Paul,

it also worked with FB1.0, but it doesn't with FB1.5

When you don't use parameters it still works with FB 1.5 but if you
use parameters, it doesn't work any more.

If you use FB 1.5 then

Try

select test_udf (1,2,2.9) from rdb$database

in IBManager and it will work as expected and bring the result 2.95

Try

select test_udf (:a,:b,:c) from rdb$database

and fill in 1 for a, 2 for b and 2.9 for c

in IBManager and it will NOT work as expected. It will bring 2.5 as
the result, because the parameter c is interpreted as an integer
value.

cu
Helmut