Re: advices on sockets

In article
<c59bd765-7adc-4e11-a022-189e4433dfd0@xxxxxxxxxxxxxxxxxxxxxxxxxxxx>,
mast4as <mast4as@xxxxxxxxx> wrote:

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
window.

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.
etc...)

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-
questions:
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 ?

Yes.

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 ?

Yes. You could write it as a subroutine in your program, and use fork()
to start a new process. In the new process you would call this
subroutine, while the original process would go back to trying to
connect.

You might also want to arrange some additional communication between the
parent and child process, so that the child can tell the parent when it
has started the server. Otherwise, if the parent tries to connect too
soon, it might think it needs to start the server again. You could use
a semaphore for this.


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) {
checkXEvent(&c);
if (c=='q')
quit = 1;
if (accept(sockfd, ...)==-1)
continue;
// 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).

You need to use select() to listen for activity on multiple sockets at
once. Or you can use threads, where one thread waits for new
connections, and other threads process existing connections. If you
only expect a small number of clients this can be an easier way to
program it.


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 ?

This is where threads may make things easier. When a connection comes
in, start a new thread that reads from that client.


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 ?

For a simple application like this, your "stupid protocol" may be best.


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

--
Barry Margolin, barmar@xxxxxxxxxxxx
Arlington, MA
*** PLEASE don't copy me on replies, I'll read them in the group ***
.

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