Explicitly rolling back transaction from within a C-Language function

Hello,

I have a transactional system built on top of
BerkeleyDB which I would like to call from within
Postgres through a C-Language function. The behavior
of the function will be read-only. Even so, the
BerkeleyDB-based system's transaction will sometimes
fail. What I would like to do is propogate that
transactional failure into the Postgres transaction
that called the C-Language function. Is this
possible?

To clarify what I think I want: I would like the
C-Language function to return an error condition to
Postgres that will cause the current Postgres
transaction to abort. I do not know how to do this.

I am not particularly worried about needing to
rollback the BerkeleyDB transaction when the Postgres
transaction fails, because the BerkeleyDB transaction
was read-only anyway, and there are no updates to roll
back. However, I might want to do this in the future
so information on this subject is also welcome.

Thank you for any help,

mark

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