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Subject: Re: [boost] [boost.process] 0.6 Alpha
From: Klemens Morgenstern (klemens.morgenstern_at_[hidden])
Date: 2016-06-07 08:23:51

Am 07.06.2016 um 13:21 schrieb Brendon Costa:
> I am vaguely following this thread as I don't have lots of time but am
> interested in the process library.
> I have also written a C++ process (and signals) library supporting
> windows/OSX/Linux/Android (for an employer though so I can't propose it for
> boost). It actually suprised me how many platform differences existed that
> needed to be worked around in writing this library compared to others I
> have written.
> I also did something similar to Klemens if what I undertood is correct, I
> did a fallback scenario of send TERM if not closed within time period send
> a KILL. I didn't really like it though and wonder if the std::thread
> destructor behavior is better to std::terminate if the child is still
> running but not detached and treat it more like a bug to destroy an active
> process instance than something to rely on to close it.
Well, I don't give a timeout, it just terminates. That is, you have to
explicitly call wait or join, or wait_for/wait_until if you want a timeout.

> Not having a SIGTERM equivilant for windows processes bothered me as
> SIGKILL is just too nasty as a default as mentioned. So I looked into this
> quite thoroughly. For GUI processes you can easily send WM_CLOSE message to
> it to simulate something akin to a SIGTERM. I think I used a WM_CLOSE from
> memory. I know there was a choice between QUIT and CLOSE, I think CLOSE was
> more suitable but I cant recall the exact reason now and may be wrong.
> For console processes I only found bad options. I found something that
> worked quite well but ended up dropping it in the end as it was not
> required and was super hacky IMO. I only ever wanted to SIGTERM my own
> processes in the end so just used a non generic solution of a named pipe to
> listen for signals and raise() to local process.
> The GenerateConsoleCtrlEvent() comes close to doing what we want but its
> got the worst behaviour in how to choose who gets the signal delivered and
> is not sufficient (even when creating processes to detach/attach to other
> consoles to generate the events). In the end I followed the basic idea
> proposed at:
> With extra ideas proposed in comment (20110401) and a few extra changes of
> my own. I got this working well in all scenarios I could think of to test
> on win32 and 64 up to Win7 (I think I went down to XP but cant recall for
> sure).
> The idea is basically to use the mechanism that GenerateConsoleCtrlEvent()
> uses to deliver the signal, but you choose the process where to deliver it
> yourself instead of using the stupid rules that GenerateConsoleCtrlEvent()
> follows.
Well, I could put each process in it's own group and just try to signal
arbitrary values. But that would break the console and might not even work.

> The idea is to create a new "helper" process, have it register a signal
> handler, raise a signal to itself and in the handler walk back the stack to
> find the entry point where the signal was generated. It turns out that the
> function where the signal was generated (I think it was
> kernel32!CtrlRoutine) this function has the correct prototype for
> CeateRemoteThread and is in the kernel32.dll so is in the same location in
> all processes on the system and is how GenerateConsoleCtrlEvent() is
> implemented.
> So you basically then find the process you care about, create a remote
> thread in it to call the specified function and presto you end up with a
> SIGTERM that can be handled by a normal signal() handler in a console
> process.
> I hope my research was useful, but this approach was not really acceptable
> to me and I dont think would be to the boost community. Maybe another idea
> might arise from it though, in which case I would love to hear about it.
It's definitely intertesting, thank you. Though you're unfortunately
right, that's a bit too much hacking for a stable library.

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