Problem with same thread ID for different threads

I am having trouble with my threads. I am trying to create 10 threads and for each thread I want to print the thread ID. I am able to print the thread ID but the problem is that all threads prints out the same thread ID.

This is my code:

main:

int main(void)
{
    int check_error;
    pthread_t clientThread[10];
    
    printf("creating a client thread..!n");
    fflush(stdout);
    
    for(int i=0; i<10;i++){
        check_error=pthread_create(&clientThread[i], NULL, mqClient, NULL);

        if(check_error!=0)
            printf("Error when creating thread: %s", strerror(check_error));
        else
            pthread_join(clientThread[i], NULL);
    }

    return EXIT_SUCCESS;
}

mqClient:

void * mqClient(void * arg){
    pthread_mutex_lock(&mutex);

    mqd_t mq_on_server;
    struct pt planet;

    char thread_id[30];
    sprintf(thread_id, "%d", pthread_self());

    usleep(1000);

    int response = MQconnect(&mq_on_server, "/servermq");

    if(response==0){
        printf("Something went wrong with MQconnectn");
    }
    else{

        printf("nEnter planet name: ");
        scanf("%s", planet.name);

        printf("nEnter planet X-position: ");
        scanf("%lf", &planet.sx);

        printf("nEnter planet Y-position: ");
        scanf("%lf", &planet.sy);

        printf("nEnter planet X-velocity: ");
        scanf("%lf", &planet.vx);

        printf("nEnter planet Y-velocity: ");
        scanf("%lf", &planet.vy);

        printf("nEnter planet mass: ");
        scanf("%lf", &planet.mass);

        printf("nEnter planet life time: ");
        scanf("%d", &planet.life);

        strcpy(planet.pid, thread_id);

        printf("id: %s", planet.pid);

        printf("n---------------------------------------");
    }


    MQwrite (mq_on_server, &planet);

    int c;
    while ( (c = getchar()) != 'n' && c != EOF);

    pthread_mutex_unlock(&mutex);

    return NULL;
}

When testing two threads this is the output (observe the same thread ID):

Planet name: dsadas
Planet X-position: 321.000000
Planet Y-position: 321.000000
Planet X-velocity: 321.000000
Planet Y-velocity: 312.000000
Planet mass: 321.000000
Planet life time: 321
Planet thread ID: 1058301696
---------------------------------------
Planet name: ytyr
Planet X-position: 3123.000000
Planet Y-position: 54.000000
Planet X-velocity: 56.000000
Planet Y-velocity: 231.000000
Planet mass: 546.000000
Planet life time: 231
Planet thread ID: 1058301696

The thread ID should be unique for each thread, so any idea why I am getting this kind of output?

Answer

In your loop in main you are creating 10 threads, but you don’t create them in parallel. You are creating one thread at a time, waiting for it to end, and only then proceeding with the loop. So you only create 0-1 threads at a time, but not 0-10 threads.

In general: two threads that exist at the same time have different IDs, but two threads that exist at different points in time can have the same ID.