Currently, I'm not thinking about distribute the GUI and daemons on different machine. So I should use named pipe? Or just using process signals and a file?

In my design, those daemons are not able to be re-configured unless they are not at "running" state. However, I think it still needs to be sychronized that configure files are written down before corresponding daemon starts.

With out a mainloop, how can a flow daemon kept itself exist? Just by while (1) {}? However it seems while(1){} is very CPU-consuming. I don't know why...