[SOLVED] Trying to get Ntp client to work

[koahnig]

Finally solved.

Stupid user error or misleading Qt doc. Whereever you want to blame this.

QHostAddress is only valid for IP addresses. However, since no IP addresses are typically given for NTP servers, I had used names found in internet. RTFM carefully helps.

Well, I did not see a point to check out QHostAddress carefully, because with QTcpSockets I am using a mix of host names and ip addresses without caring. The parameter list of connectToHost has also QHostAddress. However, the other method contains QString as parameter. After years of using successfully there was no reason ....

QHostInfo has to be involved for resolving the lookup.

[koahnig]

@SGaist @mcosta
123 is possible to use. See also above

[SGaist]

If I understand you correctly, the behavior of QHostAddress changed between two versions of Qt ?

[koahnig]

@SGaist
I am not sure about this. Also I am not sure when QHostAddress has been implemented first.

Those prototype are available in QAbstractSocket

connectToHost(const QString &, quint16, OpenMode, NetworkLayerProtocol);
connectToHost(const QHostAddress &, quint16, OpenMode);

I have started to use QTcpSocket in 2007 and I am using the same mechanism with host names and IP addresses. Being careful, it is obvious that I am using the first prototype. I have seen also the second prototype many times.
When seeing the prototype using the ntp class, it was completely clear to me that the behaviour is identical to the behaviour of QTcpSocket. The "minor detail" of QHostAddress was overlooked. I saw no warning or anything. Therefore, I did not pay attention and looked in other areas where I did not have experience with.

Finally I thought that I missed the obvious and started this thread. Unfortunately, I missed to add the section with the creation of the QHostAddress object by using the contructor QHostAddress::QHostAddress(const QString & address) with a host name.
That would have given others the chance to point out my stupid mistake.

[SGaist]

QHostAddress dates back to Qt 3 so it's a pretty old class

So if I follow you correctly, you thought you were using the first one, while in practice the second one was called ?

[koahnig]

@SGaist
Basically yes. Simply running into the trap and did not pay attention.

[SGaist]

I'm surprised that it does an automatic conversion like that...

[koahnig]

@SGaist said:

I'm surprised that it does an automatic conversion like that...

Just try HostAddress.pro

QT       += core network
QT       -= gui
TARGET = HostAddress
CONFIG   += console
CONFIG   -= app_bundle
TEMPLATE = app
SOURCES += main.cpp

main.cpp:

#include <QCoreApplication>
#include <QHostAddress>

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

    QHostAddress hadd ( "any.domain.com" );

    return a.exec();
}

This does not tell you a thing. Clearly by reading the docs that is wrong and you have to look up the ip address and use a proper ip address.
However, the constructor does notice that the input is wrong. I would expect to see at least a message. Probably the reason is that the class is rooting back to Qt3.

[SGaist]

Hi,

In fact the constructor taking a string doesn't detect anything particular. However using setAddress would return false so it might be a better option currently

[koahnig]

@SGaist
You are right. Also when reading all the details in the documentation it becomes obvious.

My point is that this is not really consistent with error reporting in other sections of Qt. In my opinion Qt's beauty is because of the error reporting, which helps to avoid problems. For instance when using a non-existing signal in a connect or the parameters are not fitting one receives a message reporting the problem. Certainly when one is not checking the connect's return value, the information might be lost. However, when you start to check the return value with an assert it is very powerful and you see immediately your problem.

For consistency I would expect to obtain at least a direct warning, when a non-IP address is handed over. Compared to other really good things in Qt, this is rather sloppy and forms a trap.

[SGaist]

Just talked with one the network kings: you should use isNull before using your QHostAddress object, then you'll know for sure that you have a valid address.

[koahnig]

@SGaist

Thanks for following up. Certainly I will take care this next time. I guess I will remember.