unix signal, briefly

A Unix signal is picked up only at start of a time slice by the recipient "cpu-driver". See P126[[art of debugging]]. Postman drops a letter on your doorstep when you step out. This is the best way to deliver the signal. It's uncommon (impossible?) to interrupt a process /midstream/ a sequence of instructions. I guess the reason is efficiency -- no efficient implementation of that interrupt mechanism. In Unix, what is common is the signal mechanism.

Unix signals can be generated from interactive user, from any other process, from kernel or from hardware, but they all have a target PID.

Unix Signal is at a lower level than threading. Thread preemption often depends on Unix signals

Unix Signal is a kernel-level mechanism.

Unix Signals target a process, not a thread.

A Unix Signal is an event. A Signal handler is an event callback function whose address is associated with one specific signal.