How to write a compare function for qsort from stdlib?

Question

I have a structure:

struct pkt_
{
  double x;
  double y;
  double alfa;
  double r_kw;
};

typedef struct pkt_ pkt;

A table of these struc

Answer 1

Something like this should work:

int porownaj(const void *p_a, const void *p_b)
{
  /* Need to store arguments in appropriate type before using */
  const pkt *pkt_a = p_a;
  const pkt *pkt_b = p_b;

  /* Return 1 or -1 if alfa members are not equal */
  if (pkt_a->alfa > pkt_b->alfa) return 1;
  if (pkt_a->alfa < pkt_b->alfa) return -1;

  /* If alfa members are equal return 1 or -1 if r_kw members not equal */
  if (pkt_a->r_kw > pkt_b->r_kw) return 1;
  if (pkt_a->r_kw < pkt_b->r_kw) return -1;

  /* Return 0 if both members are equal in both structures */
  return 0;
}

Stay away from silly tricks like:

return pkt_a->r_kw - pkt_b->r_kw;

which return un-normalized values, are confusing to read, won't work properly for floating point numbers, and sometimes have tricky corner cases that don't work properly even for integer values.