Checking if a device has an active internet connection

EricoDeMecha

Re: How to check whether the computer connects the internet?

Problem:
Check if a device has an active internet connection. The solutions I have found so far are either not cross-platform or blocking. However, while reading the network documentation in Qt, I cam across QNetworkInformation which seems to support and do this efficiently.

Code Sample:

#include <QCoreApplication>
#include <QNetworkInformation>

int main(int argc, char *argv[])
{
    QCoreApplication a(argc, argv);

    QNetworkInformation* net_info = QNetworkInformation::instance();
    
    if(net_info->reachability() == QNetworkInformation::Reachability::Online)
    {
        qDebug() << "Device is Online";
    }else
    {
        qDebug() << "Device is offline";
    }
    return a.exec();
}

The above program crashes. I have tried debugging it but could not arrive at a reasonable conclusion.

Question: If you have used this library before, would you share on a section that checks for online connectivity?

wrosecrans

As I said in the previous thread you linked,

What's the underlying question? Trying to determine if you are connected to the Internet is almost never useful information, and it's almost impossible to define well.

Is there a particular service you want to connect to? Just try to connect to that. Don't check if there's an internet connection an then only try to connect to the service if you think there's an Internet connection. You will drive users crazy when your Internet check fails despite having a route to your service. And when things work, it will be slower for no reason while you spend time trying to convince yourself there is an Internet connection.

JoeCFD

@EricoDeMecha net_info is a nullptr.

JoeCFD

@JoeCFD

    #include <QCoreApplication>
    #include <QNetworkInformation>

    int main(int argc, char *argv[])
    {
        QCoreApplication a(argc, argv);

        if ( QNetworkInformation::loadDefaultBackend() && QNetworkInformation::loadBackendByFeatures( QNetworkInformation::Features::Reachability ) ) {
            QNetworkInformation* net_info = QNetworkInformation::instance();
            if ( nullptr != net_info ) {
                if(net_info->reachability() == QNetworkInformation::Reachability::Online) {
                    qDebug() << "Device is Online";
                } 
                else {
                    qDebug() << "Device is offline";
                }  
            }
       }

        return a.exec();
    }

Cobra91151

@EricoDeMecha

Hello!

Additionally, you can use the QTcpSocket (https://doc.qt.io/qt-6/qtcpsocket.html) and connect to your website or google.com to check for the Internet connection. Please, check out my code below:

bool Dialog::checkNetConnection()
{
    bool isConnected = false;
    QTcpSocket сonnectionSocket;
    сonnectionSocket.connectToHost("google.com", 80);
    сonnectionSocket.waitForConnected(4000);

    if (сonnectionSocket.state() == QTcpSocket::ConnectedState) {
        isConnected = true;
    }

    сonnectionSocket.close();
    return isConnected;
}

qDebug() << "Is online: " << checkNetConnection();

Happy coding!

JonB

@Cobra91151 Absolutely! But what happens when google.com goes out of business? ;-)

Cobra91151

@JonB

In such case he can use his own website or other websites, for example: "1.1.1.1" (cloudflare) instead of "google.com". :)

JonB

@Cobra91151
CloudFlare is now a Google Cloud Platform Technology Partner So they will be brought down with Google :D

Cobra91151

@JonB

Ok, good point. I would suggest to use one of the Public DNS Server from this list: https://public-dns.info/ ;-)

RaptaG

@JoeCFD besides return a.exec(), what is the point of QCoreApplication?

JonB

@RaptaG
It provides all the (non-UI) services Qt requires/offers (signals, slots, lots & lots of other stuff, initializations)... basically a Qt program won;t work without one, else it's a not a program actually using Qt.

RaptaG

@JonB Can I borrow your code and use it on my open-source project (GPL3)? I will credit you.

JonB

@RaptaG
I don't know what code you are thinking of and not sure what you could want? But you are welcome to whatever, no credit necessary.

RaptaG

@JonB

Something else now, if I use it in a bool function, then return a.exec() is not necessary, right?

Also, how likely is it for the first 2 if statements not to be the case?

jsulm

@RaptaG said in Checking if a device has an active internet connection:

return a.exec() is not necessary, right?

Wrong.
If you're writing a Qt application which requires the event loop to be running (most Qt applications) then you have to call exec()...

RaptaG

@jsulm so, how do i adjust it so that it works in a bool function?

JonB

@RaptaG
I really don't think we know what you are asking about here.

There is nothing to stop you calling exec() in a function, boolean or not.

bool foo()
{
    int unusedReturnResult = someApplicationObject.exec();
    // ignore that return result
    return false;    // whatever boolean value you want
}

However, since exec() is the entry point to running the main Qt application event loop you are only going to want to call it once (and it does not exit till Qt quits event loop). So I cannot imagine what/why you want to move it to some function which returns a boolean. Makes me thing you are going to use it oddly. If you are thinking of calling this each time you want to check "if a device has an active internet connection" this is not the architecture.

RaptaG

@JonB said in Checking if a device has an active internet connection:

If you are thinking of calling this each time you want to check "if a device has an active internet connection"

That's what I was planning to do tbh 😅

jsulm

@RaptaG said in Checking if a device has an active internet connection:

That's what I was planning to do tbh

Why? You will block your application. Qt is an asynchronous framework - you should use it as such. QTcpSocket is also asynchronous...

RaptaG

@jsulm I am a newbie and async stuff look really difficult. Even then, I only need to check for internet connection very few times/day in my program