[GENERAL] psql - looking in wrong place for socket

Hi List,

I am running postgresql 8.4.20 on CentOS 6. Things have been running fine for a long time
then I rebooted. Postgres came up but when I tried to connect with psql on the local machine
I got

psql: could not connect to server: No such file or directory
        Is the server running locally and accepting
        connections on Unix domain socket "/var/run/postgresql/.s.PGSQL.5432"?

The socket is actually being created (and always has been) in /tmp

I worked around the problem temporarily by
ln -s /tmp/.s.PGSQL.5432 /var/run/postgresql/.s.PGSQL.5432

What controls where psql looks for the socket?

Thanks,
Steve