在C ++中，如何让一个pthread继续而另一个线程正在等待信号量？

Question

I am currently going over an example our professor gave us before our current assignment with semaphores and pthreading in C++. Currently, the entire program waits when one of the threads is blocked.

#include <stdio.h>
#include <stdlib.h>
#include <pthread.h>
#include <semaphore.h>
#include <unistd.h>
#include <iostream>

using namespace std;

int account = 99;
bool sent = false;
int rate = 12;
int hours = 15;
sem_t s1;
sem_t s2;


//work thread
void *work(void*){
  while(1){
      sem_wait(&s1);
      account += hours * rate;
      cout << "Account: " << account << endl;
      if(account >= 1000 && !sent){
          sem_post(&s2);
          sent = true;
      }
      sem_post(&s1);

      pthread_exit(NULL);
  }
}

void* buy(void*){
  while(1){
      sem_wait(&s2);
      sem_wait(&s1);
      account -= 1000;
      sent = false;
      cout << "iPhone bought!! Account: " << account << endl;
      sem_post(&s1);
      pthread_exit(NULL);
  }
}

int main(){

  pthread_t workt, buyt;
    sem_init(&s1, 0, 1);
    sem_init(&s2, 0, 0);

  while(1){
    pthread_create( &workt, NULL, work, NULL);
    pthread_create( &buyt, NULL, buy, NULL);

    pthread_join(workt, NULL);
    pthread_join(buyt, NULL);
  }
    sem_close(&s1);
    sem_close(&s2);

    pthread_exit(NULL);
}

The program should run the 'work' thread continuously until enough is in the account (1000), then it will buy an iPhone. My code will run until it hits the sem_wait(s2) semaphore in the 'buy' thread, which blocks the thread as it should, but my entire program waits and does not run the 'work' thread again.

Answer 1

You're calling pthread_exit(NULL); on every iteration through your loop in work . Basically it's acting like there's no loop.

Maybe you mean something more like:

  while(!sent){
      sem_wait(&s1);
      account += hours * rate;
      cout << "Account: " << account << endl;
      if(account >= 1000){
          sem_post(&s2);
          sent = true;
      }
      sem_post(&s1);

  }
  pthread_exit(NULL);