Process and child process

[SPlatten]

@jsulm , I just added:

qdbg() << mspobjProcess->readAllStandardError();
qdbg() << mspobjProcess->readAllStandardOutput();

And the output in the log file was:

QIODevice::read (QProcess): device not open
QIODevice::read (QProcess): device not open

[jsulm]

@SPlatten said in Process and child process:

I just added:

Where? And when is this code executed?

[JonB]

@SPlatten
I am not 100% sure that @jsulm has noticed you are using startDetached(), and I'm not 100% sure whether for that you must instead use setStandardErrorFile() etc. instead, as per https://doc.qt.io/qt-5/qprocess.html#startDetached ?

EDIT Hold on! The overload you are using, https://doc.qt.io/qt-5/qprocess.html#startDetached-1, is static. So you're not going to have a mspobjProcess instance? For more control over it use the https://doc.qt.io/qt-5/qprocess.html#startDetached overload I referred to earlier.

[jsulm]

@JonB Actually I noticed before, but forgot about it :-)
@SPlatten Any special reason you're using startDetached?

[SPlatten]

@jsulm To be honest, probably because the example I came across for QProcess was using it. What I want is to launch the child process and allow the launching process to carry on without being held up.

[jsulm]

@SPlatten Usually you would only use startDetached if the second process needs to continue if the parent process terminates. If this is not a requirement for your app then simply use start(...).

[JonB]

@SPlatten said in Process and child process:

What I want is to launch the child process and allow the launching process to carry on without being held up.

That happens without startDetached(), QProcess runs asynchronously. You only need startDetached basically if you want to sub-process not to e.g. get terminated when your process exits.

[SPlatten]

@JonB , @jsulm , also startDetached() obtains the PID where as start does not.

Why does startDetached take a qint64 for the PID as the last parameter but the QProcess function pid returns not a qint64 but a Q_PID?

Why are these different types?

[jsulm]

@SPlatten Do you really need the PID? In the code you posted you only check and print it out.

[SPlatten]

@jsulm , I get the PID so I can verify its actually running since neither start or startDetached return anything.

[jsulm]

@SPlatten See https://doc.qt.io/qt-5/qprocess.html#state
And check https://doc.qt.io/qt-5/qprocess.html#start

[J.Hilk]

@SPlatten said in Process and child process:

@jsulm , I get the PID so I can verify its actually running since neither start or startDetached return anything.

well, Process has the started and stateChanged signal for that.
https://doc.qt.io/qt-5/qprocess.html#started

you should actually listen to the stateChanged signal, to see if it changes from Running to NotRunning

[JonB]

@SPlatten
You have other things you can look at instead, like QProcess::stateChanged signal for clues.

While you get it working, you may find start() is easier to work with than startDetached().

EDIT Sigh, looks like 3 of us are all trying to answer :)

If it's not secret, you might like to share with us the full command you are running, including the arguments, in case we can spot anything for you....

[J.Hilk]

3 People 3 times the same thought

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[SPlatten]

@JonB I just tried start and it didn't work, I got errors in the Application Output:

QObject: Cannot create children for a parent that is in a different thread.
(Parent is QProcess(0x101e0b420), parent's thread is QThread(0x101e0b880), current thread is clsThread(0x122c1bcc0)

[jsulm]

@SPlatten Do you use threads?

[JonB]

@SPlatten said in Process and child process:

Why does startDetached take a qint64 for the PID as the last parameter but the QProcess function pid returns not a qint64 but a Q_PID?
Why are these different types?

Don't use Q_PID for this (Windows). Use qint64 QProcess::processId() const.

[JonB]

@SPlatten said in Process and child process:

QObject: Cannot create children for a parent that is in a different thread.

That would explain a lot...! :)

[SPlatten]

@JonB, I think I said from the start these are two different processes, not threads in the same process.

[jsulm]

@SPlatten said in Process and child process:

I think I said from the start these are two different processes

This is clear.
I asked because the warning you posted comes when using threads.