QNetworkAccessManager and network errors

**lukass** · 21st February 2011, 19:21

Hello.
On http://doc.qt.nokia.com/4.7/qnetworkaccessmanager.html is small example:

Qt Code:

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QNetworkRequest request;
request.setUrl(QUrl("http://qt.nokia.com"));
request.setRawHeader("User-Agent", "MyOwnBrowser 1.0");
 
QNetworkReply *reply = manager->get(request);
//if network error occur here - what happens? slotError is called or not?
 
connect(reply, SIGNAL(readyRead()), this, SLOT(slotReadyRead()));
connect(reply, SIGNAL(error(QNetworkReply::NetworkError)),
        this, SLOT(slotError(QNetworkReply::NetworkError)));
connect(reply, SIGNAL(sslErrors(QList<QSslError>)),
        this, SLOT(slotSslErrors(QList<QSslError>)));

QNetworkRequest request;
request.setUrl(QUrl("http://qt.nokia.com"));
request.setRawHeader("User-Agent", "MyOwnBrowser 1.0");

QNetworkReply *reply = manager->get(request);
//if network error occur here - what happens? slotError is called or not?

connect(reply, SIGNAL(readyRead()), this, SLOT(slotReadyRead()));
connect(reply, SIGNAL(error(QNetworkReply::NetworkError)),
        this, SLOT(slotError(QNetworkReply::NetworkError)));
connect(reply, SIGNAL(sslErrors(QList<QSslError>)),
        this, SLOT(slotSslErrors(QList<QSslError>)));

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If network error occur in line 6 of code - what happens? Probably slotError will not be triggered. Any suggestions?

**ChrisW67** · 21st February 2011, 21:39

As I understand it, the call to get() queues a request that does not start executing until the event loop is reached. This happens after making all the connections. As a result there can be no error in the location you highlighted.

Of course, I could be wrong.

**lukass** · 22nd February 2011, 07:33

Originally Posted by ChrisW67

As I understand it, the call to get() queues a request that does not start executing until the event loop is reached. This happens after making all the connections. ...

So, why http://doc.qt.nokia.com/4.7/qnetwork...nager.html#get not writing about any queue?

In addition to the above:
why condition in line 7 is true(when network error occur)?

Qt Code:

Switch view

QNetworkRequest request;
request.setUrl(QUrl("http://qt.nokia.com"));
request.setRawHeader("User-Agent", "MyOwnBrowser 1.0");
 
QNetworkReply *reply = manager->get(request);
//if network error occur here - what happens? slotError is called or not?
if(reply->error() != QNetworkReply::NoError)
  qDebug("network error!");
 
connect(reply, SIGNAL(readyRead()), this, SLOT(slotReadyRead()));
connect(reply, SIGNAL(error(QNetworkReply::NetworkError)),
        this, SLOT(slotError(QNetworkReply::NetworkError)));
connect(reply, SIGNAL(sslErrors(QList<QSslError>)),
        this, SLOT(slotSslErrors(QList<QSslError>)));

QNetworkRequest request;
request.setUrl(QUrl("http://qt.nokia.com"));
request.setRawHeader("User-Agent", "MyOwnBrowser 1.0");

QNetworkReply *reply = manager->get(request);
//if network error occur here - what happens? slotError is called or not?
if(reply->error() != QNetworkReply::NoError)
  qDebug("network error!");

connect(reply, SIGNAL(readyRead()), this, SLOT(slotReadyRead()));
connect(reply, SIGNAL(error(QNetworkReply::NetworkError)),
        this, SLOT(slotError(QNetworkReply::NetworkError)));
connect(reply, SIGNAL(sslErrors(QList<QSslError>)),
        this, SLOT(slotSslErrors(QList<QSslError>)));

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**ChrisW67** · 22nd February 2011, 08:15

From the documentation you linked:

Note: QNetworkAccessManager queues the requests it receives. The number of requests executed in parallel is dependent on the protocol. Currently, for the HTTP protocol on desktop platforms, 6 requests are executed in parallel for one host/port combination.

This example:

Qt Code:

Switch view

#include <QtCore>
#include <QtNetwork>
#include <QDebug>
 
class TestObject: public QObject {
    Q_OBJECT
public:
    TestObject(QObject *p = 0): QObject(p), reply(0) {
        QTimer::singleShot(0, this, SLOT(doIt()));
    }
public slots:
    void doIt() {
        qDebug() << "doIt called";
        QNetworkRequest request;
        // request.setUrl(QUrl("http://qt.nokia.com"));
        // request.setUrl(QUrl("NOSCHEME://qt.nokia.com"));
        // request.setUrl(QUrl("ftp://qt.nokia.com"));
        // request.setUrl(QUrl("https://site.without.ssl"));
        request.setUrl(QUrl("http://RUBBISH.nokia.com"));
 
        reply = manager.get(request);
 
