Multi-Thread----Condition Variables

等待条件有两种方式:无条件等待pthread_cond_wait()和计时等待pthread_cond_timedwait()

激发条件有两种形式:pthread_cond_signal()激活一个等待该条件的线程，存在多个等待线程时按入队顺序激活其中一个；而pthread_cond_broadcast()则激活所有等待线程

在调用pthread_cond_wait()前必须由本线程加锁（pthread_mutex_lock()），而在更新条件等待队列以前，mutex保持锁定状态，并在线程挂起进入等待前解锁。

在条件满足从而离开pthread_cond_wait()之前，mutex将被重新加锁，以与进入pthread_cond_wait()前的加锁动作对应

用互斥量锁住条件,防止同一个条件被多个线程竞争.

下面例子是创建三个线程,设定全局变量i,当i==1时唤醒线程1,当i==2时唤醒线程2,当i==3时唤醒线程3.

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

int i = -1;

pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER;
pthread_cond_t condition = PTHREAD_COND_INITIALIZER;

void thread1() {

    printf("Therad1 Begin\n");

    pthread_mutex_lock(&mutex);
    while (i != 1) {
        printf("Therad1 Waiting\n");
        pthread_cond_wait(&condition, &mutex);
    }

    printf("Thread1 doing\n");

    pthread_mutex_unlock(&mutex);

    printf("Thread1 End\n");
}

void thread2() {

    printf("Therad2 Begin\n");

    pthread_mutex_lock(&mutex);
    while (i != 2) {
        printf("Therad2 Waiting\n");
        pthread_cond_wait(&condition, &mutex);
    }

    printf("Thread2 doing\n");

    pthread_mutex_unlock(&mutex);

    printf("Thread2 End\n");
}

void thread3() {

    printf("Therad3 Begin\n");

    pthread_mutex_lock(&mutex);
    while (i != 3) {
        printf("Therad3 Waiting\n");
        pthread_cond_wait(&condition, &mutex);
    }

    printf("Thread3 doing\n");

    pthread_mutex_unlock(&mutex);

    printf("Thread3 End\n");
}

int main(int argc, const char * argv[]) {

    pthread_t tid1;
    pthread_t tid2;
    pthread_t tid3;
    void *t1 = thread1;
    void *t2 = thread2;
    void *t3 = thread3;

    pthread_create(&tid1, NULL, t1, NULL);
    pthread_create(&tid2, NULL, t2, NULL);
    pthread_create(&tid3, NULL, t3, NULL);

    while (i != 3) {
        i++;
        printf("i = %d\n",i);
        sleep(3);
        pthread_cond_broadcast(&condition);
    }

    return 0;
}

输出结果:

i = 0
Therad1 Begin
Therad1 Waiting
Therad3 Begin
Therad2 Begin
Therad3 Waiting
Therad2 Waiting
i = 1
Thread1 doing
Thread1 End
Therad3 Waiting
Therad2 Waiting
i = 2
Therad3 Waiting
Thread2 doing
Thread2 End
i = 3
Thread3 doing
Thread3 End

总结:条件不满足时,线程挂起,等待条件满足.

因为使用的pthread_cond_broadcast,所以使用while循环判断条件是否成立.

当没有用互斥量锁住条件时:

i = 0
Therad3 Begin
Therad1 Begin
Therad2 Begin
Therad3 Waiting
Therad1 Waiting
Therad2 Waiting
i = 1
Therad3 Waiting
Therad2 Waiting
Thread1 doing
Thread1 End
i = 2
i = 3//////////////或者/////////////i = 0Therad1 BeginTherad3 BeginTherad2 BeginTherad1 WaitingTherad3 WaitingTherad2 Waitingi = 1Thread1 doingThread1 Endi = 2i = 3