4. 阻塞线程

#include <pthread.h> struct timespec {

time_t tv_sec;/* seconds */ // 秒 longtv_nsec; /* nanosecondes*/ // 纳秒

}

// pthread_cond_timedwait(&cond, &t);

/**

• 阻塞等待一个条件变量

*1. 阻塞等待条件变量cond（参1）满足

pthread_mutex_lock(&mutex);

• @param cond 指向要初始化的条件变量指针.
• @param mutex 互斥锁.
• @return成功: 0; 失败: 非0错误号.

*/ int pthread_cond_wait(pthread_cond_t *restrict cond,

pthread_mutex_t *restrict mutex);

/**

• 限时等待一个条件变量.
• @param cond 指向要初始化的条件变量指针.
• @param mutex 互斥锁.
• @param abstime 绝对时间.
• @return成功: 0; 失败: 非0错误号.

*/ int pthread_cond_timedwait(pthread_cond_t *restrict cond,

pthread_mutex_t *restrict mutex, const struct *restrict abstime);

1. 条件变量流程分析

1、条件变量的优缺点

相较于mutex而言，条件变量可以减少竞争。

如直接使用mutex，除了生产者、消费者之间要竞争互斥量以外，消费者之间也需要竞争互斥量;

但如果汇聚（链表）中没有数据，消费者之间竞争互斥锁是无意义的。

有了条件变量机制以后，只有生产者完成生产，才会引起消费者之间的竞争。提高了程序效率。

代码如下：

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

#include <stdlib.h> #define NUM_OF_TASKS 3

#define MAX_TASK_QUEUE 11 char tasklist[MAX_TASK_QUEUE]="ABCDEFGHIJ";

int head = 0;

int tail = 0;

int quit = 0;

pthread_mutex_t g_task_lock;

pthread_cond_t g_task_cv;

void *coder(void *notused)

{ pthread_t tid = pthread_self();

while(!quit){ pthread_mutex_lock(&g_task_lock);

while(tail == head){ if(quit){

pthread_mutex_unlock(&g_task_lock);

pthread_exit((void *)0);

} printf("No task now! Thread %u is waiting!\n", (unsigned int)tid);

pthread_cond_wait(&g_task_cv, &g_task_lock);

printf("Have task now! Thread %u is grabing the task !\n", (unsigned int)tid);

} char task = tasklist[head++];

pthread_mutex_unlock(&g_task_lock);

printf("Thread %u has a task %c now!\n", (unsigned int)tid, task);

sleep(5);

printf("Thread %u finish the task %c!\n", (unsigned int)tid, task);

} pthread_exit((void *)0);

}

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

{ pthread_t threads[NUM_OF_TASKS];

int rc;

int t;

pthread_mutex_init(&g_task_lock, NULL);

pthread_cond_init(&g_task_cv, NULL);

printf("ERROR; return code from pthread_create() is %d\n", rc);

exit(-1);

} }

sleep(5);

for (t=1;t<=4;t++) { pthread_mutex_lock(&g_task_lock);

tail+=t;

printf("I am Boss, I assigned %d tasks, I notify all coders!\n", t);

pthread_cond_broadcast(&g_task_cv);

pthread_mutex_unlock(&g_task_lock);

sleep(20);

} pthread_mutex_lock(&g_task_lock);

quit = 1;

pthread_cond_broadcast(&g_task_cv);

pthread_mutex_unlock(&g_task_lock);

pthread_mutex_destroy(&g_task_lock);

pthread_cond_destroy(&g_task_cv);

pthread_exit(NULL);

}

下面是对于每个输出的分析：

No task now! Thread 1461831424 is waiting!

No task now! Thread 1453438720 is waiting!

No task now! Thread 1445046016 is waiting!

这是主线程新建的三个工作线程，这时候工作线程都是先获得锁，判断tail==head，阻塞，cond_wait释放了锁，三个工作线程都等待信号来唤醒

I am Boss, I assigned 1 tasks, I notify all coders!

主线程增加了一个任务，引起了head!=tail，这时候broadcast唤醒了所有工作线程，之后主线程释放锁，开始睡眠 20s

Have task now! Thread 1453438720 is grabing the task !

1工作线程在竞争中抢到了锁，退出了while(tail==head)循环，并开始完成A任务，这时候把头加1，释放锁

Have task now! Thread 1445046016 is grabing the task !

No task now! Thread 1445046016 is waiting!

