写入串行端口时Qt崩溃

Question

Hi I have a program in QT that seems to be crashing whenever I write to a Serial Port. I am using Mac OSx 15 inch retina. Here is the relevant code:

MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent)
{
    //set central widget for the Main Window
    centralWidget = new QWidget(this);
    this->setCentralWidget(centralWidget);

    //creation and attribution of slider
    slider = new QSlider();
    slider->resize(255, 20);
    slider->setOrientation(Qt::Horizontal);
    slider->setRange(0, 255); //0-255 is range we can read

    //layout with slider and lcd
    main_layout = new QVBoxLayout();
    main_layout->addWidget(slider);

    //set layout to the widget inside MainWindow
    centralWidget->setLayout(main_layout);

    /*Connection Events*/
    //connection between the slider event and the transmission function
    QObject::connect(slider, SIGNAL(valueChanged(int)), this, SLOT(transmitCmd(int)));
}

void MainWindow::init_port()
{
    port = new QSerialPort("COM3");     //create port

    port->open(QIODevice::ReadWrite | QIODevice::Unbuffered); //open port
    if(!port->isOpen())
    {
        QMessageBox::warning(this, "port error", "Can't open port!");
    }

    //set port properties
    port->setBaudRate(QSerialPort::Baud9600); //9600 FOR ARDUINO
    port->setFlowControl(QSerialPort::NoFlowControl);
    port->setParity(QSerialPort::NoParity);
    port->setDataBits(QSerialPort::Data8);
    port->setStopBits(QSerialPort::OneStop);
}

void MainWindow::transmitCmd(int value)
{
    //if value wasn't between 0-255, no transmission
    if(value < 0 || value > 255)
        return;
    char *buffer = (char*) new int(value);

    //send buffer to serial port
    port->write(buffer);
}

It crashes on the line port->write(buffer). I am using QT 5.5 and using QTSerialPort.

Answer 1

Try

port->write (buffer, sizeof (int));

You used the QIODevice::write(const char* data) overload, which expects a null-terminated string (which your buffer isn't). So naturally, the io-device doesn't know when to stop...

This should fix your crash in any case. It is, btw., equally possible to just say:

port->write (reinterpret_cast<const char*> (&value), sizeof (int))

However, be aware that both commands above will send 4 bytes (sizeof int) of data over your port (in the byte order of your system, probably little-endian). Perhaps (judging from your 0-255 check at the beginning of your function), this is not actually what you want. If you just want to send a single byte:

unsigned char valueCh = static_cast<unsigned char> (value);
port->write (reinterpret_cast<const char*> (&valueCh), 1)

Addendum:

As you wrote, you simply forgot the init-call. Good catch @perencia! But it is still worth understanding why your transmitCmd() does indeed work -- because at first glance, it shouldn't.

I still maintain that you are using the wrong write() call -- but as it happens, it works nontheless. What happens is:

Let's say we have value == 17 . Then, on a little-endian architecture, your buffer looks like this:

// "17" little-endian
11 00 00 00
^
|
buffer

and your call to write(buffer) will see the correct data byte you want to send, followed by a nul-byte which causes it to stop.

Answer 2

You are not calling init_port . At least in the code you submitted.