QProcess segmentation fault in QThread

E106JZ

I'm battling a bug where I sometimes get a segmentation fault on QProcess readyReadStandardOutput()/readyReadStandardError() signals when I run it inside a thread and I'm not sure what I'm doing wrong. Below is a minimal example of the crash in a contrived example:

class Process : public QObject
{
    Q_OBJECT
public:
    Process() = default;
    ~Process() = default;
    void start();
};

void Process::start()
{
    QProcess process;

    connect(&process, &QProcess::readyReadStandardOutput, this, [&]()
    {
        QByteArray readStdOut = process.readAllStandardOutput();
    });
    connect(&process, &QProcess::readyReadStandardError, this,[&]()
    {
        QByteArray readStdErr = process.readAllStandardError();
    });

    QStringList args;

    args << ("+vm");
    args << ("-hFs1");
    args << ("file.txt");

    process.setProgram("D:\\test\\program.exe");
    process.setArguments(args);
    process.start();

    if (!process.waitForStarted(-1))
    {
        Q_ASSERT(false);
    }
    if (!process.waitForFinished(-1))
    {
        Q_ASSERT(false);
    }

}

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    QThread* thread = new QThread;
    Process process;
    process.moveToThread(thread);
    thread->start();
    process.start();

    thread->quit();
    thread->wait();
    delete thread;

    MainWindow w;
    w.show();
    return a.exec();
}

Can anyone spot what I'm doing wrong?
Thanks

Christian Ehrlicher

@E106JZ said in QProcess segmentation fault in QThread:

process.start();

This is wrong - it executes the function in the main thread. You have to use signals and slots to call a function in another thread as clearly described in the documentation to QThread. But why do you need a thread here at all?

E106JZ

@Christian-Ehrlicher Thanks, I didn't realise I was doing that. In my case yes I need a background thread to run this lengthy QProcess so I can update the main thread GUI with the progress.

Christian Ehrlicher

@E106JZ said in QProcess segmentation fault in QThread:

yes I need a background thread to run this lengthy QProcess so I can update the main thread GUI with the progress.

Still no reason for a QThread - QProcess is async.

E106JZ

@Christian-Ehrlicher How so? I create a progress window (a QDialog that calls exec()) and inside it has a worker that calls the QProcess. Without a QThread, the QProcess runs and blocks the dialog from appearing until its finished. Can it be done without a QThread?

Christian Ehrlicher

A QProcess does not block anything - it's async.

E106JZ

@Christian-Ehrlicher But if we call waitForFinished() on it then it will block the current thread I mean.

Christian Ehrlicher

@E106JZ said in QProcess segmentation fault in QThread:

But if we call waitForFinished() on it then it will block the current thread I mean.

Then don't call it - it's not needed.

J.Hilk

@E106JZ said in QProcess segmentation fault in QThread:

But if we call waitForFinished() on it then it will block the current thread I mean.

that's the point, waitforfinished is the synchronous version of QProcess, you want to listen to the finished signal, https://doc.qt.io/qt-5/qprocess.html#finished, which is asynchronous

kuzulis

Besides, your QProcess instance destroyed after the start() called...

jsulm

@kuzulis No, he waits for finished:

if (!process.waitForFinished(-1))