Hi everyone,

Yes I spend quite some time (2 days) reading tutorials, man pages on
sockets and related topics but still can't seem to find the proper way
of doing what I need. It's not so much that it's impossible, it's just
that I don't have the knowledge and seek over people's experiences on
this topic ;-)

The little app I need to write is a x-window to which a graphic
program sends an image. The graphic program renders a frame and send
it to the x-window in tiles (blocks of pixels). I ended up deciding to
implement that with a server (x-window) -client (graphic app) type of
model. The reason is that eventually several "instances" of the
graphic program can run on the computer at the same time and the
images that each "instance" works on, needs to be send to the same

The other condition is that the x-window stays active even though the
graphic app has finished processing the frame. The user can therefore
continue manipulating the frame in the x-window (zooming, panning.

Now I must say I am trying to prototype things here, some I just want
to find a simple, elegant and robust way to do that. I am not a socket
expert (obviously), have very modest programming skills in comparison
to you all... So I am just really asking for a bit of supervision
here, with something I can't seem to find any answers for in posts
tutorials on the web. So please be kind ;-)

I have many question regarding this problem...

1/ on the client side (the graphic app), I'd like to start to the
server (the x-window) if it's not running yet. This yields 2 sub-

1.1 what's the best way to find out that the server is not running ?
Is that fact that a call to connect() fails is enough ?

1.2 is there a way I can start my server/x-window code by another mean
that calling the function exec() ? I am asking that question because
it means the code for the server has to be compiled as a separate app.
Can I avoid that? Can I write the code for the x-window/server with
the code of the client and start the x-window from within the graphics
app as a *separate process* that won't die when the graphics app has
finished processing the image ?

2/ my other problem (thing I am not clear about) is the way I am
passing the data from the client (graphics app) and the server (x-
window). I have also 2 questions for this.

2.1 ok i read in the docs that by default, accept() is blocking.
Remember that I wrote at the beginning of the post that I want the x-
window app to run as an independent process from the graphics app and
that the user can manipulate the frame in the x-window using shortkeys
for example (z = zoom in, Z = zoom out, or 'q' to quit the x-window
program). So the x-display/server program needs to check for both new
data coming through the socket but also X events. If I use a loop to
check for new connections from clients, because accept() is blocking,
it stops me from checking X-events.

// this for example doen't work because accept is blocking
while(1) {
if (c=='q')
quit = 1;
if (accept(sockfd, ...)==-1)
// we have an incoming connection with a client, proceed..., fork
to create a
// a process child and treat packed of incoming data

so my question here is the following. Is the only way to do this is by
making the socket non-blocking ? But in that case, isn't the fact that
having a non-blocking socket uses a lot of unnecessary CPU resources
(because it never stops checking if there's an incoming connection).

I played with select() (commented part of my code) but it didn't work.
In particular my understanding is that if I use select() is can't use
the fork technique anymore. In other words if 2 instances of the
graphics app processing 2 images are running at the same time, the
file descriptor set will contain 2 file descriptors (one for each
connection it has had from the 2 running clients). Meaning if the 2
clients send data to the sever at the same time, I will have to read
the data from each client and do i need to do with that data sent. Is
that a better way than fork ?

2.2 to pass the data i didn't find anything better that coming up with
a stupid type of protocol. I send a request to the server first that
tells it what it should expect to read next (some info about the size
of the incoming tile, or the RGB data for the tile). In the little
prototype I worked on it seems to work but I am not sure it's
reliable ? Is it how you would do it ? Is there a better way ?

I put the code online with you have the patience and the kindness to
have a look

http://www.scratchapixel.com/docs/proto.cpp // sim graphics app
http://www.scratchapixel.com/docs/server.cpp // sim server/x-window

I really apologize for that long post, but hopefully it will have some
interesting answers and will help me and other people in the future,
as a part from basic client-server code examples that deals with
buff[1024], I didn't find anything else out there that answered
clearly those questions.

Thank you so much -coralie