Thread 1453438720 has a task A now!

调度程序调度到1线程执行，打印了这句话，开始睡眠5s

Have task now! Thread 1461831424 is grabing the task !

No task now! Thread 1461831424 is waiting!

3工作线程抢到了锁（虽然这时候只有它在抢锁），3得到了锁，但是这时候已经没有工作了。while(tail==head)循环没有退出，因此释放锁，等待唤醒

Thread 1453438720 finish the task A!

No task now! Thread 1453438720 is waiting!

20工作线程执行完任务，这时候还是没有工作。先上锁，后进入while(tail==head)循环没有退出，因此释放锁，等待唤醒

I am Boss, I assigned 2 tasks, I notify all coders!

主线程醒了，又找了两个工作B,C，唤醒了所有工作线程去做

Have task now! Thread 1461831424 is grabing the task !

Thread 1461831424 has a task B now!

24号线程抢到了锁，执行头加1，释放锁，然后睡眠了

Have task now! Thread 1445046016 is grabing the task !

Thread 1445046016 has a task C now!

16号线程抢到了锁，执行头加1，释放锁，然后睡眠了

Have task now! Thread 1453438720 is grabing the task !

No task now! Thread 1453438720 is waiting!

20号线程抢到了锁，没任务了，就释放锁，等待唤醒

Thread 1445046016 finish the task C!

No task now! Thread 1445046016 is waiting!

调度16号线程执行，16号线程完成任务，得到锁，发现没任务，释放锁，等待唤醒

Thread 1461831424 finish the task B!

I am Boss, I assigned 3 tasks, I notify all coders!

主线程醒了，又找了三个个工作D,E,F，唤醒了所有工作线程去做

Have task now! Thread 1461831424 is grabing the task !

Thread 1461831424 has a task D now!

24号线程抢到了锁，执行头加1，释放锁，然后睡眠了

Have task now! Thread 1445046016 is grabing the task !

Thread 1445046016 has a task E now!

16号线程抢到了锁，执行头加1，释放锁，然后睡眠了

Have task now! Thread 1453438720 is grabing the task !

Thread 1453438720 has a task F now!

20号线程抢到了锁，执行头加1，释放锁，然后睡眠了

Thread 1461831424 finish the task D!

No task now! Thread 1461831424 is waiting!

调度24号线程执行，24号线程完成任务，得到锁，发现没任务，释放锁，等待唤醒

Thread 1445046016 finish the task E!

No task now! Thread 1445046016 is waiting!

调度16号线程执行，16号线程完成任务，得到锁，发现没任务，释放锁，等待唤醒

Thread 1453438720 finish the task F!

No task now! Thread 1453438720 is waiting!

调度20号线程执行，20号线程完成任务，得到锁，发现没任务，释放锁，等待唤醒

I am Boss, I assigned 4 tasks, I notify all coders!

主线程醒了，又找了三个个工作G,H,I,J，唤醒了所有工作线程去做

Have task now! Thread 1453438720 is grabing the task !

Thread 1453438720 has a task G now!

24号线程抢到了锁，执行头加1，释放锁，然后睡眠了

Have task now! Thread 1445046016 is grabing the task !

Thread 1445046016 has a task I now!

16号线程抢到了锁，执行头加1，释放锁，然后睡眠了

Thread 1453438720 finish the task G!

Thread 1453438720 has a task J now!

20号线程执行完任务，下一轮while中，持有了锁，发现还有任务，就继续做J，执行头加1，释放了锁，开始睡眠 5s

Thread 1461831424 finish the task H!

No task now! Thread 1461831424 is waiting!

调度24号线程执行，24号线程完成任务，得到锁，发现没任务，释放锁，等待唤醒

Thread 1445046016 finish the task I!

No task now! Thread 1445046016 is waiting!

调度16号线程执行，16号线程完成任务，得到锁，发现没任务，释放锁，等待唤醒

Thread 1453438720 finish the task J!

No task now! Thread 1453438720 is waiting!

20线程完成任务，下一轮while中持有锁，发现没任务了，释放锁，等待唤醒主线程抢锁，设置退出标志quit为1，并通知三个工作线程

Have task now! Thread 1461831424 is grabing the task !

Have task now! Thread 1445046016 is grabing the task !

Have task now! Thread 1453438720 is grabing the task !

三个工作线程分别抢到了锁，发现quit为1，就释放锁，然后终止各自的线程主线程销毁了锁和条件变量

终止了main线程