Re: advices on sockets

1/ open the display
2/ create a thread to catch x-event and manipulate the framebuffer
accordingly (zoom in...)
3/ create a thread that "accept" new connections
4/ each connection starts a thread that listen to what's coming in on
that connection

It would be ok but I was hoping for something simpler... I don't want
that x-window/server app to use too much memory and CPU cycles to
leave as much processing power for the graphics app...

Why not use a UDP based message passing. In that way you don't have to
handle several threads for each connection. UDP is connectionless.
- the x app opens a socket and listen on it for messages
- the graphic programs send messages to that socket
- the x app processes incoming messages one by one
That MAY do the job in a simple way.

If all your progs run on the same machine you can also consider using
a message queue: