Qt - QTcpserver is not working properly

I am doing a simple server-client application. But, the client gets some undefined behaviour from the server side. After I retrieved the error code, I came to know that the server cut the connection.

This is server-side main.cpp

#include <QApplication>
#include <QFile>
#include <QFileDialog>
#include <QMessageBox>
#include <QtNetwork/QTcpServer>
#include <QtNetwork/QTcpSocket>

class MyMessageBox:public QMessageBox
{
public:
    MyMessageBox(std::string message,QWidget *parent=0):QMessageBox(QMessageBox::NoIcon,QString("ErrorMessage"),QString(message.c_str()),QMessageBox::Ok,parent,Qt::Widget)
    {
    }
};

class My_Server:public QTcpServer
{
    Q_OBJECT
public:
    My_Server();
public slots:
    void on_Connection();
};

My_Server::My_Server():QTcpServer()
{
    connect(this,SIGNAL(newConnection()),this,SLOT(on_Connection()));
}

void My_Server::on_Connection()
{
    MyMessageBox mm("Connection Established");
    mm.exec();
    QTcpSocket * my_Socket = this->nextPendingConnection();

    my_Socket->waitForBytesWritten(30000);

    QByteArray block("Hi all");

    my_Socket->write(block);
}

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

    My_Server tcp_Server;
    tcp_Server.listen(QHostAddress("127.0.0.1"),15000);

    return app.exec();
}

#include "main.moc"

This is client-side main.cpp

#include <QApplication>
#include <QDataStream>
#include <QFile>
#include <QFileDialog>
#include <QHostAddress>
#include <QMessageBox>
#include <QtNetwork/QTcpServer>
#include <QtNetwork/QTcpSocket>

class MyMessageBox:public QMessageBox
{
public:
    MyMessageBox(std::string message,QWidget *parent=0):QMessageBox(QMessageBox::NoIcon,QString("ErrorMessage"),QString(message.c_str()),QMessageBox::Ok,parent,Qt::Widget)
    {
    }
};

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

    QTcpSocket client_Socket;

    client_Socket.connectToHost(QHostAddress("127.0.0.1"),15000);

    QDataStream in(&client_Socket);
    in.setVersion(QDataStream::Qt_4_开发者_Go百科7);

    client_Socket.waitForReadyRead(30000);

    char buf[100]={'\0'};
    client_Socket.read(buf,(quint16)sizeof(buf));
    QString nothing(buf);

    MyMessageBox mm((QString("++ ")+nothing+" ++").toStdString());
    mm.exec();

    MyMessageBox mn(QString::number(client_Socket.error()).toStdString());
    mn.exec();

    return app.exec();
}

This is the pro file( same for both)

TEMPLATE = app
TARGET = 
DEPENDPATH += .
INCLUDEPATH += .

# Input
SOURCES += main.cpp

QT += network

I can't find out that why the server side is cutting the connection. If anybody help me to find out the cause, I will be thankful to them.

Note: I am using Qt-4.7.2 in windows platform

The connection is getting closed because your server is exiting right after it's sent that message.

This is probably because you're using QApplication, but don't actually have a long-lasting GUI widget. So the event loop stops short once you've finished displaying that first dialog box.

Try either:

• using QCoreApplication on the server side instead. (And don't start a message box - no GUI allowed with QCoreApplication.)
• give your server a main widget that is responsible for starting the TCP server.

Check quitOnLastWindowClosed property of QApplication, this will prevent your application to quit after closing the dialog.

Also your server is "waiting for bytes to be written" before actual write. You should first do write(block), than call waitForBytesWritten