Cannot get host address from name

rackover

Hi,

I'm new to Qt. I'm trying to make a very simple console application using Qt Network.
In this application, I want to get the host IP from its address.
I want to get this IP in a blocking way : I am not interested in callbacks and asynchronous.

But both ways, asynchronous QHostInfo::lookupHost(); and synchronous QHostInfo::fromName();, fail to me. I do not understand why and I would need at least one of them to work.

Here is the code :

const QString serverName = "lobby.faforever.com";
QHostAddress serverAddress;
quint16 serverPort = 8001;
QTcpSocket *socket = nullptr;

class Task : public QObject
{
public:
    void resolveDNS(QString DNS, QHostAddress& address) {
        QHostInfo::lookupHost(DNS, this, SLOT(finished(address, QHostInfo)));
    };
    void finished(QHostAddress& address, QHostInfo hostInfo){
        address = hostInfo.addresses()[0];
    }
};


int main()
{
    qDebug() << "Program started" << endl;


    qStdOut() << "->RESOLVING ADDRESS FOR " << serverName.toUtf8() << endl;

    // NON-BLOCKING WAY :
    //
    // Task thisTask;
    // thisTask.resolveDNS(serverName, serverAddress);
    // while (serverAddress.isNull()){
    //    //do nothing
    // }
    //

    // BLOCKING WAY :
    //
    // QHostInfo servInf = QHostInfo::fromName(serverName);
    // serverAddress = servInf.addresses()[0];
    //


    qStdOut() << "->CONNECTING TO " << serverAddress.toString() << ":" << serverPort << endl;

    socket->connectToHost(serverAddress, serverPort);
    socket->setSocketOption(QTcpSocket::KeepAliveOption, 1);

    // we need to wait...
    if(socket->waitForConnected(1000))
    {
        qDebug() <<  "Connected!";
    }
   return 0;
}

The blocking way, "fromName", is returning this upon fire :

QObject::connect: Cannot connect (null)::destroyed() to QHostInfoLookupManager::waitForThreadPoolDone()

The non-blocking way, via my resolveDNS() function, returns this :

QHostInfo::lookupHost() called with no event dispatcher

What can I do ?
Regards,

6thC

Have you seen? http://doc.qt.io/qt-5/qdnslookup.html

I'm sorry I don't have the effort to understand, to me it looks like you are not properly using / trying to abuse signals and slots - or something. I'd just use them in the conventional way.

I myself would use a connection using the new syntax like:

connect(dns, QDnsLookup::finished, this, &MyObject::handleServers);

jsulm

@rackover This simply cannot work as you dont't have QApplication instance and you do not start event loop. Please take a look at Qt basics and examples.

Don't do such things in event driven applications like Qt application! You block the event loop!

while (serverAddress.isNull()){
    do nothing
}

Taz742

@rackover
Your socket is allways null. Where is a socket = new QTcpSocket() ??
Also

@rackover said in Cannot get host address from name:

class Task : public QObject
{
public:

Signals and slots are available only in QObject derived classes, and a Q_OBJECT macro is needed. You forget Q_OBJECT.
Correct Is

class Task : public QObject
{
     Q_OBJECT
public:

Also your finished function does not a slot:
correct is :

public slots:
    void finished(QHostAddress& address, QHostInfo hostInfo){
        address = hostInfo.addresses()[0];
    }

Use this example to solve your problem:

void MainWindow::lookUp(QHostInfo info)
{
    if (info.error()  != QHostInfo::NoError) {
        qDebug() << info.errorString();
        return;
    }

    for (int i = 0; i < info.addresses().size(); i++) {
        qDebug() << info.addresses().at(i).toString();
    }
}

void MainWindow::on_pushButton_clicked() //on_pushButton_clicked() is a your void resolveDNS(QString DNS, QHostAddress& address)  
{
    QHostInfo::lookupHost("qt.io", this, SLOT(lookUp(QHostInfo)));
}

private slots:
    void lookUp(QHostInfo info);

rackover

Thank you for all your answers.

Given the small app I want to build, the blocking synchronous way is what I need to make work.

So I've enhanced my code a bit and deleted the other :

//#include <QCoreApplication>
#include <QtCore>
#include <QtNetwork>

//Debugging
#include <QDebug>

using namespace std;

const QString serverName = "lobby.faforever.com";
QHostAddress serverAddress;
quint16 serverPort = 8001;
string firstQuery = "{'command':'ask_session','version':'1.0','user_agent':'faf-client'}";

QTcpSocket *socket = new QTcpSocket();


QTextStream& qStdOut()
{
    static QTextStream ts( stdout );
    return ts;
}

int main()
{
    qDebug() << "Program started" << endl;


    qStdOut() << "->RESOLVING ADDRESS FOR " << serverName.toUtf8() << endl;

    // BLOCKING WAY :
    //

     QHostInfo servInf = QHostInfo::fromName(serverName);
     QList<QHostAddress> addressesList = servInf.addresses();

     for (int i = 0; i < addressesList.size(); ++i) {
         if (addressesList.at(i) != QHostAddress::LocalHost &&
             addressesList.at(i).toIPv4Address() ) {
             serverAddress = addressesList.at(i);

             break;
         }
     }
    //


    qStdOut() << "->CONNECTING TO " << serverAddress.toString() << ":" << serverPort << endl;

    socket->connectToHost(serverAddress, serverPort);
    socket->setSocketOption(QTcpSocket::KeepAliveOption, 1);

    // we need to wait...
    if(socket->waitForConnected(1000))
    {
        qDebug() <<  "Connected!";
    }
   return 0;
}

Adding " new QTcpSocket();" instead of starting the socket as a null value makes the thing work. But despite it working, the same error is still displaying :

->RESOLVING ADDRESS FOR lobby.faforever.com
QObject::connect: Cannot connect (null)::destroyed() to QHostInfoLookupManager::waitForThreadPoolDone()
->CONNECTING TO 148.251.238.131:8001
Connected!

Why that ?
You said the socket should be up before doing a host lookup : but how can my socket be connected to Host if I dont know host address first ?

jsulm

@rackover said in Cannot get host address from name:

but how can my socket be connected to Host if I dont know host address first ?

Using the URL?
http://doc.qt.io/qt-5/qabstractsocket.html#connectToHost

rackover

This post is deleted!

jsulm

@rackover The host name is the URL: "lobby.faforever.com"...

rackover

Using the servername directly, i'm still getting the same error.
QObject::connect: Cannot connect (null)::destroyed() to QHostInfoLookupManager::waitForThreadPoolDone()

Why that ?