QtConcurrent::run Access violation reading location

bagipro

Hello all, I have a code
@for(int i=0;i<countThreads;i++)
{
QFuture<void> future = QtConcurrent::run(this,&MainWindow::thread);
threads.push_back(future);
}@

@void MainWindow::thread()
{
while(lastLink < countLinks)
{
::EnterCriticalSection(&editLastLink);
int i = lastLink;
incLastLink();
::LeaveCriticalSection(&editLastLink);

    QString req = doRequest(links[i]);
    if(req.contains("something"))
    {
        ::EnterCriticalSection(&editGoodLink);
        out <&lt; links[i] &lt;&lt; endl;
        incGoodLinks();
        ::LeaveCriticalSection(&editGoodLink);
    }
}

}@

@QString MainWindow::doRequest(QString url)
{
QString out;
QTimer timer;
QNetworkAccessManager* manager = new QNetworkAccessManager;
QEventLoop loop;
QObject::connect(&timer,SIGNAL(timeout()),&loop,SLOT(quit()));
QObject::connect(manager,SIGNAL(finished(QNetworkReply*)),&loop,SLOT(quit()));
QNetworkReply* reply = manager->get(QNetworkRequest(QUrl(url)));
timer.start(TIMEOUT);
loop.exec();
if(!timer.isActive())
{
reply->abort();
}
else
{
if(reply->error() == QNetworkReply::NoError)
{
out = static_cast<QString>( reply->readAll() );
}
reply->close();
}
delete manager;
return out;
}@

And for example after some minutes of work i get error
@Unhandled exception at 0x02e4c518 in progr.exe: 0xC0000005: Access violation reading location 0x76c29e98@
Where is an error? And how to fix it?

SGaist

Hi,

Your application is trying to read an invalid memory location.

Run you application through a debugger, it will show you where the error happens.

bagipro

Yes, i can read ;)
So 1h passed in debug mode, but the error doesn't appear.

bagipro

!http://clip2net.com/clip/m0/1373023940-clip-17kb.png(Text)!

SGaist

Out of curiosity, why don't you use QMutex to protect your data access ?

Also, I am surprised of this:

@out = static_cast<QString>( reply->readAll() )@

Why not use the QString::QString(const QByteArray) constructor ?

static_cast doesn't transform the QByteArray into a QString

bagipro

@void MainWindow::thread()
{
while(lastLink < countLinks)
{
mutex1.lock();
int i = lastLink;
incLastLink();
mutex1.unlock();

    QString req = doRequest(links[i]);
    if(req.contains("something"))
    {
        mutex2.lock();
        out <&lt; links[i] &lt;&lt; endl;
        incGoodLinks();
        mutex2.unlock();
    }
}

}
@

@QString MainWindow::doRequest(QString url)
{
QString out;
QTimer timer;
QNetworkAccessManager* manager = new QNetworkAccessManager;
QEventLoop loop;
QObject::connect(&timer,SIGNAL(timeout()),&loop,SLOT(quit()));
QObject::connect(manager,SIGNAL(finished(QNetworkReply*)),&loop,SLOT(quit()));
QNetworkReply* reply = manager->get(QNetworkRequest(QUrl(url)));
timer.start(TIMEOUT);
loop.exec();
if(!timer.isActive())
{
reply->abort();
}
else
{
if(reply->error() == QNetworkReply::NoError)
{
out = QString::QString( reply->readAll() );
}
reply->close();
}
delete manager;
return out;
}@

!http://clip2net.com/clip/m0/1373130879-clip-19kb.png(1)!

SGaist

Is it the function you use to increment your lastLink variable ? (You might use a QAtomicInt for that)

bagipro

Oh no, it's some system function

Its my functions
@void MainWindow::incLastLink()
{
lastLink++;
ui->label_7->setNum(lastLink);
}

void MainWindow::incGoodLinks()
{
goodLinks++;
ui->label_5->setNum(goodLinks);
}@

SGaist

Since it's pointer related, try to replace delete manager with
@manager->deleteLater();@

And maybe check that reply is a valid pointer (could be something related to it since it's the only other pointer visible in your code).

On a related note, I don't know for sure but I think It's your job to delete reply.

bagipro

How i can check reply pointer on valid? I didn't find some method like isValid. Or check like
@reply != NULL@

SGaist

Since it's a pointer:

@if (reply)@

Unless something is wrong, reply should either be NULL or pointing to a valid QNetworkReply (AFAIK, It shouldn't be NULL, but it's better to check to ensure that's not that that is causing you the access violation)

bagipro

I edit the code
@QString MainWindow::doRequest(QString url)
{
QString out;
QTimer timer;
QNetworkAccessManager* manager = new QNetworkAccessManager;
QEventLoop loop;
QObject::connect(&timer,SIGNAL(timeout()),&loop,SLOT(quit()));
QObject::connect(manager,SIGNAL(finished(QNetworkReply*)),&loop,SLOT(quit()));
QNetworkReply* reply = manager->get(QNetworkRequest(QUrl(url)));
timer.start(TIMEOUT);
loop.exec();
if(!timer.isActive())
{
reply->abort();
}
else
{
if(!reply) ::Beep(1000,1000);
if(reply->error() == QNetworkReply::NoError)
{
out = QString::QString( reply->readAll() );
}
reply->close();
}
reply->deleteLater();
manager->deleteLater();
return out;
}@

And i got error
!http://clip2net.com/clip/m0/1373183493-clip-9kb.png(1)!

So, i never heard beep

SGaist

I would check reply sooner, you might be calling abort on an invalid reply (though it would be strange)

bagipro

I edit
@loop.exec();
if(!reply)
{
::Beep(10000,10000);
exit(123456);
}@
But i got same violation error
Maybe I need to create exeptions?

SGaist

Are you sure everything you are accessing is valid (like links[i]) ?

bagipro

I think yes, i get errors in the middle of work process

bagipro

I runned it from cdb debugger. I see many errors, its showed at startup
@:0: предупреждение: Exception at 0x7c812aeb, code: 0x6ba: RPC server unavailable, flags=0x1 (execution cannot be continued) (first chance)

:0: предупреждение: Exception at 0x7c812aeb, code: 0x6ba: RPC server unavailable, flags=0x1 (execution cannot be continued) (first chance)

:0: предупреждение: Exception at 0x7c812aeb, code: 0x406d1388: Startup complete, flags=0x0 (first chance)

:0: предупреждение: Exception at 0x7c812aeb, code: 0x6ba: RPC server unavailable, flags=0x1 (execution cannot be continued) (first chance)

:0: предупреждение: Exception at 0x7c812aeb, code: 0x6a6: Invalid RPC server, flags=0x1 (execution cannot be continued) (first chance)@

So, on my pc RPC service is runned

SGaist

Then are you sure your server is running and that you can make as much connection that you are trying ?

bagipro

Oh, i haven't server, i have only this code.
And i don't click any buttons, i only run application and see errors

SGaist

Then I would :

First step: ensure the server is responding
Second step: try to access the server with one query
Third step: try with several queries in a row
Forth step: Try to parallelize your queries

That might include refactoring your code so that for example you'll use QtConcurrent::map to call your function with all the queries from a QList rather than accessing them yourself