Question

I’m asked to write a program that will have 2 threads and print 5 random integers such that the first thread will generate a number, the second will print it. Then the first will generate the 2nd number, the second thread will print it… etc. using a mutex.

My code now execute it for one cycle. How can I extend it to make threads excute the methods 5 times?

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

void* generate (void*);
void* print (void*);

pthread_mutex_t m;
int number = 5;
int genNumber;


int main()
{
    int i;
    srandom(getpid());
    pthread_t th[2];

    pthread_mutex_init(&m,NULL);

    pthread_create(&th[0],NULL,generate,NULL);
    pthread_create(&th[1],NULL,print, NULL);

    for (i = 0; i < 2; i++)
        pthread_join(th[i], NULL);

    pthread_mutex_destroy(&m);

    return 0;
}

void* generate(void* arg)
{
    pthread_mutex_lock(&m);
    genNumber = random() % 9;
    printf("Generated #1 \n");
    pthread_mutex_unlock(&m);
}

void* print(void* arg)
{
    pthread_mutex_lock(&m);
    printf("The number is %d " , genNumber);
    pthread_mutex_unlock(&m);
    pthread_exit(NULL);
}

Answer 1

Use condition variables to synchronize the two threads. When a thread has completed its work, it signals to the other thread to wake up, and then it goes to sleep to wait for more work. So something like this:

// Pseudocode
pthread_cond_t c1, c2;
pthread_mutex_t mutex;

// Thread 1 (producer):
for(int i = 0; i < 5; i++)
{
    lock(mutex);
    genNumber = random() % 9;
    signal(c2);
    wait(c1, mutex);
    unlock(mutex);
}

// Thread 2 (consumer):
for(int i = 0; i < 5; i++)
{
    lock(mutex);
    wait(c2, mutex);
    print("The number is %d\n", genNumber);
    signal(c1);
    unlock(mutex);
}