QProcess::StartDetached, but the child process closes when the parent exits

mewster

I have a QProcess

QProcess playerProcess;
playerProcess.setProgram("/bin/sh");
playerProcess.setArguments(QStringList{"script.sh", "param1 param2 param3});

that starts detached through

if (playerProcess.startDetached())
{
	sleep(10);
	QCoreApplication::quit();
}

but when the main application quits, the script gets closed too (I put the sleep to verify that the script get executed while the main application is still running).

I checked through ps -ef that the spawned process doesn't have the main application's parent pid (it has 1).

I'm using QT 5.12.3

Any suggestion?

Thanks.

KroMignon

@mewster said in QProcess::StartDetached, but the child process closes when the parent exits:

Any suggestion?

From documentation:

The called process inherits the console window of the calling process. To suppress console output, redirect standard/error output to QProcess::nullDevice().

Maybe you have to redirect standard I/O to null:

playerProcess.setStandardErrorFile(QProcess::nullDevice());
playerProcess.setStandardInputFile(QProcess::nullDevice());
playerProcess.setStandardOutputFile(QProcess::nullDevice());

mewster

I don't think it has anything to do with the console output....

KroMignon

@mewster said in QProcess::StartDetached, but the child process closes when the parent exits:

I don't think it has anything to do with the console output....

But it is the only link between the running process and forked process.
Have you tried it out?

mewster

Yep, it still closes.
I'm thinking it's something about the process groups; is it possible to set a different group to a process through QT?

KroMignon

@mewster said in QProcess::StartDetached, but the child process closes when the parent exits:

I'm thinking it's something about the process groups; is it possible to set a different group to a process through QT?

This don't made sense to me. QCoreApplication::quit() stops application and not any forked process.
Perhaps you have to setup working directory?

playerProcess.setWorkingDiretory(pathOfScript);

mewster

Found a dirty fix: changing the group id on the child sh script itself lets it live after the parent's exit.
Maybe there is some kind of indirect reference outside of QT's process management that keeps the two processes entwined...

JoeCFD

@mewster Try to use pointer without destroying it to see it helps.

auto player_process = new QProcess;
player_process->setProgram("/bin/sh");
player_process->setArguments(QStringList{"script.sh", "param1 param2 param3});

I use pointer in my code and the program still runs after qt app exits.

JonB

@mewster
I had typed in a whole answer, but what you have just said changes everything!

It certainly should not be anything "Qt" or "QProcess" which is at issue; I would expect it to behave the same from any parent program.

I'm intrigued by your "changing the group id on the child sh script itself lets it live after the parent's exit". I don't know about that. Nothing to do with the terminal group then?

JonB

@JoeCFD said in QProcess::StartDetached, but the child process closes when the parent exits:

I use pointer in my code and the program still runs after qt app exits.

To me this change makes no sense, because on program exit the new QProcess is still destroyed.

Did you actually check that the child, whatever you are using, did get terminated with a stack variable QProcess and did not get terminated with a heap variable QProcess, or did you just check the heap case and not the stack case?

mewster

@JonB Unfortunately I'm not that expert on the system's process management, I don't even know what is the terminal group...
I tried a possible solution I found after searching different keywords on Stackoverflow, but at this point I'm positive it has nothing to do with QT, I too would have expected a standard behaviour and it would have been a pretty solid bug (that should have been already found) otherwise

JonB

@mewster
Understood :) I just wanted to know why the solution! :)

You ought just briefly try what @JoeCFD has said to see if it fixes, instead of your "group id on the child sh script". I have said I would find it "surprising" if what he suggests fixes your situation (I would have thought it was to do with whatever your script does), but I could be wrong, and I'd like to hear if it magically fixes for you!? In which case, I would raise a separate question about it in this forum... :)

mewster

Ok, I found the problem.

While the QT application was working correctly, it was launched through a systemd service.
Even if the application would start a detached process, the underlying system would kill it anyway when the parent process would exit (keyword: "KillMode=process").
Changing the group id of the detached process would effectively detach it even from QT's parent, systemd.

JoeCFD

@JonB my process call start() to launch a gstreamer pipeline which has to be killed manually when my app exits.
I see in this case it is a bit different. Destroying the process will not kill the pipeline. Instead CTRL+C is needed to stop the pipeline.

JonB

@JoeCFD
My thought is that QProcess::start() does require the QProcess instance to stay in scope as long as the process is running, but QProcess::startDetached() should presumably arrange that it doesn't matter if it goes out of scope.

JoeCFD

@JonB That is true.