使用多个线程时，Qt Creator崩溃

Question

I'm writing a Qt (5.3) program which has a joystick test UI in it, but I need a separate thread for an infinite while loop looking for joystick axis/button value changes through SDL. That part of the code is working fine as I can have the thread qDebug() messages and it seems to work. But from the main window, when I try to open the test joystick UI, the program crashes. I've had the test joystick UI running separation WITHOUT the JoystickThread thread and it seems to open up fine.

The error messages are inconsistent though - some times, I just get

The program has unexpectedly finished. /home/narendran/QtWorkspace/build-LinkControl-Desktop-Debug/LinkControl crashed

This has shown up once:

QXcbWindow: Unhandled client message: "_GTK_LOAD_ICONTHEMES"

And a few other times:

[xcb] Unknown sequence number while processing queue [xcb] Most likely this is a multi-threaded client and XInitThreads has not been called [xcb] Aborting, sorry about that. star: ../../src/xcb_io.c:274: poll_for_event: Assertion `!xcb_xlib_threads_sequence_lost' failed.

I found that this was common if XInitThreads(); is not run in the main function, but even with it on there, it crashes with the same error(s).

main.cpp

#include <qsplashscreen.h>
#include "linkcontrol.h"
#include "configure.h"
#include <unistd.h>
#include <QApplication>
#include <QPixmap>
#include <QStyle>
#include <QDesktopWidget>
#include "linkports.h"
#include "joystickthread.h"
#include <X11/Xlib.h>

int main(int argc, char *argv[])
{
    XInitThreads();
    QApplication a(argc, argv);

    QPixmap pix(":splash.png");

    QSplashScreen splash(pix);
    splash.show();

    a.processEvents();

    JoystickThread jsThread;
    jsThread.start();

    LinkControl linkcontrol;
    usleep(1000000);
    splash.finish(&linkcontrol);
    usleep(100000);
    linkcontrol.show();

    linkcontrol.setGeometry(QStyle::alignedRect(Qt::LeftToRight, Qt::AlignCenter,linkcontrol.size(),a.desktop()->availableGeometry()));

    return a.exec();
}

The actual thread is in joystickthread.cpp

#include "joystickthread.h"
#include "global.h"
#include "unistd.h"

/* JoystickThread::JoystickThread(int _interval)
{
    this->interval_us = _interval;
} */

void JoystickThread::run()
{
    while(1)
    {
        if(joystick->connected)
        {
            joystick->updateJSData();
            // Check for changed values
            for(int i=0; i<joystick->axis.count(); i++)
            {
                if(joystick->axis.value(i) != joystick->axis_last[i])
                {
                    joystick->axisUpdateEmit(i);
                    // qDebug() << "AXIS: " << i << "\tVALUE: " << joystick->axis.value(i);
                }
                joystick->axis_last[i] = joystick->axis.value(i);
            }
            for(int i=0; i<joystick->button.count(); i++)
            {
                if(joystick->button.value(i) != joystick->button_last[i])
                {
                    joystick->btnUpdateEmit(i);
                    // qDebug() << "BUTTON: " << i << "\tVALUE: " << joystick->button.value(i);
                }
                joystick->button_last[i] = joystick->button.value(i);
            }
        }
        usleep(2500);
    }
}

The function that causes the program to crash is in linkcontrol.cpp

void LinkControl::on_actionJoystick_Test_triggered()
{
    qDebug() << "STARTING CHECK";
    if(!js_test->initialized) {
        qDebug() << "NOT INIT";
        js_test = new TestJoystick();
        js_test->initialized = true;
         qDebug() << "FININSH INIT";
    }
    if(joystick->connected) {
         qDebug() << "SHOWING UI";
        js_test->show();
    } else {
        QMessageBox::critical(this, tr("No Joystick Connected!"), tr("Please connect a joystick first..."));
    }
}

Where js_test is declared as a TestJoystick object in the linkcontrol.h file

public:
    explicit LinkControl(QWidget *parent = 0);
    QSlider *portSliders[16];
    QLineEdit *setVals[16];
    SerialTerminal *ser_term;
    TestJoystick *js_test;
    ~LinkControl();

Thank you very much! Please let me know if you need anymore information.

Answer 1

QThread s are a little tricky to get used to initially, and have their share of gotchas.

You should construct and connect appropriate items at the top of your run function.

If you do it other places, you need to make sure that you don't use Qt::AutoConnection , but instead use Qt:QueuedConnection .

http://qt-project.org/doc/qt-5/qt.html#ConnectionType-enum

Certain elements are only accessible from the "GUI" thread or the main thread of the program. This is the thread that has QApplication::exec(); ran on. It has an event loop that propagates messages around.

Look at the Application output for runtime errors that Qt will tell you about.

When crossing thread boundaries, be sure to use signals and slots.

And if you are accessing a member of your thread class from outside that thread, be sure to use thread synchronization, practices, such as prefacing all access to these members with QMutexLocker locker(m_mutex); .

http://qt-project.org/doc/qt-5/threads.html

And as implied by the title "GUI thread", it is the only thread that is allowed to do certain things such as drawing QPixmap s and accessing certain parts of QWidget s.

Hope that helps.