void* blue(struct image *img)
{
    printf("Launching blue filtern");
    if (filter_blue(img))
    {
    perror("Error calling filter_blue");
    pthread_exit((void*)1);
}
    pthread_exit(NULL);
}

// in main file:
pthread_t thread[parameters.num_threads];
pthread_attr_t attr;
pthread_attr_init(&attr);
pthread_attr_setdetachstate(&attr, PTHREAD_CREATE_JOINABLE);
for (j=parameters->num_threads - parameters->num_threads_left; count < img->blue_threads ; j++)
{
    pthread_create(&thread[j], &attr, (void*)blue, img);
    printf("blue foundn");
    count++;
}

void *status;
for (i=0; i<num_thread; i++)
{
    int a = pthread_join(thread[i], &status);
    printf("pthread join return value: %d n",a);
}
pthread_attr_destroy(&attr);