text: #include <stdio.h>
  #include <stdlib.h>
  #include <errno.h>
  #include <string.h>
  
  /** Our old friend die from ex17. */
  void die(const char *message)
  {
      if (errno) {
          perror(message);
      } else {
          printf("ERROR: %s\n", message);
      }
  
      exit(1);
  }
  
  // a typedef creates a fake type, in this
  // case for a function pointer
  typedef int (*compare_cb) (int a, int b);
  
  /**
   * A classic bubble sort function that uses the 
   * compare_cb to do the sorting. 
   */
  int *bubble_sort(int *numbers, int count, compare_cb cmp)
  {
      int temp = 0;
      int i = 0;
      int j = 0;
      int *target = malloc(count * sizeof(int));
  
      if (!target)
          die("Memory error.");
  
      memcpy(target, numbers, count * sizeof(int));
  
      for (i = 0; i < count; i++) {
          for (j = 0; j < count - 1; j++) {
              if (cmp(target[j], target[j + 1]) > 0) {
                  temp = target[j + 1];
                  target[j + 1] = target[j];
                  target[j] = temp;
              }
          }
      }
  
      return target;
  }
  
  int sorted_order(int a, int b)
  {
      return a - b;
  }
  
  int reverse_order(int a, int b)
  {
      return b - a;
  }
  
  int strange_order(int a, int b)
  {
      if (a == 0 || b == 0) {
          return 0;
      } else {
          return a % b;
      }
  }
  
  /** 
   * Used to test that we are sorting things correctly
   * by doing the sort and printing it out.
   */
  void test_sorting(int *numbers, int count, compare_cb cmp)
  {
      int i = 0;
      int *sorted = bubble_sort(numbers, count, cmp);
  
      if (!sorted)
          die("Failed to sort as requested.");
  
      for (i = 0; i < count; i++) {
          printf("%d ", sorted[i]);
      }
      printf("\n");
  
      free(sorted);
  }
  
  int main(int argc, char *argv[])
  {
      if (argc < 2) die("USAGE: ex18 4 3 1 5 6");
  
      int count = argc - 1;
      int i = 0;
      char **inputs = argv + 1;
  
      int *numbers = malloc(count * sizeof(int));
      if (!numbers) die("Memory error.");
  
      for (i = 0; i < count; i++) {
          numbers[i] = atoi(inputs[i]);
      }
  
      test_sorting(numbers, count, sorted_order);
      test_sorting(numbers, count, reverse_order);
      test_sorting(numbers, count, strange_order);
  
      free(numbers);
  
      return 0;
  }
  