How to convert a double value to a numeric datum type in pgsql?

Hi all,
I'm talking about the source code of pgsql and I want to know how the
typing system works in pgsql.
A few functions can help us do the type conversion, for example:

Int32GetDatum -- convert a integer to a datum
Float8GetDatum -- convert double to a datum
cstring_to_text -- convert a string to a text
...

but there are a lot of types in pgsql,how to choose the corresponding
function? or is there a more automatic way to do the type conversion(raw
chars to a datum, suppose I have the corresponding Form_pg_type instance)?

I ask this question because I don't know how to convert a double value to a
numeric datum(the field's type is numeric(10,2)), pg_type shows that
numeric's typlen is -1 whose length is variable thus Float8GetDatum is not
working..