Process and child process

[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

Up Top🙌

[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.

[SPlatten]

@JonB I tried start and it didn't work at all, so I'm back to startDetached which does launch the application and I can see it creates a log file then disappears.

[SPlatten]

@J-Hilk , @JonB , @jsulm, I've modified the function:

bool clsMainWnd::blnLaunch(QString strApp, QStringList& slstArgs, qint64& int64PID) {
    Q_ASSERT_X(clsMainWnd::mspobjProcess!=nullptr, "blnLaunch", "mspobjProcess is null!");
    QString strFullPath(clsDebugService::strGetUserFolder(strApp));
    int64PID = 0;    
    qdbg() << "Checking for PID for: " << strFullPath;
    if ( blnGetPID(strFullPath, int64PID) == true ) {
    //Process already running, no action required
    } else {
        int intLastSep = strFullPath.lastIndexOf(QDir::separator());

        if ( intLastSep > 0 ) {
            QString strName = strFullPath.mid(intLastSep + 1)
                   ,strPath = strFullPath.mid(0, intLastSep + 1);
            qdbg() << "Launching: " << strFullPath;
            clsMainWnd::mspobjProcess->setArguments(slstArgs);
            clsMainWnd::mspobjProcess->setWorkingDirectory(strPath);
            clsMainWnd::mspobjProcess->setProgram(strName);
            clsMainWnd::mspobjProcess->startDetached(&int64PID);
        }
    }    
    if ( int64PID > 0 ) {
        qdbg() << "Process: " << strFullPath << ", is running PID: " << QString::number(int64PID);
        return true;
    }
    return false;
}

I can see that a valid PID is returned and that the log files are created, but when I check for the process:

ps -A

The PID isn't present.

[JonB]

@SPlatten
As we said before, if you want help on this you really need to put in more debugging information calls. Like slots for errorOccurred, finished, started, stateChanged, and reading of anything appearing on stdout/stderr, e.g. via readyReadStandardError/Output. And there is the exitCode(). Sorry, but that's how it is. I also offered to look for clues if you wanted to tell us the program you are executing and what arguments you pass , but you didn't reply yea or nay.

And btw you have something fishy going on if you can run this command via startDetached() but not via start().

And one other thing: the fact that you can't find the pid in itself proves nothing. It might be running fine. Processes can spawn sub-processes and exit.

[SPlatten]

@JonB, I'll give it a go now.

[SPlatten]

@JonB , I've connected up slots to the following signals:

connect(clsMainWnd::mspobjProcess, &QProcess::errorOccurred, this, &clsMainWnd::qprocErrorOccurred);
connect(clsMainWnd::mspobjProcess, &QProcess::started, this, &clsMainWnd::qprocStarted);
connect(clsMainWnd::mspobjProcess, &QProcess::stateChanged, this, &clsMainWnd::qprocStateChanged);

The implementation:

void clsMainWnd::qprocErrorOccurred(QProcess::ProcessError error) {
   qdbg() << "QProcess errorOccurred:" << error;
}
void clsMainWnd::qprocStarted() {
    qdbg() << "QProcess started";
}
void clsMainWnd::qprocStateChanged(QProcess::ProcessState newState) {
    qdbg() << "QProcess stateChanged, newState:" << newState;
}

I also put breakpoints in each slot, none of these got triggered. I'm not surprised though because I'm not suggesting the problem is with QProcess. I'm just trying to find out why the process when started this way is terminating where as if I launch it in Qt Creator it continues to run.

I would have though the "started" and "stateChanged" signals would get raised, but I don't get anything.