Call a function when a User (dis)connects from the Server/Database

Hello,
I search a way to realize following:
When a user connects to a databse, a function of my should be executed. The
best would be a system table where the active Databaser Users are in, and
creating a Trigger on it. But there is not such a table.

Sometold me to modifying the source, but i feel a bit lost in such a big
source. Please help me :)

The best would be a modification which stores active Users in table. I think
this would be a nice feature for following PostgreSQL Versions, or?

Sorry, for my bad english :/

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Johannes 'robotron' Barop