CODEWITHSUNDEEP
pthreadsconditional-statements
Sara
Sara

Reputation: 2436

pthread and condition variable

I'm wondering why the following code gives unexpected output: a can get 110...!

pthread_t th[nbT];
void * func(void *d)
{       
    while(a<100)
    {
            pthread_mutex_lock(&l);
            cout <<a <<" in thread "<<pthread_self()<<"\n";
            a+=1;
            pthread_mutex_unlock(&l);
    }  
    return NULL;
 } 
  int main(int argc, const char* argv[])
 {
    for(int i=0;i<nbT;i++)
            pthread_create(&(th[i]), NULL, func, NULL);

    for(int i=0;i<nbT;i++)
            pthread_join(th[i],NULL);
 }

Upvotes: 0

Views: 64

Answers (1)

DaoWen
DaoWen

Reputation: 33019

The problem is that you get the lock (mutex) after checking the condition, so you don't know if it's still true or not once you get the lock. You should just do a simple double-check:

while(a<100)
{
        pthread_mutex_lock(&l);
        cout <<a <<" in thread "<<pthread_self()<<"\n";
        if (a<100) a+=1; // <== Added condition here!
        pthread_mutex_unlock(&l);
}

Upvotes: 2

Related Questions