QLabel can't display it's text real-time after it is setText successfully

[Limer]

#ifndef WIDGET_H
#define WIDGET_H

#include <QWidget>
#include <QPushButton>
#include <QLabel>
#include <QVBoxLayout>
#include <stdio.h>
#include <stdlib.h>

/* ubuntu 18.04 */
#include <sys/types.h>
#include <sys/socket.h>
#include <errno.h>
#include <unistd.h>
#include <netinet/in.h>
#include <arpa/inet.h>
/* ubuntu 18.04 */

class Widget: public QWidget
{
    Q_OBJECT

public:
    Widget(QWidget* parent = 0) : QWidget(parent)
    {
        m_label = new QLabel;
        m_label->setStyleSheet("border-width: 1px;"
                               "border-style: dashed;"
                               "border-color: black;");
        m_btn = new QPushButton("Start");
        connect(m_btn, SIGNAL(clicked()), this, SLOT(connectToServer()));
        QVBoxLayout* layout = new QVBoxLayout(this);
        layout->addWidget(m_btn);
        layout->addWidget(m_label);

        this->resize(500, 200);
    }

private slots:
    void connectToServer()
    {
        m_label->setText("this is the first information\n");

        memset(&m_client_addr, 0, sizeof(m_client_addr));
        m_client_addr.sin_family      = AF_INET;
        m_client_addr.sin_port        = htons(5000);
        m_client_addr.sin_addr.s_addr = inet_addr("192.168.70.8");

        if ((m_fd = socket(AF_INET, SOCK_STREAM, 0)) < 0)
        {
            m_label->setText(m_label->text() + QString("socket failed\n"));
            return;
        }

        // assume the ip "192.168.70.8" is not existed, so connect function below will block
        if (::connect(m_fd, (sockaddr*)&m_client_addr, sizeof(m_client_addr)) < 0)
        {
            m_label->setText(m_label->text() + QString("connect failed\n"));
            return;
        }

        m_label->setText(m_label->text() + QString("connect to server successfully\n"));
    }

private:
    sockaddr_in  m_client_addr;
    int          m_fd;
    QLabel*      m_label;
    QPushButton* m_btn;
};

#endif // WIDGET_H

When I click the pushbutton, the m_label should display this is the first information immediately, but it don't.

m_label will not display this is the first information until the connect return it's value.

Why?

Mark : I use the socket in ubuntu 18.04.

[JonB]

@Limer
I presume after setText() Qt needs the event loop to spin to re-paint the label. Your code goes straight into the ::connect() without doing so. If you want to do it this way (using all the C socket stuff), try a http://doc.qt.io/qt-5/qcoreapplication.html#processEvents immediately after setting the text and see if that updates?

[Limer]

@JonB You are right, it success. Thanks .

[JonB]

@Limer
Before you go any farther with your code:

You are using the C level socket calls. Given that you are writing a Qt app, you should consider whether you wouldn't be better using Qt's own http://doc.qt.io/qt-5/qtcpsocket.html for all this instead. Among other things, it will work in a more event-driven fashion (signals/slots) than the C calls, which will fit in more naturally with Qt's way of doing things, e.g. you wouldn't have to spin that event loop for refreshes, as you'd write it differently.

[Limer]

@JonB Thanks for your wram reminding. The server is made by my colleague, so I have to do it with the c socket.

[JonB]

@Limer
No, you don't. Regardless of how the server is written (C, C++, socket, whatever), it's a server with TCP sockets. You are writing a client to connect to that server. In your client you use whatever is most suitable for that. In this case, being a Qt application, your client would be much better off using the Qt classes & methods. http://doc.qt.io/qt-5/qabstractsocket.html lists all the methods inherited by QTcpSocket, e.g. http://doc.qt.io/qt-5/qabstractsocket.html#connectToHost-1 would replace your C ::connect().