Unique Session ID in PGSQL?

Hi!

Is there any function in PGSQL that returns an unique Session
identifier of the actual session?
For example a Bigint, or GUID, or etc?
Can I get the living Session identifiers from PG?

We have a little application that uses these infos which are basically
came from another database system, and I will port this, but I don't
know, how to do in PGSQL...
In this app. we associated (stored) the session id to a special field
in some tables.
We can retreive the living session ids with a system functions.

If a session died, these records have become irrelevant - we can
delete them. If the session still alive, we don't touch them, only can
read the content...

I see adv_locks in pg, but because the quantity of locks are
pre-determined by the server, I don't choose them.
I better like a technic that cannot exhaust the number of persistent elements.

Thanks:
dd