Question

I’m trying to implement this simple example of how to synchronize threads using pthread library:

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

using namespace std ;

static pthread_mutex_t locker;
pthread_cond_t cond;
volatile bool ok=false;


void *func2(void *data)
{
int i;
for(i=0;i<100;i++)
{
    pthread_mutex_lock (&locker);
    cout << "1";
    pthread_mutex_unlock(&locker);
    if(i==10)
    {
        ok=true;
        pthread_cond_signal(&cond);
    }

}

pthread_exit(0);

  }

void *fun1(void *data)
{
int i;
for(i=0;i<100;i++)
{

    if(ok==false){
    pthread_cond_wait(&cond, &locker);
    }

    pthread_mutex_lock (&locker);
    cout << "2";
    pthread_mutex_unlock(&locker);
}

pthread_exit(0);
   }




   int main(void)
  {


pthread_t thread1, thread2;
void *retour_thread;


pthread_mutex_init (&locker, NULL);
pthread_cond_init(&cond, NULL);

if(pthread_create (&thread1, NULL, fun1, NULL) < 0)
{
    cout << "problem thread";
    exit(1);
}
if(pthread_create (&thread2, NULL, func2, NULL) < 0)
{
    cout << "problem thread";
    exit(1);
}


(void)pthread_join(thread1,&retour_thread);
(void)pthread_join(thread2,&retour_thread);

return 0;
    }

what should I see is func1 wait until the condition (ok==true) then process func2…but what I’m getting is unpredictable and not synchrinised!!!

any help and thanks in advence

Answer 1

You need to acquire the mutex before you call pthread_cond_wait, and after pthread_cond_wait returns, the mutex is reacquired. Your fun1() should look something like:

void *fun1(void *data)
{
    int i;
    for(i=0;i<100;i++)
    {
        pthread_mutex_lock (&locker);
        if(ok==false)
            pthread_cond_wait(&cond, &locker);

        cout << "2";
        pthread_mutex_unlock(&locker);
    }
}

Update:

You have a race where func2() can signal faster than fun1() can consume. If you want alternate “1” and “2” outputs, you need feedback in both directions.