Certain special events are handled at a very low level—as soon
as they are read. The
read-event function processes these
events itself, and never returns them. Instead, it keeps waiting for
the first event that is not special and returns that one.
Special events do not echo, they are never grouped into key
sequences, and they never appear in the value of
(this-command-keys). They do not
discard a numeric argument, they cannot be unread with
unread-command-events, they may not appear in a keyboard macro,
and they are not recorded in a keyboard macro while you are defining
Special events do, however, appear in last-input-event immediately after they are read, and this is the way for the event’s definition to find the actual event.
The events types
user signals like
sigusr1 are normally handled in this way.
The keymap which defines how to handle special events—and which
events are special—is in the variable special-event-map
(see Active Keymaps).