attr_t threadattr;
        int i, rand;
        int readercnt = 1, writercnt = 1;
        int halfmax = RAND_MAX / 2;
        if(pthread_rwlock_init(&rwlock, NULL)) {                     //初始化读写锁
                printf("initialize rwlock error!\n");
        }
        pthread_rwlock_wrlock(&rwlock);
        pthread_attr_init(&threadattr);
        pthread_attr_setdetachstate(&threadattr, PTHREAD_CREATE_DETACHED);
        for(i = 0; i < THREADCNT; i++) {
                rand = random();
                if(rand < halfmax) {
                        pthread_create(&reader_id, &threadattr, reader,
                                                (void*)readercnt);
                        printf("Created reader %d\n", readercnt++);
                } else {
                        pthread_create(&writer_id, &threadattr, writer,
                                                (void*)writercnt);
                        printf("Created writer %d\n", writercnt++);
                }
        }
        pthread_rwlock_unlock(&rwlock);
        pthread_exit(0);
        return 0;
}

 

运行结果：

 

Created writer 1
Created reader 1
Created writer 2
Created writer 3
Created writer 4
Created reader 2
Created reader 3
Created writer 5
Created reader 4
Created writer 6
writer 3 worked. global_var = 1
writer 4 worked. global_var = 2
writer 5 worked. global_var = 3
writer 2 worked. global_var = 4
writer 6 worked. global_var = 5
writer 1 worked. global_var = 6
reader 2 worked. global_var = 7
reader 3 worked. global_var = 7
reader 4 worked. global_var = 7
reader 1 worked. global_var = 7