GDM also provides a FIFO called .gdmfifo in the
ServAuthDir directory
(usually <var>/gdm/.gdmfifo).  You must be
root to use this protocol, and it is mostly used for internal GDM
chatter.  It is a very simple protocol where you just echo a command on
a single line to this file.  It can be used to tell GDM things such as
restart, suspend the computer, or restart all X servers next time it has
a chance (which would be useful from an X configuration application).
Full and up to date documentation of the commands and their use is
contained in the GDM source tree in the file
daemon/gdm.h.  Look for the defines starting with
GDM_SOP_.  The commands which require the
pid of the slave as an argument are the ones that are really used for
internal communication of the slave with the master and should not be
used.
