POSIX 条件变量详细解析

条件锁pthread_cond_t

说明，
等待线程
1。使用pthread_cond_wait前要先加锁
2。pthread_cond_wait内部会解锁，然后等待条件变量被其它线程激活
3。pthread_cond_wait被激活后会再自动加锁

激活线程：
1。加锁（和等待线程用同一个锁）
2。pthread_cond_signal发送信号
3。解锁
激活线程的上面三个操作在运行时间上都在等待线程的pthread_cond_wait函数内部。

程序示例：

#include <stdio.h>#include <pthread.h>#include <unistd.h>pthread_mutex_t count_lock;pthread_cond_t count_nonzero;unsigned count = 0;void *decrement_count(void *arg){    pthread_mutex_lock(&count_lock);    printf("decrement_count get count_lock\n");    while(count == 0)    {        printf("decrement_count count == 0 \n");        printf("decrement_count before cond_wait \n");        pthread_cond_wait(&count_nonzero, &count_lock);        printf("decrement_count after cond_wait \n");        printf("decrement_count count = %d \n",count);    }    count = count + 1;    pthread_mutex_unlock(&count_lock);}void *increment_count(void *arg){    pthread_mutex_lock(&count_lock);    printf("increment_count get count_lock \n");    if(count == 0)    {        printf("increment_count before cond_signal \n");        pthread_cond_signal(&count_nonzero);        printf("increment_count after cond_signal \n");    }    count = count + 1;    printf("huangcheng \n");    printf("increment_count count = %d \n",count);    pthread_mutex_unlock(&count_lock);}int main(void){    pthread_t tid1, tid2;    pthread_mutex_init(&count_lock, NULL);    pthread_cond_init(&count_nonzero, NULL);    pthread_create(&tid1, NULL, decrement_count, NULL);    sleep(2);    pthread_create(&tid2, NULL, increment_count, NULL);    sleep(10);    pthread_exit(0);    return 0;}

运行结果：

# gcc -o pthread_cond pthread_cond.c -lpthread
#./pthread_cond
decrement_count get count_lock
decrement_count count == 0
decrement_count before cond_wait
increment_count get count_lock
increment_count before cond_signal
increment_count after cond_signal
huangcheng
increment_count count = 1
decrement_count after cond_wait
decrement_count count = 1

 

 

多线程编程,条件变量pthread_cond_t应用

 

程序代码：

#include <stdio.h>#include <pthread.h>#include <unistd.h>pthread_mutex_t counter_lock;pthread_cond_t counter_nonzero;int counter = 0;int estatus = -1;void *decrement_counter(void *argv);void *increment_counter(void *argv);int main(int argc, char **argv){    printf("counter: %d/n", counter);    pthread_t thd1, thd2;    int ret;    ret = pthread_create(&thd1, NULL, decrement_counter, NULL);    if(ret){        perror("del:/n");        return 1;    }    ret = pthread_create(&thd2, NULL, increment_counter, NULL);    if(ret){        perror("inc: /n");        return 1;    }    int counter = 0;    while(counter != 10){        printf("counter: %d/n", counter);        sleep(1);        counter++;    }    return 0;}void *decrement_counter(void *argv){    pthread_mutex_lock(&counter_lock);    while(counter == 0)        pthread_cond_wait(&counter_nonzero, &counter_lock);    counter--;    pthread_mutex_unlock(&counter_lock);    return &estatus;}void *increment_counter(void *argv){    pthread_mutex_lock(&counter_lock);    if(counter == 0)        pthread_cond_signal(&counter_nonzero);    counter++;    pthread_mutex_unlock(&counter_lock);    return &estatus;}

运行结果：
#gcc -o pthread_cond2 pthread_cond2.c -lpthread
#./pthread_cond2                              
counter: 0
counter: 0
counter: 1
counter: 2
counter: 3
counter: 4
counter: 5
counter: 6
counter: 7
counter: 8
counter: 9

 

调试程序的运行过程：

1、开始时 counter 为0 （main）

2、ret = pthread_create(&thrd1, NULL, decrement_counter, NULL)处生成一个thrd1线程运行decrement_counter(),

此线程内函数运行流程为:

先锁定 互斥锁（count_lock） 如果counter为0,此线程被阻塞在条件变量（count_nonzero）上.同时释放互斥锁count_lock（wait内部会先释放锁，等待signal激活后自动再加上锁）.

 

3、与此同时主程序还在运行,创建另一个线程thrd2运行 increment_counter,

此线程内的函数流程如下:

先锁定 互斥锁（count_lock）【wait内部释放锁的互斥锁】 如果counter为0, 唤醒在条件变量（count_nonzero）上的线程即thrd1.但是由于有互斥锁count_lock【signal激活后，wait内部又自动加上锁了】, thrd1还是在等待. 然后count++,释放互斥锁,.......thrd1由于互斥锁释放,重新判断counter是不是为0,如果为0再把线程阻塞在条件变量count_nonzero上,但这时counter已经为1了.所以线程继续运行.counter--释放互斥锁......（退出后，运行主线程main）

4、与此主程序间隔打印counter运行一段时间退出.

 注：更清晰的运行流程请详见如下“改进代码”

后记,在编译的时候加上 -lpthread

改进代码：

 

#include <stdio.h>#include <pthread.h>#include <unistd.h>pthread_mutex_t counter_lock;pthread_cond_t counter_nonzero;int counter = 0;int estatus = -1;void *decrement_counter(void *argv);void *increment_counter(void *argv);int main(int argc, char **argv){    printf("counter: %d/n", counter);    pthread_t thd1, thd2;    int ret;    ret = pthread_create(&thd1, NULL, decrement_counter, NULL);    if(ret){        perror("del:/n");        return 1;    }    ret = pthread_create(&thd2, NULL, increment_counter, NULL);    if(ret){        perror("inc: /n");        return 1;    }    int counter = 0;    while(counter != 10){        printf("counter(main): %d/n", counter);        sleep(1);        counter++;    }    return 0;}void *decrement_counter(void *argv){    printf("counter(decrement): %d/n", counter);    pthread_mutex_lock(&counter_lock);    while(counter == 0)        pthread_cond_wait(&counter_nonzero, &counter_lock); //进入阻塞(wait)，等待激活(signal)        printf("counter--(before): %d/n", counter);        counter--; //等待signal激活后再执行    printf("counter--(after): %d/n", counter);        pthread_mutex_unlock(&counter_lock);     return &estatus;}void *increment_counter(void *argv){    printf("counter(increment): %d/n", counter);    pthread_mutex_lock(&counter_lock);    if(counter == 0)        pthread_cond_signal(&counter_nonzero); //激活(signal)阻塞(wait)的线程(先执行完signal线程，然后再执行wait线程)    printf("counter++(before): %d/n", counter);        counter++;     printf("counter++(after): %d/n", counter);        pthread_mutex_unlock(&counter_lock);    return &estatus;}

 

运行结果：

# gcc -o pthread_cond2 pthread_cond2.c -lpthread
#./pthread_cond2                              
counter: 0
counter(main): 0
counter(decrement): 0
counter(increment): 0
counter++(before): 0
counter++(after): 1
counter--(before): 1
counter--(after): 0
counter(main): 1
counter(main): 2
counter(main): 3
counter(main): 4
counter(main): 5
counter(main): 6
counter(main): 7
counter(main): 8
counter(main): 9