带Linux的TCP客户端/服务器

Question

This may be a very basic question/design but I am struggling with the correct method to handle the system I am going to define here.

I have a system with a single client (PC) that will connect to an embedded Linux board (Raspberry Pi) via TCP/IP protocol. This will be a command/response system where the PC will ask for something and the raspberry PI will respond with the results.

For Example:
CMD => Read/Return ADC Channel X

RSP => ADC Channel X Data

For this type of system I have already defined a packet protocol that will allow for this interaction. My problem is how to handle this on the Raspberry PI. I envision having a single thread handling the TCP connection; placing incomming data into a thread safe queue and pulling outgoing data from a thread safe queue. Then the main thread would poll the queue periodically looking for data. When data is found the command would be processed and a response will be generated. All commands have a response.

The main thread will also be doing other time critical tasks (PID control loop) so it cannot wait for incoming or outgoing data.

My guess is this type of system is fairly common and there is probably a good approach to implementing this type of system. I am very new to Linux programming but I have been programming highly embedded systems (No OS) forever. Just struggling with the correct approach for this type of design.

Note I chose TCP/IP because it handles retying in case of failure. In my case every command has a response so UDP could be used if it makes the design easier/more flexible.

Any help is greatly appreciated.

Answer 1

I tend to avoid threads if I can and only use them if I have to because they make debugging the program harder. They turn a determinsitic problem into a non-deterministic one. So my initial approach would be to see if I can do this without a thread and still achieve concurrency. This is possible using select which will notify your main program when there is something on the socket that needs to be read. Then, when there is something on the socket, it can read the data, process it, and wait for the next event. Problems with this approach is if the computation on the received data takes longer than the acceptable time wanted to process the next element of data, you could end up with a backlog of unprocessed data on the socket. If this is going to happen then you can go ahead and run the receive loop in thread, and the work function in another thread, or fork a new process and deal with a copy of the data from the new process.

Answer 2

the ultra classic linux approach is to have a listener program that forks a new copy of itself for each new client. Linux even has a built in demon that does that for you (initd - although that might have changed with all the systemd stuff). Thats how sshd, telnetd, ftpd all work. No threads