I wrote a stored proc to do some null handling, realised it was not
returning the expected results, so I modified it for testing:

DECLARE EXTERNAL FUNCTION TEMP_UDF_SNVL
VARCHAR(500),
VARCHAR(500),
VARCHAR(500)
RETURNS PARAMETER 3
ENTRY_POINT 'sNvl' MODULE_NAME 'fbudf';

set term ^;
CREATE PROCEDURE TEMP_SP_FIX1
(PAR_STRING VARCHAR(500))
RETURNS
(RET_STRING VARCHAR(500))
AS
DECLARE VARIABLE VAR_STRING VARCHAR(500);
begin
var_string = temp_udf_snvl(par_string,'was_null');
ret_string = var_string;
end^
set term ;^

This SQL works:

select TEMP_UDF_SNVL(a_field,'was_null') from some_table;

correctly replacing any null records in a_field with 'was null'.

However this SQL fails:

execute procedure TEMP_SP_FIX1(null);

It does not return 'was null'.

However,

execute procedure TEMP_SP_FIX1('test');

correctly returns 'test'.

So it appears that you cannot use variable values for SNVL, only
valid fields.

If this is the case, a note should be put into the docs pointing it
out: that the NVL function won't handle null variables, only null
field values.

Regards,
Andrew Ferguson
-- Creating a better tomorrow using the leftovers from yesterday.