udf in fluent
 

hello everyone
I'm studing about function udf in the softwave fluent. Do you know with this chunk code ?. Can you interpret for me all line.
thank you very much !
this chunk code
real hy_get_face_force(int tid1)
{
Thread *t1;
face_t f1;
real force1;
real NV_VEC (A1);

t1 = Lookup_Thread(root_domain, tid1);
force = 0.0;
begin_f_loop (f1, t1)
{
F_AREA (A1, f1, t1);
force1 += F_P (f1, t1) * NV_MAG (A1);
}
end_f_loop (f1, t1)

return force1;
}

try here