c++ - 在 c/c++ 中使用 pthread_cond_wait 和 pthread_cond_signal 以循环方式执行多个线程

Question

我创建了 10 个线程，想要以循环方式执行 3 次。最初全部 线程正在等待。主线程向线程 0 发送信号，收到信号后线程 0 被唤醒并 然后执行一些任务，然后向线程 1 发送信号，这会像 thread1-> thread2->thread3 一样重复 ....->线程9。然后线程 9-> 线程 0。

我正在尝试实现这个

线程 i 执行一些任务，然后向线程 (i+1) 发送信号，然后线程 i 进入休眠状态。线程 (i+1) 将在 t 秒后唤醒(意味着线程 i+1 唤醒时间 - 线程 i sleep 时间 = t 秒)，线程 (i+1) 将执行一些任务，向线程 (i+2) 发送信号然后去 sleep ，这会重复几次(3)次。

虽然我能够从线程 0-> 线程 1 -> .... 线程 9 发送信号(循环仅执行一次)，我无法发送信号线程 9 -> 线程 0并且这就是为什么我无法重复这个循环3次。

我在哪里犯了错误？

任何帮助将不胜感激。 我在 linux 内核 2.6.32 下使用 g++ 4.6.3。

这是我的预期输出

    Create 5 threads
    Thread created=0
    Thread created=1
    Thread created=2
    Thread created=3
    Thread created=4
    Thread 4 blocked
    Thread 3 blocked
    Thread 2 blocked
    Thread 1 blocked
    Thread 0 blocked
    Wake up all waiting threads...
    Thread 0 unblocked
    Thread 1 unblocked
    Thread 2 unblocked
    Thread 3 unblocked
    Thread 4 unblocked

    Thread 4 blocked // repeataion of same sequence
    Thread 3 blocked
    Thread 2 blocked
    Thread 1 blocked
    Thread 0 blocked
    Thread 0 unblocked
    Thread 1 unblocked
    Thread 2 unblocked
    Thread 3 unblocked
    Thread 4 unblocked


    Wait for threads and cleanup
    Main completed

这是我的实际输出

Create 5 threads
Thread created=0
Thread created=1
Thread created=2
Thread created=3
Thread created=4
Thread 4 blocked
Thread 3 blocked
Thread 2 blocked
Thread 1 blocked
Thread 0 blocked
Wake up all waiting threads...
Thread 0 unblocked
Thread 1 unblocked
Thread 2 unblocked
Thread 3 unblocked
Thread 4 unblocked
Wait for threads and cleanup
Main completed

这是我的代码

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


/* For safe condition variable usage, must use a boolean predicate and   */
/* a mutex with the condition.                                           */
int                 conditionMet = 0;
pthread_cond_t      cond[5];

pthread_mutex_t     mutex = PTHREAD_MUTEX_INITIALIZER;

#define NTHREADS    5

void *threadfunc(void *parm)
{

  int i;
  long my_id = (long)parm;
  int           rc;

// Initially all threads will wait 
rc = pthread_mutex_lock(&mutex);
printf("Thread %d blocked\n",my_id);
rc = pthread_cond_wait(&cond[my_id], &mutex);
printf("Thread %d unblocked\n", my_id);
rc = pthread_mutex_unlock(&mutex);

int count=0;

while(count++<3) // This line makes no sense, no repeatation as expected. 
{

  rc = pthread_mutex_lock(&mutex);

  while (!conditionMet) {
    printf("Thread %d blocked\n",my_id);
    rc = pthread_cond_wait(&cond[my_id], &mutex);
    printf("Thread %d unblocked\n", my_id);
  }

  rc = pthread_mutex_unlock(&mutex);

  // sending signal to next thread i+1
 rc = pthread_mutex_lock(&mutex);
    rc = pthread_cond_signal(&cond[(my_id+1)%NTHREADS]);
    rc = pthread_mutex_unlock(&mutex);

   }
      return NULL;
   }

    int main(int argc, char **argv)
    {
  int                   rc=0;
  int                   i;
  pthread_t             threadid[NTHREADS];

    for(rc=0;rc<NTHREADS;rc++)
    cond[rc]= PTHREAD_COND_INITIALIZER;

  printf("Enter Testcase - %s\n", argv[0]);

  printf("Create %d threads\n", NTHREADS);
  for(i=0; i<NTHREADS; ++i) {
    rc = pthread_create(&threadid[i], NULL, threadfunc, (void *)i);
    printf("Thread created=%d\n", i);
  }

  sleep(5);  /* Sleep is not a very robust way to serialize threads */
  rc = pthread_mutex_lock(&mutex);

  /* The condition has occured. Set the flag and wake up any waiting threads */
  conditionMet = 1;
  printf("Wake up all waiting threads...\n");
  rc = pthread_cond_signal(&cond[0]);    
  rc = pthread_mutex_unlock(&mutex);      

  printf("Wait for threads and cleanup\n");
  for (i=0; i<NTHREADS; ++i) {
    rc = pthread_join(threadid[i], NULL);        
  }
  pthread_cond_destroy(&cond[0]);
  pthread_mutex_destroy(&mutex);

  printf("Main completed\n");
  return 0;
}

Answer 1

引用pthread_wait后我自己解决了我的问题。

所有线程按循环顺序运行一次，然后程序终止/完成执行，因为没有线程等待下一轮，全部完成。

因此，要多次(3)运行相同的序列，需要正确修改等待变量，以便每个线程将等待下一轮直到第3次执行。

这是修改后的程序:

void *threadfunc(void *parm)
{

  int i;
  long my_id = (long)parm;
  int           rc;

   /*
 DELETE THIS PORTION, OTHERWISE NO OUTPUT, AGAIN HANGED BEFORE SINGLE SEQUENCE.
 IT WILL NOT ENTER INSIDE LOOP while(count++<3)

    // Initially all threads will wait 
    rc = pthread_mutex_lock(&mutex);
    printf("Thread %d blocked\n",my_id);
    rc = pthread_cond_wait(&cond[my_id], &mutex);
    printf("Thread %d unblocked\n", my_id);
    rc = pthread_mutex_unlock(&mutex);
    */


int count=0;

while(count++<3)
{

  rc = pthread_mutex_lock(&mutex);
  while ( conditionMet != my_id) 
   {
    printf("Thread %d blocked\n",my_id);
    rc = pthread_cond_wait(&cond[my_id], &mutex);
    printf("Thread %d unblocked\n", my_id);
   }

    rc = pthread_mutex_unlock(&mutex);     
    rc = pthread_mutex_lock(&mutex);
    conditionMet = (my_id+1)%NTHREADS ; // This is important to wait for next time
    rc = pthread_cond_signal(&cond[(my_id+1)%NTHREADS]);
    rc = pthread_mutex_unlock(&mutex);
}
  return NULL;
}