        // This message is not triggered
        if(reply->error() != QNetworkReply::NoError)
          qDebug() << "network error!" << reply->error();
 
        connect(reply, SIGNAL(readyRead()), this, SLOT(slotReadyRead()));
        connect(reply, SIGNAL(finished()), this, SLOT(slotFinished()));
        connect(reply, SIGNAL(error(QNetworkReply::NetworkError)),
                this, SLOT(slotError(QNetworkReply::NetworkError)));
        qDebug() << "doIt finished";
    }
    void slotReadyRead() { 
        qDebug() << "slotReadyRead"; 
    }
    void slotFinished() { 
        qDebug() << reply->readAll();
        reply->deleteLater();
    }
    void slotError(QNetworkReply::NetworkError e) { 
        qDebug() << "slotError" << e ; 
    }
private:
 
    QNetworkAccessManager manager;
    QNetworkReply *reply;
};
 
int main(int argc, char *argv[])
{
    QCoreApplication app(argc, argv);
 
    TestObject t;
    return app.exec();
}
#include "main.moc"

#include <QtCore>
#include <QtNetwork>
#include <QDebug>

class TestObject: public QObject {
    Q_OBJECT
public:
    TestObject(QObject *p = 0): QObject(p), reply(0) {
        QTimer::singleShot(0, this, SLOT(doIt()));
    }
public slots:
    void doIt() {
        qDebug() << "doIt called";
        QNetworkRequest request;
        // request.setUrl(QUrl("http://qt.nokia.com"));
        // request.setUrl(QUrl("NOSCHEME://qt.nokia.com"));
        // request.setUrl(QUrl("ftp://qt.nokia.com"));
        // request.setUrl(QUrl("https://site.without.ssl"));
        request.setUrl(QUrl("http://RUBBISH.nokia.com"));

        reply = manager.get(request);

        // This message is not triggered
        if(reply->error() != QNetworkReply::NoError)
          qDebug() << "network error!" << reply->error();
        
        connect(reply, SIGNAL(readyRead()), this, SLOT(slotReadyRead()));
        connect(reply, SIGNAL(finished()), this, SLOT(slotFinished()));
        connect(reply, SIGNAL(error(QNetworkReply::NetworkError)),
                this, SLOT(slotError(QNetworkReply::NetworkError)));
        qDebug() << "doIt finished";
    }
    void slotReadyRead() { 
        qDebug() << "slotReadyRead"; 
    }
    void slotFinished() { 
        qDebug() << reply->readAll();
        reply->deleteLater();
    }
    void slotError(QNetworkReply::NetworkError e) { 
        qDebug() << "slotError" << e ; 
    }
private:

    QNetworkAccessManager manager;
    QNetworkReply *reply;
};

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

    TestObject t;
    return app.exec();
}
#include "main.moc"

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Doesn't fail at your debug statement with any of the examples. All the error messages come from the slot code. Some of these should fail immediately so there is no network delay.

Can you provide an example that does fail?

**lukass** · 22nd February 2011, 09:40

Originally Posted by ChrisW67

Can you provide an example that does fail?

Yes, here:

Qt Code:

Switch view

#include <QtCore>
#include <QtNetwork>
#include <QDebug>
 
class TestObject: public QObject {
    Q_OBJECT
public:
    TestObject(QObject *p = 0): QObject(p), reply(0) {
        QTimer::singleShot(0, this, SLOT(doIt()));
    }
public slots:
    void doIt() {
        qDebug() << "doIt called";
        QNetworkRequest request;
        request.setUrl(QUrl("http://213.251.170.205/~david/index.htm"));
 
        reply = manager.get(request);
 
        // This message is not triggered
        if(reply->error() != QNetworkReply::NoError)
          qDebug() << "network error!" << reply->error();
 
        connect(reply, SIGNAL(finished()), this, SLOT(slotFinished()));
        connect(reply, SIGNAL(error(QNetworkReply::NetworkError)),
                this, SLOT(slotError(QNetworkReply::NetworkError)));
        qDebug() << "doIt finished";
    }
 
    void slotFinished() {
        qDebug() << "slotFinished";
        qDebug() << reply->readAll();
        reply->deleteLater();
 
        //reconnect after 2 seconds
        QTimer::singleShot(2000, this, SLOT(doIt()));
    }
    void slotError(QNetworkReply::NetworkError e) {
        qDebug() << "slotError" << e ;
    }
private:
 
    QNetworkAccessManager manager;
    QNetworkReply *reply;
};
 
int main(int argc, char *argv[])
{
    QCoreApplication app(argc, argv);
 
    TestObject t;
    return app.exec();
}
#include "main.moc"

