pg_on_connection_loss creates, changes, or cancels a request to execute a callback command if an unexpected loss of connection to the database occurs. With a callbackCommand parameter, the request is established, or the command string of an already existing request is replaced. With no callbackCommand parameter, a prior request is canceled.
The callback command string is executed from the Tcl idle loop. That is the normal idle state of an application written with Tk. In non-Tk Tcl shells, you can execute update or vwait to cause the idle loop to be entered.