There are two parts to the problem - how to write the code, and how to compare the packet types. You must ensure you always return a value. Your code should also always be such that:
porownaj(&pkt_a, &pkt_b) == -porownaj(&pkt_b, &pkt_a)
Your outline comparison does not handle cases such as:
pkt_a->alfa > pkt_b->alfa && pkt_a->r_kw <= pkt_b->r_kw
pkt_a->alfa < pkt_b->alfa && pkt_a->r_kw >= pkt_b->r_kw
pkt_a->alfa == pkt_b->alfa && pkt_a->r_kw != pkt_b->r_kw
There is one more problem - is it appropriate to compare floating point values for exact equality? That will depend on your application.
Mechanically, you have to convert the const void pointers to const structure pointers. I use the explicit cast - C++ requires it, and I try to make my code acceptable to a C++ compiler even when it is really C code.
int porownaj(const void *vp1, const void *vp2)
{
const pkt *pkt_a = (const pkt *)vp1;
const pkt *pkt_b = (const pkt *)vp2;
if (pkt_a->alfa > pkt_b->alfa && pkt_a->r_kw > pkt_b->r_kw) return 1;
if (pkt_a->alfa == pkt_b->alfa && pkt_a->r_kw == pkt_b->r_kw) return 0;
if (pkt_a->alfa < pkt_b->alfa && pkt_a->r_kw < pkt_b->r_kw) return -1;
return 0;
}
This does not deal with the bits that I cannot resolve since I am not party to the necessary information. Note that, in general, multi-dimensional objects (such as complex numbers, or (x,y) or (x,y,z) coordinates) cannot simply be compared for greater than or less than or equal to.