[Interest] [Development] Stopping a QProcess gracefully

Thiago Macieira thiago.macieira at intel.com
Thu Aug 7 16:28:37 CEST 2014

On Wednesday 06 August 2014 23:19:49 achartier at fastmail.fm wrote:
> Hello,
> I am trying to find the proper way to gracefully exit a QProcess. The
> particular process that I am running does some work and then is in a
> waiting state, exiting only when the user presses the Enter key. This
> works fine when the executable is run from the command line.

This is not a question about Qt's own development. Please use the interest 
mailing list next time. (and drop dev@ when replying to this email)

> The problem I am having is that every way I have tried to stop the
> QProcess results in the error signal being emitted; in such cases,
> errorString() always returns "Process crashed". I have tried:
> (1) Calling QProcess::terminate()
> (2) Calling QProcess::kill()

Those send a signal to the process, which means it will understand the process 
as crashed. That's normal.

If the target process gracefully handles SIGTERM, then you can use it.

> (3) Writing the return key to the process with QProcess::write("\r") and
> QProcess::write("\\r)

As Ossi said, you need to verify that the byte was actually sent. Use 
waitForBytesWritten or waitForFinished() to ensure it has finished.

Also, please make sure that the process is actually waiting for a Carriage 
Return. I doubt that it is. The Enter key in a terminal does send a CR, but 
the terminal translates it to a LF for the application. So you should try 
sending "\n" instead.

> (4) Sending a Qt::Key_Return using qApp->sendEvent() to the QProcess
> object (though it's not clear to me that QKeyEvents sent to the QProcess
> get automatically forwarded to the executable they have spawned).

That doesn't do anything. QProcess is not a graphical widget, so it doesn't 
handle key events.

Option (5): closeWriteChannel(). Most shell processes terminate execution when 
they get an EOF on stdin.

> In summary, how can I stop this QProcess properly without having the
> error signal emitted?

Thiago Macieira - thiago.macieira (AT) intel.com
  Software Architect - Intel Open Source Technology Center

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