What's the real order? Order in which friend threads were spawned or the ticket id that they came back with?

friends.c

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

static int ticketid = 0;
pthread_mutex_t lock;

// buyticket function remote dial a central store to obtain ticket id
void * buyticket() {
    int *id = (int *)malloc(sizeof(int));
    pthread_mutex_lock(&lock);
    *id = ticketid++;
    pthread_mutex_unlock(&lock);
    return id;
}

void * friend(void *arg) {
    return buyticket();
}

int main() {
    // N friends one by one to go buy ticket
    int N = 10;
    int i;
    pthread_t friends[N];
    for (i=0; i<N; i++) {
        // imagine spawn of thread as u saying to friends on Node A, B, C ... to buy a ticket in order
        pthread_create(&friends[i], NULL, friend, NULL);
    }

    // wait for friends to come back
    int *id;
    for (i=0; i<N; i++) {
        pthread_join(friends[i], (void **)&id);
        printf("friend id %d, ticket id %d\n", i, *id);
        free(id);
    }
}

Try as:

make friends && ./friends

This code is not to say that above strategy is correct, but more to imply that how ordering of operations are simply lost as soon as you start to multi-thread the client side.