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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 = 0
Therad1 Begin
Therad3 Begin
Therad2 Begin
Therad1 Waiting
Therad3 Waiting
Therad2 Waiting
i = 1
Thread1 doing
Thread1 End
i = 2
i = 3
Multi-Thread----Condition Variables
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原文地址：http://www.cnblogs.com/xwjack1554239786/p/5405594.html
