gistfile1.c

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

typedef struct node {
  int level;
} node;

int compare_nodes(const void * a, const void * b) {
  int a_lvl = (*(node **)a)->level;
  int b_lvl = (*(node **)b)->level;

  if (a_lvl < b_lvl)
    return -1;
  else if (b_lvl > a_lvl)
    return 1;
  else
    return 0;
}

int main(int argc, char *argv[]) {
  int i;
  node **nodes = malloc(sizeof(node *) * 5);

  for (i=0;i<5;i++) {
    nodes[i] = malloc(sizeof(node));
    nodes[i]->level = 5-i;
  }

  for (i=0;i<5;i++) {
    printf("%d\n", nodes[i]->level);
  }

  // prints 5, 4, 3, 2, 1

  qsort(nodes, 5, sizeof(node *), compare_nodes);


  for (i=0;i<5;i++) {
    printf("%d\n", nodes[i]->level);
  }

  // still prints 5, 4, 3, 2, 1
  // expecting 1, 2, 3, 4, 5

  return 0;
}