A simple communicate c/s app using QtUdpSocket - c++

Question

I am a beginner of Qt.I want to use QUdpSocket to write a server app without a GUI.I have writen the client GUI app and server GUI app.They work well.But the server without GUI doesn't work well.
I try to use a while(true) loop to make the app running ,because I think after the code 'return a.exec()' the app will stop.But my server app does't work.
Here is my code:
client with GUI:
Widget::Widget(QWidget *parent) :
QWidget(parent),
ui(new Ui::Widget)
{
ui->setupUi(this);
socket=new QUdpSocket(this);
//click pushbutton send message to server
connect(this->ui->pushButton,SIGNAL(clicked()),this,SLOT(send()));
}
Widget::~Widget()
{
delete ui;
}
//send message
void Widget::send()
{
QByteArray msg="client2:hello world";
socket->writeDatagram(msg.data(),msg.size(),QHostAddress::LocalHost,6666);
}
The client app and server with GUI app can work well.But When I try to make a server app without GUI,I found server can not get the message from client.
And here is my server.cpp without GUI:
#include "server.h"
Server::Server(QObject *parent) : QObject(parent)
{
socket=new QUdpSocket(this);
array=new QByteArray();
socket->bind(QHostAddress::LocalHost,6666,QAbstractSocket::DontShareAddress);
socket->open(QIODevice::ReadWrite); //without this line, the app will show:
//'QIOBevice::read(QUdpSocket):device not open',but it still can get the message from client.
connect(socket,SIGNAL(readyRead()),this,SLOT(printMsg()));
}
void Server::printMsg()
{
if(socket->hasPendingDatagrams()) //I forgot this line before.
{
array->resize(socket->pendingDatagramSize());
socket->readDatagram(array->data(),array->size());
socket->readAll();
qDebug()<<"yesyesyes";
}
}
I try many times,I think the problem is in my main.cpp:
#include <QCoreApplication>
#include "server.h"
int main(int argc, char *argv[])
{
QCoreApplication a(argc, argv);
Server s;
return a.exec();//after a.exec(),the app is still working!!!
}
In a GUI app, if I don't close the windows, the server app will always run, and the SIGNAL and SLOT I can understand.But in a console app, it seems that the app end fast and can't get the message from client(can't display the message).

Answer 1

You should not have a while(true) in your main, otherwise, you are effectively just freezing your thread and Qt application. This is exactly what app.exec(); is for, it will return when your application is closed (more details here)
If your application (with or without GUI) closes after calling app.exec(), it is due to one of the following:
You are closing it manually, e.g. with qApp->quit();
You are forcing an exit, e.g. with exit(1);
A fatal error or exception prematurely ends your application