> Which is the correct sintax to create external udf into fdb?
>
> if I have a function p_round into a pudf.dll library under UDF

firebird

> installation directory

This is the correct syntax.

DECLARE EXTERNAL FUNCTION pround
DOUBLE PRECISION, INTEGER
RETURNS DOUBLE PRECISION BY VALUE
ENTRY_POINT 'p_round' MODULE_NAME 'pudf';


Sean

P.S. I strongly recommend that you prefix UDF's with "F_", since
un-prefixed functions could become duplicated/conflict if the engine
implements the same function.