多线程同步之条件变量

客官°小女子只卖身不卖艺 2022-01-05 10:43 341阅读 0赞

条件变量是线程同步的另一种手段，主要逻辑就是等待和唤醒。条件不满足时，线程等待；条件满足，线程被(其他线程)唤醒。条件变量一般和互斥量一起使用，因为需要保证多线程互斥地修改条件。

涉及到的函数有：

int pthread_cond_init(pthread_cond_t *restrict cond,
    const pthread_condattr_t *restrict attr);
    
    int pthread_cond_destroy(pthread_cond_t *cond);
    
    int pthread_cond_wait(pthread_cond_t *restrict cond,
    pthread_mutex_t *restrict mutex); 
    
    int pthread_cond_signal(pthread_cond_t *cond); 
    
    int pthread_cond_broadcast(pthread_cond_t *cond);

实战训练：

有三个线程分别打印A、B、C,请用多线程编程实现，在屏幕上循环打印10次ABCABC…

solution：

#include <stdio.h>
    #include <stdlib.h>
    #include <pthread.h>
    #define N 10
    #define M 3          //number of threads
    pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER;
    pthread_cond_t cond;
    int m = 0;
    
    void* thr_fn(void *arg)
    {
        int index = (int)arg;
        char ch = 'A' + index;
        int i;
        for (i = 0; i < N; ++i)
        {
             pthread_mutex_lock(&mutex);
             while (index != m)
                 pthread_cond_wait(&cond, &mutex);
    
             printf("%c", ch);
             m = (m+1) % M;
             pthread_mutex_unlock(&mutex);
    
             pthread_cond_broadcast(&cond);
        }
        return (void*)0;
    }
    
    int main()
    {
        pthread_cond_init(&cond, NULL);
        pthread_t tid[M];
        int i;
        for (i = 0; i < M; ++i)
            pthread_create(&tid[i], NULL, thr_fn, (void*)i);
        for (i = 0; i < M; ++i)
            pthread_join(tid[i], NULL);
        pthread_cond_destroy(&cond);
        printf("\n");
        return 0;
    }

REFERENCE:

[【机试】华为2014校招机试：多线程循环打印十次ABC][2014_ABC]

转载于:https://www.cnblogs.com/gattaca/p/4732623.html

[2014_ABC]: http://blog.csdn.net/wangyf101/article/details/12614421