Pthread synchronization - print even odd numbers - c

Question

I am new to pthreads. I am trying to print even and odd numbers from two threads. What is wrong with below code? Its intention is to create two threads - one will print odd numbers and other will print even numbers. The numbers have to be printed in order. It seems to get stuck (time limit exceeded in ideone).. I have spent a lot of time staring at it. Just can't figure out what is wrong..
#include <stdio.h>
#include <pthread.h>
pthread_mutex_t lock;
int n = 0;
int max = 10;
pthread_cond_t even;
pthread_cond_t odd;
void* print_odd(void *x)
{
while(1)
{
pthread_mutex_lock(&lock);
while(n%2 != 0)
{
pthread_cond_wait(&even, &lock);
}
if(n >= max)
{
pthread_mutex_unlock(&lock);
pthread_exit(NULL);
}
printf("Thread A : %d", ++n);
pthread_cond_signal(&odd);
pthread_mutex_unlock(&lock);
}
}
void* print_even(void *x)
{
while(1)
{
pthread_mutex_lock(&lock);
while(n%2 == 0)
{
pthread_cond_wait(&odd, &lock);
}
if(n >= max)
{
pthread_mutex_unlock(&lock);
pthread_exit(NULL);
}
printf("Thread B : %d", ++n);
pthread_cond_signal(&even);
pthread_mutex_unlock(&lock);
}
}
main()
{
pthread_t t1, t2;
pthread_create(&t1, NULL, print_odd, NULL);
pthread_create(&t2, NULL, print_even, NULL);
pthread_join(t1, NULL);
pthread_join(t2, NULL);
exit(0);
}

Answer 1

There are multiple issues with your program-
As suggested in the comments, the lock and the conditional variables need to be initialized.
pthread_mutex_t lock = PTHREAD_LOCK_INITIALIZER;
pthread_cond_t even = PTHREAD_COND_INITIALIZER;
pthread_cond_t odd = PTHREAD_COND_INITIALIZER;
You might get lucky accidentally here even without initialization since you've declared them as global and they will be zero-inited and pthread implementations might actually be zero-initing when you properly initialize them.
Your printf doesn't have \n and hence output is not flushed to screen. Just add the newline and you'll see your threads are indeed running.
When n reaches 10, ie when print_odd threads increments from 9, it simply exits without signaling the even thread. Hence your even thread is hung in the cond_wait and your main thread is hung in pthread_join. You can fix this by waking up the even thread by signalling it before exiting the odd thread.
EDIT I found one more issue
Even if the odd thread signals the even thread just before exiting, since n=10, the even thread does NOT exit the while(n%2 == 0) loop and goes back to sleep again. This time, there's no one to wake up the poor soul. It is for this reason that you need to test the termination condition n>=max inside the while loop

Answer 2

pthread_cond_wait is blocking the calling threads. In your case, you have asked the threads to wait on the true conditions of odd and even. Instead, they should wait on the incorrect conditions.
While i%2 == 0, the odd thread should call the wait function inside of the routine.
While i!=2, the even thread should call wait function.