Lock manager: couldn't set uid to superuser

This warning message shows up when you are trying to use a direct connection to database file on Linux using Firebird Classic, and you are not the root user. If you are trying to use Classic as embedded database, this is normal behavior and you can safely ignore it. If you hate seeing it, you can either:

a) attach to any database as root to start the lock manager (make sure you use the same server version)
b) attach to it earlier (error message appears only once when lock manager is started)
c) add the user to firebird group (this works with Firebird 2)

If you are not trying to use Firebird as embedded engine (but rather as a regular server), than you should disconnect and connect using the hostname: prefix in the connection string (to learn what is a connection string, read FAQ #260). Something like:

localhost:/path/to/database.fdb

instead of just:

/path/to/database.fdb

Please note that you will need to provide username and password in that case.

isql localhost:/path/to/database.fdb -user sysdba -pass ******