qsort-undefined.c

/*
 * A small sample shows undefined behaviour in 'qsort()'
 * when all elements *comparing on* are equal. I.e. all
 * the array elements are not necessarily equal; only the one we
 * care about. It seems a 'swap()' operation is performed anyway (?)
 *
 * Compile:
 *   c:\> clang -Wall -o qsort-undefined.exe qsort-undefined.c
 * or
 *   c:\> gcc -Wall -o qsort-undefined.exe qsort-undefined.c
 * or
 *   c:\> cl -nologo -W3 -Fe./qsort-undefined.exe qsort-undefined.c
 *
 * Output:
 *   c:\> qsort-undefined.exe
 *   qsort-undefined.c(43): Before qsort():
 *   qsort-undefined.c(45):   ip:         7.7.7.7  metric: 10
 *   qsort-undefined.c(45):   ip:         8.8.8.8  metric: 10
 *   qsort-undefined.c(45):   ip:         9.9.9.9  metric: 10
 *   qsort-undefined.c(49): After qsort():
 *   qsort-undefined.c(51):   ip:         8.8.8.8  metric: 10
 *   qsort-undefined.c(51):   ip:         9.9.9.9  metric: 10
 *   qsort-undefined.c(51):   ip:         7.7.7.7  metric: 10
 */
#include <stdio.h>
#include <stdlib.h>

#define TRACE(fmt, ...)  do {                          \
                           printf ("%s(%u): " fmt,     \
                                   __FILE__, __LINE__, \
                                   ## __VA_ARGS__);    \
                         } while (0)
typedef struct {
        unsigned metric; /* The metric we sort them by. */
        char     ip[20];
      } Address;

static int compareAddresses (const void *arg1,
                             const void *arg2)
{
  const Address *left  = arg1;
  const Address *right = arg2;

  if (left->metric < right->metric)
     return -1;
  if (left->metric > right->metric)
     return 1;
  return 0;
}

int main (void)
{
  Address addresses[] = {
        { 10, "7.7.7.7" },
        { 10, "8.8.8.8" },
        { 10, "9.9.9.9" },
      };
  int i, num = sizeof(addresses) / sizeof(addresses[0]);

  TRACE ("Before qsort():\n");
  for (i = 0; i < num; i++)
     TRACE ("  ip: %15s  metric: %u\n", addresses[i].ip, addresses[i].metric);

  qsort (&addresses, num, sizeof(addresses[0]), compareAddresses);

  TRACE ("After qsort():\n");
  for (i = 0; i < num; i++)
      TRACE ("  ip: %15s  metric: %u\n", addresses[i].ip, addresses[i].metric);

  return (0);
}