// To compile:  gcc -g3 -pthread THIS_FILE
#define  _DEFAULT_SOURCE /* for drand48 */

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

pthread_mutex_t mymutex = PTHREAD_MUTEX_INITIALIZER;

void * thread_doit(void *thread_num) {
  int thread_num_int = *(int *)thread_num;
  int len = (int) ((drand48() * 5) + 1);
  sleep(len);
  pthread_mutex_lock(&mymutex);
  printf("THREAD %d did a task\n", thread_num_int);
  pthread_mutex_unlock(&mymutex);
  return NULL;
}

int main(int argc, char* argv[]) {
  if (argc < 2) {
    printf("Expected usage: %s THREAD_NUM\n", argv[0]);
    return -1;
  }

  int thread_num = atoi(argv[1]);
  pthread_t *pthread_array = malloc(sizeof(pthread_t) * thread_num);
  pthread_t *thread_num_array = malloc(sizeof(pthread_t) * thread_num);
  srand48(42); // arbitrary seed value

  for (int i = 0; i < thread_num; i++) {
    // We pass thread_num as an argument.
    thread_num_array[i] = i;
    // SUPPOSE LAST ARUMENT IS &i instead of &thread_doit, &thread_num_array[i]
    // TRY IT, AND SEE THE BUG.  WHY DOES THE BUG OCCUR?
    pthread_create(&pthread_array[i], NULL, &thread_doit, &thread_num_array[i]);
  }

  for (int i = 0; i < thread_num; i++) {
    pthread_join(pthread_array[i], NULL);
  }

  free(pthread_array);
  return 0;
}