C Multithreading | Thread creation in for loop uses args from last iteration

I’m new to multithreading and not the best in C in general so bare with me.

I have a for loop that creates a number of threads to which I’m passing arguments as such:

  for(int i = 0; i < NO_OF_THREADS; i++) {

    int ordered_product = (rand() % NO_OF_PRODUCTS);
    int ordered_quantity = (rand() % 10) + 1;
    int customer = (rand() % NO_OF_CUSTOMERS);

    printf("%d %d %d\n", customer+1, ordered_quantity, ordered_product+1);

    ThreadArgs myargs = {customer, ordered_product, ordered_quantity};

    int rc = pthread_create(&mythreads[i], NULL, thread_function, &myargs);
    if(rc != 0) {
      perror("Pthread create");
      exit(1);
    }

  }

and I have the function "thread_function" which is written as such:

void* thread_function(void* arg) {

  ThreadArgs* args = (ThreadArgs*) arg;
  ThreadArgs myargs = *args;
  int customer_id = myargs.customer_id + 1;
  int product_quantity = myargs.product_quantity;
  int product_id = myargs.product_id +1;

  printf("Customer %d purchased %d of Product %d\n", customer_id, product_quantity, product_id);

  //pthread_exit(NULL)   // I tried this too...
  return NULL;
}

This is the output that I’m getting:

4 8 4
3 3 9
8 1 9
Customer 8 purchased 1 of Product 9
Customer 8 purchased 1 of Product 9
Customer 8 purchased 1 of Product 9

each thread was supposed to print out its respective arguments but instead, all three threads are printing the arguments of the last iteration.

For some reason the problem goes away if I add a sleep() call to the bottom of the for loop but I don’t want it to sleep.

Would Appreciate any help.

>Solution :

When the loop pass is over, it’s undefined behaviour to access myargs.

One solution:

ThreadArgs myargs[ NO_OF_PRODUCTS ];

for ( int i = 0; i < NO_OF_THREADS; ++i ) {
   …
   myargs[i].… = …;
   …
   pthread_create( mythreads+i, NULL, thread_function, myargs+i )
   …
}

Another would be to use malloc to allocate the struct.