#include <QtCore>
#include <QtNetwork>
#include <QDebug>

class TestObject: public QObject {
    Q_OBJECT
public:
    TestObject(QObject *p = 0): QObject(p), reply(0) {
        QTimer::singleShot(0, this, SLOT(doIt()));
    }
public slots:
    void doIt() {
        qDebug() << "doIt called";
        QNetworkRequest request;
        request.setUrl(QUrl("http://213.251.170.205/~david/index.htm"));

        reply = manager.get(request);

        // This message is not triggered
        if(reply->error() != QNetworkReply::NoError)
          qDebug() << "network error!" << reply->error();

        connect(reply, SIGNAL(finished()), this, SLOT(slotFinished()));
        connect(reply, SIGNAL(error(QNetworkReply::NetworkError)),
                this, SLOT(slotError(QNetworkReply::NetworkError)));
        qDebug() << "doIt finished";
    }

    void slotFinished() {
        qDebug() << "slotFinished";
        qDebug() << reply->readAll();
        reply->deleteLater();

        //reconnect after 2 seconds
        QTimer::singleShot(2000, this, SLOT(doIt()));
    }
    void slotError(QNetworkReply::NetworkError e) {
        qDebug() << "slotError" << e ;
    }
private:

    QNetworkAccessManager manager;
    QNetworkReply *reply;
};

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

    TestObject t;
    return app.exec();
}
#include "main.moc"

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When my PPP network interface is down, I start example:

doIt called
network error! 99
doIt finished

[program running but not do enything, not calling slots etc. I shutdown example.]

PPP network interface is up, I start example:

doIt called
doIt finished
slotFinished
"<html><body>web contents</body></html>
"

[In this moment, I shutdown PPP network interface]

doIt called
doIt finished

[program running but not do enything, not calling slots etc.]

**ChrisW67** · 22nd February 2011, 09:52

If you use the finished() signal emitted by QNetworkAccessManager rather than the specific QNetworkReply you could check that the reply was successful in the slot before using the result. The signal connection is done one once, and before any requests are issued thereby eliminating the race condition you have here. You can also check the networkAccessible property of the manager.

**lukass** · 22nd February 2011, 12:00

Originally Posted by ChrisW67

If you use the finished() signal emitted by QNetworkAccessManager rather than the specific QNetworkReply you could check that the reply was successful in the slot before using the result. The signal connection is done one once, and before any requests are issued thereby eliminating the race condition you have here. You can also check the networkAccessible property of the manager.

Thanks, I will check QNetworkAccessManager::finished(). But in the meantime: What wrong with my code example? Where is the begin of race condidtion?

Is this it?

Qt Code:

Switch view

//...
//reconnect after 2 seconds
QTimer::singleShot(2000, this, SLOT(doIt()));
//...

//...
//reconnect after 2 seconds
QTimer::singleShot(2000, this, SLOT(doIt()));
//...

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Added after 20 minutes:

I used QNetworkAccessManager::finished(QNetworkReply *):

Qt Code:

Switch view

#include <QtCore>
#include <QtNetwork>
#include <QDebug>
 
class TestObject: public QObject {
    Q_OBJECT
public:
    TestObject(QObject *p = 0): QObject(p)
    {
      connect(&manager, SIGNAL(finished(QNetworkReply *)), this, SLOT(slotFinished(QNetworkReply *)));
      QTimer::singleShot(0, this, SLOT(doIt()));
    }
public slots:
    void doIt() {
        qDebug() << "doIt called";
        QNetworkRequest request;
        request.setUrl(QUrl("http://213.251.170.205/~david/index.htm"));
 
        manager.get(request);
 
        qDebug() << "doIt finished";
    }
 
    void slotFinished(QNetworkReply * reply) {
      qDebug() << "slotFinished";
      if(reply->error() != QNetworkReply::NoError)
        qDebug() << "network error! " << reply->error();
      else
        qDebug() << reply->readAll();
 
      reply->deleteLater();
 
      //reconnect after 2 seconds
      QTimer::singleShot(2000, this, SLOT(doIt()));
    }
private:
    QNetworkAccessManager manager;
};
 
int main(int argc, char *argv[])
{
    QCoreApplication app(argc, argv);
 
    TestObject t;
    return app.exec();
}
#include "main.moc"

#include <QtCore>
#include <QtNetwork>
#include <QDebug>

class TestObject: public QObject {
    Q_OBJECT
public:
    TestObject(QObject *p = 0): QObject(p)
    {
      connect(&manager, SIGNAL(finished(QNetworkReply *)), this, SLOT(slotFinished(QNetworkReply *)));
      QTimer::singleShot(0, this, SLOT(doIt()));
    }
public slots:
    void doIt() {
        qDebug() << "doIt called";
        QNetworkRequest request;
        request.setUrl(QUrl("http://213.251.170.205/~david/index.htm"));

        manager.get(request);

        qDebug() << "doIt finished";
    }

    void slotFinished(QNetworkReply * reply) {
      qDebug() << "slotFinished";
      if(reply->error() != QNetworkReply::NoError)
        qDebug() << "network error! " << reply->error();
      else
        qDebug() << reply->readAll();

      reply->deleteLater();

