event_id
The first argument passed to an event handler is always the event ID.
It is useful if there are several GPMI events with the same arguments and
the script wants to bind them all to the same event handler. In that case
using event_id the event handler can determine which event triggered the
call. This design is similar to the signal handler documented in
signal(2).
The case described above (dispatcher in the event handler) is rare.
Most users will bind one event to one function and can safely ignore this
argument. Nevertheless it must present as the first argument
on the event handler's argument list.