In C on Linux, how would I go about using 2 programs, the latter sending text data to the first displayed using stdout?

Question

I am writing a simple instant messenger program in C on Linux.

Right now I have a program that binds a socket to a port on the local machine, and listens for text data being sent by another program that connected to my local machine IP and port.

Well, I can have this client send text data to my program, and have it displayed using stdout on my local machine; however, I cannot program a way to send data back to the client machine, because my program is busy listening and displaying the text sent by the client machine.

How would I go about either creating a new process (that listens and displays the text sent to it by the client machine, then takes that text and sends it to the other program's stdout, while the other program takes care of stdin being sent to the client machine) or create 2 programs that do the separate jobs (sending, receiving, and displaying), and sends the appropriate data to one another?

Sorry if that is weirdly worded, and I will clarify if need be. I looked into exec, execve, fork, etc. but am confused as to whether this is the appropriate path to look in to, or if there is a simpler way that I am missing.

Any help would be greatly appreciated, Thank you.

EDIT: In retrospect, I figured that this would be much easier accomplished with 2 separate programs. One, the IM server, and the others, the IM clients.

The IM Clients would connect to the IM server program, and send whatever text they wanted to the IM server. Then, the IM server would just record the data sent to it in a buffer/file with the names/ip's of the clients appended to the text sent to it by each client, and send that text (in format of name:text) to each client that is connected.

This would remove the need for complicated inter-process/program communication for stdin and stdout, and instead, use a simple client/server way of communicating, with the client programs displaying text sent to it from server via stdout, and using stdin to send whatever text to the server.

With this said, I am still interested in someone answering my original question: for science. Thank you all for reading, and hopefully someone will benefit from my mental brainstorming, or whatever answers come from the community.

Answer 1

however, i cannot program a way to send data back to the client machine, because my program is busy listening and displaying the text sent by the client machine.

The same socket that was returned from a listening-socket by accept() can be used for both sending and receiving data. So your socket is never "busy" just because you're reading from it ... you can write back on the same socket.

If you need to both read and write concurrently, then share the socket returned from accept() across two different threads. Since two different buffers are being used by the networking stack for sending and receiving on the socket, a dedicated thread for reading and another dedicated thread for writing to the socket will be thread-safe without the use of mutexes.

Answer 2

I would go with fork() - create a child process and now you have two different processes that can do two different things on two different sockets- one can receive and the other can send. I have no personal experience with coding a client/server like this yet, but that would be my first stab at solving your issue...

Answer 3

As @bdonlan mentioned in a comment, you definitely need a multiplexing call like select or preferably poll (or related syscalls like pselect, ppoll ...). These multiplexing calls are the primitive to wait on several channels at once (with pselect and ppoll able to atomically wait for both I/O events and signals). Read also the select tutorial man page. Of course, you can wait for several file descriptors, and you can wait for both reading & writing abilities (even on the same socket, if needed), in the same select or poll syscall.

All event-based loops and frameworks are using these multiplexing calls (like poll or select). You could also use libevent, or even (particularly when coding a graphical user interface application) some GUI toolkit like Gtk or Qt, which are all based around a central event loop.

I don't think that having a multi-process or multi-threaded application is useful in your case. You just need some event loop.

You might also ask to get a SIGIO signal when data arrives on your socket using fcntl with F_SETOWN, but this is not very useful for you. Then you often want to have your socket non-blocking.