Call a script in parallel in Qt C++

Question

I am trying to run a script in parallel to my Qt program and am having trouble starting it as a separate process. Check out my attempts below and let me know what you see wrong.

The first attempt was just a system call:

system("python3 startprocess.py");

This works but it also stops the program while running it.

I then followed this guy https://forum.qt.io/topic/92205/writing-commands-to-linux-bash with no success. No errors, just no start of my script.

I am trying this after I saw the documentation and have the below code.

QProcess process;
process.start("python3 startprocess.py");
process.waitForStarted();

I am just wanting to start this script and have it run at the same time as my C++ code. Perhaps I am using the QProcess wrong?

UPDATE: It was a lot easier to use QThreading and the original system call.

Using QThread might be easier, but could lead to bugs. I suggest trying again with the QProcess! — Andrea Ricchi
– Andrea Ricchi, Commented Jan 9, 2020 at 7:14
As my rules of thumb on how to use QProcess without trouble: 1. Avoid all the waitForXxx methods (they cause event loop to be re-entered, which can lead to issues in complex applications, which means all GUI applications). 2. Connect all the signals of QProcess, at least to a lambda with qDebug print, so you will notice when things don't work the way you expect. 3. Pass the parameters as QStringList to avoid surprises with splitting at spaces. — hyde
– hyde, Commented Jan 9, 2020 at 22:54

Andrea Ricchi · Accepted Answer · 2020-01-09 07:24:57Z

1

I think the issue is that the QProcess doesn't have the file path and fails to find and start it! I suggest first to use the full file path! Also check the QProcess::setWorkingDirectory and QProcess::setProcessEnvironment that are useful to handle this case!

Update

In order to prevent the QProcess to be killed while running and without freezing the GUI, you need to define it as a pointer, then connect the QProcess::finished event; in the slot, you can check the exit code and delete the sender using QObject::deleteLater method. Check both the Qt example and the QProcess::finished.

Update 2

Try this code:

auto process = new QProcess(this);
connect(process, QOverload<int,QProcess::ExitStatus>::of(&QProcess::finished), 
[this](int exitCode, QProcess::ExitStatus exitStatus) 
{
    if (exitStatus == QProcess::ExitStatus::CrashExit 
        || exitCode != 0) {
        // Process error!
    } else {
        // Process OK!
    }
});
process->setWorkingDirectory("startprocess.py folder location");
process->start("python3 startprocess.py");
if (!process->waitForStarted(-1)) {
    // Failed to start process
    delete process;
}

edited Jan 9, 2020 at 7:24

answered Jan 8, 2020 at 17:56

Andrea Ricchi

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1 Comment

Charles Curt Over a year ago

Ok tried setworkingdirectory right above the process.start and QProcess "destroyed it while python was still running" which is odd with the waitforstarted in there. I'm trying but don't understand the setProcessEnviroment but I'll give it a shot. Thank you.

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