Process and child process

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.

JonB

@SPlatten
You have been changing: with the code you now display in this latest post, can you show how you start the sub-process, please paste the line.

SPlatten

@JonB , the actual 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;
}

The call itself:

std::string strPort = std::to_string(clsMainWnd::intGetServerPort());
const char* cpszPort = strPort.data();
QString strFullPath(strModulesPath + strModule.toString());
QStringList slstArgs;
qint64 int64PID;
slstArgs << cpszPort;                                           //RX port
slstArgs << QString::number(clsSocketThread::uint16NextPort()); //TX port

if ( clsMainWnd::blnLaunch(strFullPath, slstArgs, int64PID) == true ) {

This is all working and the process is started I can see that because when the application starts it creates a log file:

-rw-r--r--  1 simonplatten  staff   201 21 Oct 17:33 mdFileIO20201021.001

And this is its content:

D00000000000000000001:mdFileIO Version: 1.00
D00000000000000000002:Setting up socket on ip: 127.0.0.1, port: 8124
D00000000000000000003:Setting up socket on ip: 127.0.0.1, port: 8123, for heartbeat

Then it dies...

JonB

@SPlatten
At least you are using an instance! bool QProcess::startDetached(qint64 *pid = nullptr)

Returns true on success; otherwise returns false.

Please at least check the return result, we're trying to debug here....

I don't know why whatever you are running exits. If you really say you cannot start it instead via QProcess::start() I would want to know why. Although it may not matter with startDetached(), what is the scope/duration of your clsMainWnd::mspobjProcess object? I presume you're sure it does persist?

I would verify that my Qt code successfully launches some other program. Then I don't know what it is different about whatever you are trying to run. I would put the very small amount of code you have for launching this process into a very simple standalone program and see how that behaves.

SPlatten

@JonB , like I said, its obviously working because the application is started, the log file created and contains content, so how could startDetached return false if it launched the process.

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());
        qdbg() << "Process is NOT running, lanching";

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

Why don't the signals started and/or stateChanged get raised?

SPlatten

@JonB , @jsulm , Found it!

Finally! Turned out to be a silly error, obvious when I found it, should have realised. One of the sockets is being used by the parent and the child terminates if it cannot listen to the socket.

Thank you for your time and input, it helped me along the way.

This now raises question, is there a better way to communicate between processes instead of using sockets?

I plan to add lots of other processes the relationship will be parent launching application will always be the consumer and all the processes it launches will provide data services.

At some point in the future processes may also be on remote systems but there is no plan for that just yet.

SPlatten

@JonB , @jsulm, just found:
https://doc.qt.io/qt-5/ipc.html