I am in a Systems programming class this semester, and our current project is to write a program utilizing sockets and fork. For the project, I decided to make my own instant messaging program. I have the code completed, but I have a problem that keeps old clients from communicating with new clients. The way it is currently set up has 2 programs, a client and a server. The server is always running. Each time a client connects, it forks off a child process to handle communication with that client, then the parent goes back to listening for new clients. On the client side, a connection to the server is made and communication is done with its associated server child process. Each time the server forks, it passes the updated table of file descriptors to its new child. However, child processes that already exist don't have record of the new file descriptor, which keeps them from being able to communicate to other users.

Take this example. Client A signs on. Then Client B. Client B can send messages to Client A, but Client A cannot send messages to client B because Client B signed on after client A, so he doesn't have the file descriptor for B. Client C then signs on. He can communicate with A and B because he has a current list of file descriptors for the users. But B cannot communicate with C and A can't communicate with either. I obviously would like all clients to communicate with one another no matter when they signed on.

Essentially, I would like a way to update file descriptors on all of the children each time a new client signs on. But that may not be the best solution to the problem. All I really need is a way for clients to communicate no matter when they signed on and without creating a seperate socket between each client. Any ideas?

Thanks for your help.

Please make sure that you have read our rules. And note:
(6) Do not post classroom or homework problems.

Once in a while we bend the rules a bit, so I'll make one suggestion before closing the thread. Your server children processes need to have a Unix domain socket to talk with the master server. (Use AF-UNIX instead of AF-INET.) Then read the man pages for the system calls sendmsg() and recvmsg(). Where they talk about sending "access rights" is what you want. Bear in mind that you will be creating a new fd that references the same open file as the old fd. So the sending process may send, say, fd 9 and the receiving process might establish, say, fd 21. This works for and fd, including any socket. And there is no other standard way to transfer an fd between processes.

Sorry, didn't realize that school related stuff was against policy. For the record, my Prof. is cool with us asking for help outside of the classroom. He's even puzzled as to what to do for the situation. But I do understand, while my prof may be alright with it, others may not, and thus I would be setting an example...

I'm not trying to have someone else "do my homework", I just wanted a pointer as to which direction to go for this particular issue. I've already completed the program, just some slight troubleshooting.

I'll look into the solution. Thanks for the help.

That's all about inter-process communication ... well i think!
What if the main server listen on another socket serving the updated descriptor table, you then signal all child that a new refreshed table is available on socket x whenever a new child is joining the party ...