Is kill function synchronous?

Question

Is the kill function in Linux synchronous? Say, I programatically call the kill function to terminate a process, will it return only when the intended process is terminated, or it just sends the signal and return. If that is the case, how can I make it wait for the intended process to be killed?

Answer 1

No, since it doesn't kill anything, it only sends a signal to the process.

By default this signal can even be blocked or ignored.

You can't block kill -9 which represents sending SIGKILL

To wait for the process to die:

while kill -0 PID_OF_THE_PROCESS 2>/dev/null; do sleep 1; done 

Answer 2

kill cannot be synchronous as it only sends a signal. The target process might ignore incoming signals (cf. SIG_IGN) so there's no guarantee regarding kill's effect.

It shouldn't be difficult to make an experiment verifying this hypothesis. Start process A and make it handle SIGTERM with a 10 second sleep before dying. Then start process B, which delivers a SIGTERM to A and exits immediately.