PITR Signalling the Archiver

A few questions may help to speed up my work

I need to send a signal from a backend to the archiver process.

1. What signal should I use? 

2. How do I give the processid of the archiver to the backend? The
archiver may restart at any time, so its pid could change after a
backend is forked.

I have answers, but I strive for the best answer.

Thanks very much, Best regards, Simon Riggs