      //reconnect after 2 seconds
      QTimer::singleShot(2000, this, SLOT(doIt()));
    }
private:
    QNetworkAccessManager manager;
};

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

    TestObject t;
    return app.exec();
}
#include "main.moc"

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But slotFinished is not called when I shutdown network interface.

**lukass** · 22nd February 2011, 14:47

The QNetworkAccessManager::networkAccessible() always return -1(QNetworkAccessManager::UnknownAccessibility).
So, how can I get signal if network error occur? Can anyone help?

**lukass** · 23rd February 2011, 10:23

This problem is probably related to:
http://bugreports.qt.nokia.com/browse/QTBUG-3443

I found solution. I added timer and function state(), which check connection state:

Qt Code:

Switch view

#include <QtCore>
#include <QtNetwork>
#include <QDebug>
 
class TestObject: public QObject {
    Q_OBJECT
public:
    TestObject(QObject *p = 0): QObject(p) {
      conn_check = new QTimer(this);
      conn_check->setInterval(30000); //30 seconds
      connect(conn_check, SIGNAL(timeout()), SLOT(state()));
      connect(&manager, SIGNAL(finished(QNetworkReply *)), this, SLOT(slotFinished(QNetworkReply *)));
      QTimer::singleShot(0, this, SLOT(doIt()));
    }
public slots:
    void doIt() {
        qDebug() << "doIt called";
        QNetworkRequest request;
        request.setUrl(QUrl("http://213.251.170.205/~david/index.htm"));
 
        reply = manager.get(request);
        conn_check->start();
 
        qDebug() << "doIt finished";
    }
 
    void slotFinished(QNetworkReply * reply) {
      conn_check->stop();
      qDebug() << "slotFinished";
      if(reply->error() != QNetworkReply::NoError)
        qDebug() << "network error! " << reply->error();
      else
        qDebug() << reply->readAll();
 
      reply->deleteLater();
 
      //reconnect after 2 seconds
      QTimer::singleShot(2000, this, SLOT(doIt()));
    }
 
    void state() {
      conn_check->stop();
      qDebug() << "isFinished: " << reply->isFinished();
      qDebug() << "isRunning: " << reply->isRunning();
      if(reply->isFinished())
        slotFinished(reply);
      else
        reply->abort(); //emits finished(/*...*/) signal
    }
 
private:
    QNetworkAccessManager manager;
    QNetworkReply * reply;
    QTimer * conn_check;
};
 
int main(int argc, char *argv[]) {
    QCoreApplication app(argc, argv);
 
    TestObject t;
    return app.exec();
}
#include "main.moc"

#include <QtCore>
#include <QtNetwork>
#include <QDebug>

class TestObject: public QObject {
    Q_OBJECT
public:
    TestObject(QObject *p = 0): QObject(p) {
      conn_check = new QTimer(this);
      conn_check->setInterval(30000); //30 seconds
      connect(conn_check, SIGNAL(timeout()), SLOT(state()));
      connect(&manager, SIGNAL(finished(QNetworkReply *)), this, SLOT(slotFinished(QNetworkReply *)));
      QTimer::singleShot(0, this, SLOT(doIt()));
    }
public slots:
    void doIt() {
        qDebug() << "doIt called";
        QNetworkRequest request;
        request.setUrl(QUrl("http://213.251.170.205/~david/index.htm"));

        reply = manager.get(request);
        conn_check->start();

        qDebug() << "doIt finished";
    }

    void slotFinished(QNetworkReply * reply) {
      conn_check->stop();
      qDebug() << "slotFinished";
      if(reply->error() != QNetworkReply::NoError)
        qDebug() << "network error! " << reply->error();
      else
        qDebug() << reply->readAll();

      reply->deleteLater();

      //reconnect after 2 seconds
      QTimer::singleShot(2000, this, SLOT(doIt()));
    }

    void state() {
      conn_check->stop();
      qDebug() << "isFinished: " << reply->isFinished();
      qDebug() << "isRunning: " << reply->isRunning();
      if(reply->isFinished())
        slotFinished(reply);
      else
        reply->abort(); //emits finished(/*...*/) signal
    }

private:
    QNetworkAccessManager manager;
    QNetworkReply * reply;
    QTimer * conn_check;
};

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

    TestObject t;
    return app.exec();
}
#include "main.moc"

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