#include <complex.h>
#include <stdio.h>

void f(long double complex a, long double complex b, long double complex c, long double complex d)
{
 long double complex a_, b_, u, v;

 //a_ = conjl(a);
 //b_ = conjl(b);

 a_ = creall(a) - I*cimagl(a);
 b_ = creall(b) - I*cimagl(b);

 u = (d - c)*(a_*b - a*b_);
 v = (a_*b - a*b_)*(d - c);

 printf("u = (%Lg,\t%Lg)\n", creall(u), cimagl(u));
 printf("v = (%Lg,\t%Lg)\n", creall(v), cimagl(v));
}

int main(void){
 f(410.891l + 291.848l*I,
   476.607l + 294.149l*I,
   327.0l + 360.0l*I,
   439.0l + 192.0l*I);

 return 0;
}

dmc cbug.c -o & cbug
u = (-nan,      -nan)
v = (-6.1265e+06,       -4.08433e+06)

DMC v8.47

Brian