Concurrency in non-blocking socket?

Question

Context :

Linux debian 64. I am self learning how sockets work and it is very interesting. Single threaded application.

Question :

I am not in the clear with an async/non-blocking question. The socket is set up with fcntl(s, F_SETFL, O_NONBLOCK);

1) What exactly happens if two connections arrive exactly at the same time on one socket ?

When sockets are non blocking, will the code running the socket (call it reception) read the datas and do others stuffs with it in parallel or sequentially ? (single thread app)

I mean, with a non blocking socket, are the incoming requests treated sequentially but very fast (non-blocking operations so only the network becomes the bottleneck not the cpu) or in parallel ?

I ask because I would like to maintain a stack array in the script and cannot afford race condition when pushing datas into it.

Thanks in advance

Answer 1

"Blocking" is very specific term that pertains the scheduler of the operating system: If a user space call blocks, then the operating system deschedules the process (or thread) that is making the call. If an operation is non-blocking, that means that it won't automatically and voluntarily cause your process to go to sleep if the operation cannot succeed, but instead the call returns immediately with a status indicating that there was nothing to be done. (Of course your process can still be descheduled forcefully by preemption, but that's a separate issue.)