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

// Simplest way to create a mutex, no function calls needed
pthread_mutex_t lock=PTHREAD_MUTEX_INITIALIZER;

// When two threads must use the same memory, you must use
// mutex calls to make it safe.  It's not just a lock, it's also a
// "memory barrier", guaranteeing memory values between different CPU cores
// are synchronized properly.
int running=1;

void *thread(void *arg)
{
        fprintf(stderr, "Thread beginning\n");
        while(1)
        {
                int r;
                fprintf(stderr, "thread waiting for lock\n");
                // If main already has the mutex, thread will sleep.
                // when main unlocks it, the thread will lock it and continue.
                pthread_mutex_lock(&lock);
                        fprintf(stderr, "We have the mutex, it is safe to get value of 'running'\n");
                        r=running;
                        fprintf(stderr, "Value of 'running' is %d\n", r);
                pthread_mutex_unlock(&lock); // What gets locked MUST be unlocked later!
                fprintf(stderr, "thread has given up the lock\n");
                if(r == 0) break;
                sleep(1);
        }
        fprintf(stderr, "thread finishing\n");

        return((void *)0xdeadbeef);
}

int main()
{
        void *ret;
        pthread_t tid;
        running=1;
        pthread_mutex_lock(&lock); // Stop thread from getting the mutex
        fprintf(stderr, "main now has the mutex\n");

        pthread_create(&tid, NULL, thread, NULL);
        sleep(2); // Thread will begin, but wait for mutex

        fprintf(stderr, "main is unlocking the mutex\n");
        pthread_mutex_unlock(&lock); // Let the thread run for a while
        sleep(5);

        // Lock the mutex, making it safe to alter 'running'
        pthread_mutex_lock(&lock);
                fprintf(stderr, "main now has the mutex\n");
                running=0;
                fprintf(stderr, "main is unlocking the mutex\n");
        pthread_mutex_unlock(&lock);

        pthread_join(tid, &ret);
        fprintf(stderr, "Thread finished with return value %p\n", ret);
}