C - Concurrency issue with threads, arguments are being printed twice

Question

I wrote this program that's supposed create a new thread in every iteration of the main loop and have it print the iteration variable. Eg. in the first iteration, a new thread is made and it should print "arg: 0", next iteration prints "arg: 1", etc.

As you can see in the screenshot below, arg 3 & 7 are being printed twice, while it should only print once. Now it works if I add sleep(1) to every iteration, so it's some kind of concurrency issue. Note that the result is completely random every time, it's not just 3 and 7 every time.

Anyone have an idea? Thanks!

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

static void * mutex_thread(void * arg);
int threads_amt = 10;
static pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER;

int main (void)
{
    pthread_t threads[threads_amt];

    // Create threads.
    for(int i = 0; i < threads_amt; i++) 
    {
        printf ("Starting thread [%d]...\n", i);

        // Create thread.
        pthread_create (&threads[i], NULL, mutex_thread, &i);

        // IT WORKS IF I ENABLE sleep(1).
        //sleep (1);

        printf ("Thread created.\n\n");
    }

    // Join threads.
    printf("Joining threads...\n");
    for(int i = 0; i < threads_amt; i++) 
    {
        pthread_join (threads[i], NULL);
    }

    return (0);
}

static void * mutex_thread(void * arg) 
{
    // Lock mutex.
    pthread_mutex_lock (&mutex);

    // Print arg.
        int *number = (int*)arg;
        printf("arg: %d\n", *number);

    // Unlock mutex.
    pthread_mutex_unlock (&mutex);

    return (NULL);
}

Answer 1

You have data race - because you are passing the same address (of variable i ) to all the threads.

Pass a different address (for example use an array or use a `malloc'ed value) to each thread.