reliable lock inside stored procedure

Hi,

I have a stored procedure and I need a SHARE ROW EXCLUSIVE lock in
this procedure (otherwise data will get corrupted).

According to the documentation the LOCK statement is useless and will
silently fail if not executed inside a transaction. (btw: this sounds
dangerous to me)

Also it is not possible to start a transaction inside a stored
procedure.

How can I make the code of this stored procedure safe?

is there a way to let the procedure fail with an error if not executed
within a transaction.

does adding a SAVEPOINT the trick?
documentation says that savepoints can only be established inside
transactions, but does it fail fatal enough so that the procedure
getss aborted? (more fatal than LOCK does?)

thank you very much.
/sebastian