Program runs normally but gets Segmentation Fault in debugger

Mike_W

I have a simple console app that looks for ports and checks their status for writing.

#include <QCoreApplication>
#include <QSerialPort>
#include <QSerialPortInfo>
#include <QIODevice>
#include <iostream>

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

    QSerialPort port;
    QList<QSerialPortInfo> qports = QSerialPortInfo::availablePorts();

    std::cout << qports.length() << " serial ports found\n" << std::endl;

    for(const QSerialPortInfo &portinfo : qports) {
        std::cout << "Checking port " << portinfo.portName().toStdString() << std::endl;

        port.setPortName(portinfo.portName());


        port.setBaudRate(QSerialPort::Baud9600);
        port.setDataBits(QSerialPort::Data8);
        port.setParity(QSerialPort::NoParity);
        port.setStopBits(QSerialPort::OneStop);
        port.setFlowControl(QSerialPort::HardwareControl);
        bool isOpen = port.open(QIODevice::WriteOnly);
        std::cout << "Port opened: " << ((isOpen) ? "TRUE" : "FALSE") << std::endl;
        std::cout << "Port writable: " << ((port.isWritable()) ? "TRUE" : "FALSE") << std::endl;

        std::cout << "Port is " << ((isOpen && port.isWritable()) ? "" : "not ") << "ready\n" << std::endl;

        port.close();
    }
    return a.exec();
}

The program runs successfully on Windows 10 and my output screen looks like this:

3 serial ports found

Checking port COM2
Port opened: FALSE
Port writable: FALSE
Port is not ready

Checking port COM3
Port opened: TRUE
Port writable: TRUE
Port is ready

Checking port COM4
Port opened: FALSE
Port writable: FALSE
Port is not ready

I set a breakpoint and start debugger I enter the debugger and expand the port variable to see its members. Then expanding some of those variables (d and properties) causes a Segmentation Fault dialog window to pop up. After I cancel the dialog I see that the issue occured in qserialport_p.h at the start ofthe QSerialPortPrivate class definition.

class QSerialPortPrivate : public QIODevicePrivate
{
    Q_DECLARE_PUBLIC(QSerialPort)

I realize that these members have bad addresses. But is this normal or a problem? After I dispose of the dialog the app can continue fine. I want to know if this is a budding problem or if it is just a quirk of the Debug system. I declare my QSerialPort as a variable rather than construct it since I don't have a parent QObject for it. I want to know if I can ignore this and simply not debug into these QSerialPort members or if this is a future issue.

Christian Ehrlicher

but gets Segmentation Fault

So where does it crash?

I realize that these members have bad addresses.

Qt is compiled in release mode - the debug information can not be accurate then.

Mike_W

@Christian-Ehrlicher The program is compiled in Debug mode and runs fine if I don't try to debug it. However when I set a breakpoint (before executing port.close) and go into the debugger. I cannot expand some of the memebers of the port variable without getting a segmentation fault dialog.

Christian Ehrlicher

@Mike_W said in Program runs normally but gets Segmentation Fault in debugger:

I cannot expand some of the memebers of the port variable without getting a segmentation fault dialog.

So not your program crashes but the debugger? What compiler and debugger do you use?

Mike_W

@Christian-Ehrlicher I am using MinGW 7.3.0 32-bit for C and C++. They are installed at C:\Qt5.12.6\Tools\mingw730_32\bin on my PC. They were installed with Qt Creator and Qt. The debugger is GNU gdb 8.1 for MinGW 7.3.0 32-bit in the same directory.

Christian Ehrlicher

What version of QtCreator? Does creator or the debugger crash?