A program that prints itself

#include <stdio.h>

#include <assert.h>

int main(void)

{

  FILE *fp;

  int c;

  fp = fopen(__FILE__, "r");

  assert(fp != NULL);

  while ((c = fgetc(fp)) != EOF)

    putchar(c);

  fclose(fp);

  return 0;

}

Subtraction as a comparator

This is why you should not use subtraction as a comparator.

http://www.cplusplus.com/reference/clibrary/cstdlib/qsort/

/* qsort example */
#include <stdio.h>
#include <stdlib.h>

int values[] = {-2147483640, 50, 100};

int compare (const void * a, const void * b)
{
  return ( *(int*)a - *(int*)b );
}

int main ()

{

  int n;

  qsort (values, 3, sizeof(int), compare);

  for (n = 0; n < 3 ; n++)

     printf ("%d ",values[n]);

  return 0;

}

The output is.

50 100 -2147483640