HTTP and HTTPS in Qt

published at 07.08.2014 15:45 by Jens Weller
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Last week I started to work on an old project again: My own feed reader. I found the code 2 weeks a go on an old USB Stick, and decided to refactor it into a useful state. This involved dealing with HTTP via QNetworkAccessManager.

QNetworkAccessManager: HTTP in Qt

The QNetworkAccessManager class replaces the old QHttp classes. If you work with HTML, there is also the webkit module, but for raw HTTP, QNetworkAccessManger is the fitting choice. The API is asynchronous, your program will not block during the HTTP Request. A simple example:

QNetworkAccessManager manager;
QNetworkRequest req(url);
req.setRawHeader( "User-Agent" , "Meeting C++ RSS Reader" );
QNetworkReply* reply = manager.get(req);

Each request is done via the class QNetworkRequest, you can set various parameters for such a request. For my feed reader it was important to give the request a user agent. Otherwise some pages don't accept the connection and you will receive an RemoteHostClosed error. When you initiate the HTTP request with the QNetworkAccessManager(QNAM), you get a Pointer to a QNetworkReply object, corresponding to this request. You then either can connect to the Signals of QNetworkReply, or to the finished(QNetworkReply*) Signal of the QNAM. When dealing with multiple requests it can be better to bind to the QNAM signal, as you otherwise would have to dyncast QObjects::sender() for getting the context.

Which brings me back to my feed reader. None of the above is interesting for the feed reader. It has some feed url, and is actually interested in reading this feed. Its not interested to deal with QNAMs issues, as downloading via HTTP is such a common task, that this should be handled by a different class. So I created HttpDownloader, a class which currently downloads via get, and hands the result as a QByteArray in a signal to the caller. The interesting things then happen in the finished(QNetworkReply*) slot.

First the error handling:

if ( reply->error() != QNetworkReply::NoError ) {
    qWarning() <<"ErrorNo: "<< reply->error() << "for url: " << reply->url().toString();
    qDebug() << "Request failed, " << reply->errorString();
    qDebug() << "Headers:"<<  reply->rawHeaderList()<< "content:" << reply->readAll();
    runningreplies.erase(reply);
    return;
}

So, Qt does not use exceptions, and first one has to check if there is an error, currently all I do then is some logging and erasing the reply from an internal map.

But we aren't done yet, that there is no error does not mean that the call was successful. The HTTP Response also could be a redirect. QNAM does not automatically handle this, so the handler has to test for the redirect, and then issue a new request:

QUrl redirect = reply->attribute(QNetworkRequest::RedirectionTargetAttribute).toUrl();

if(redirect.isValid() && reply->url() != redirect)
{
    if(redirect.isRelative())
        redirect = reply->url().resolved(redirect);
    QNetworkRequest req(redirect);
    req.setRawHeader( "User-Agent" , "Meeting C++ RSS Reader" );
    QNetworkReply* reply = manager.get(req);
    runningreplies.insert(std::make_pair(reply,id));
    return;
}

Currently all I do is following the redirect. This is a working solution, but maybe not the best. It does not prevent redirect loops. QNetworkReply and QNetworkRequest have a few more attributes, but I only see the need to handle redirects currently.

Now, that errors and redirects are handled, the code for handling the content and request are rather short:

QByteArray array = reply->readAll();
emit rawDataAvailable(array,id);

runningreplies.erase(reply);
reply->deleteLater();
if(runningreplies.size() == 0)
    emit downloadFinished();

The content part of the HTTP call is easily read with readAll, as this class only deals with downloading it, it simply emits a signal that the content now can be processed. It is important also to delete the QNetworkReply instance now, first I have to delete it from a local map which binds a QVariant id to all active requests. Instead of using a local map I also could store this id directly as an attribute inside QNetworkReply.

This id is very important, as once the content is downloaded from the net, I need to do something with it. This id servers this purpose, the actual content handling then happens in the clients slot.

HTTPS and Qt

I hoped to get around this, as for the first version I would not need to rely on HTTPS, later when building apps with my feed reader, SSL support would be a must. But some of the feeds I want to have in my feed reader work over HTTPS, and one even is only available via HTTPS. Qt does not come with OpenSSL binaries, and when searching the net all kind of things show up. For example that you need to build Qt your self, as HTTPS is not enabled by default.

So, when requesting a HTTPS url, the following errors show up with QNetworkAccessManager in Qt5:

QSslSocket: cannot call unresolved function SSLv23_client_method
QSslSocket: cannot call unresolved function SSL_CTX_new
QSslSocket: cannot call unresolved function SSL_library_init
QSslSocket: cannot call unresolved function ERR_get_error
QSslSocket: cannot call unresolved function ERR_get_error

So, Qt5 does recognize the https url correctly, and tries to open a SSL Socket for it, which fails. The reason for this behavior is the missing OpenSSL binaries, which you need to provide to Qt in order to be able to properly open a https link. On windows this are the DLLs libeay32 and ssleay32.

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