Killing a process synchronously

if you use spawn_monitor/3 you will remove the race condition on starting the process

Also if you call Process.monitor on a pid that is not alive, the down message will contain :noproc as the reason, so you will know your process was not alive.

Well, then it seems to be a good way to implement what I want

From the documentation of Process.monitor/1:

If the process is already dead when calling Process.monitor/1 , a :DOWN message is delivered immediately.

So you will receive the :DOWN message in that case as well.

Here is what I try to achieve : I want to run a process and be able to kill it if needed. When the process is either killed or terminates normally, there is a piece of code that I always want to run.

The issue I see with @lud’s solution is the following : To monitor a process, you need a pid. So when you call the monitor function the process is normally already running. It may even be over by the time you call the monitor function, in which case you will never receive the DOWN message you expect (this DOWN message would trigger the piece of code you always want to execute when a process finishes).

Am I making sense ?

Because the Elixir documentation (as well as the Erlang documentation of :erlang.exit/2, which this function delegates to) only mention “sends an exit signal”, I am fairly certain that the function is asynchronous.

The approach mentioned by @lud works. However, this will not (always) prevent other code from running on pid before it exits. It will only prevent code running on the process calling Process.exit to wait with continuing before pid has exited.

However, another question would be: what are you trying to do? It seems like a rather odd request. Maybe your actual problem can be resolved in a wholly different manner.

I am not sure that the function is synchronous, but to be sure, you could first set a monitor on the process with Process.monitor, then kill it, then wait for the :DOWN message from the monitor.