Getting SIGSEGV on "finished" signal from QNetwowkAccessManager

[Fernando]

Hi everyone,

long time lurker, first time poster. :-)

I'm using QT 5.4.1 under Windows7-64.

I have a serious problem with QNetworkAccessManager: I got SIGSEGV when the "finished" signal fires, but only in Debug mode!! :-(

My sample:

mainwindow.h

#ifndef MAINWINDOW_H
#define MAINWINDOW_H

#include <QMainWindow>
#include <QNetworkAccessManager>
#include <QNetworkReply>
#include <QNetworkRequest>
#include <QFile>
#include <QDebug>

#define TEST_URL "http://www.gnu.org/licenses/gpl-3.0.txt"

namespace Ui {
class MainWindow;
}

class MainWindow : public QMainWindow
{
Q_OBJECT

public:
explicit MainWindow(QWidget *parent = 0);
~MainWindow();

private:
Ui::MainWindow *ui;

QNetworkAccessManager* mpNetManager;

void initializeNetwork();

void testDownload();

private slots:
void downloadFinished(QNetworkReply*);
};

#endif // MAINWINDOW_H

mainwindow.cpp

#include "mainwindow.h"
#include "ui_mainwindow.h"

MainWindow::MainWindow(QWidget *parent) : QMainWindow(parent), ui(new Ui::MainWindow)
{
ui->setupUi(this);
initializeNetwork();
testDownload();
}

MainWindow::~MainWindow()
{
delete ui;
mpNetManager->deleteLater();
}

void MainWindow::initializeNetwork()
{
mpNetManager = new QNetworkAccessManager;
}

void MainWindow::testDownload()
{
if (mpNetManager)
{
QNetworkRequest netRequest;
QUrl downloadURL(TEST_URL);
netRequest.setUrl(downloadURL);
connect(mpNetManager, SIGNAL(finished(QNetworkReply*)), this, SLOT(downloadFinished()));
mpNetManager->get(netRequest);
}
}

void MainWindow::downloadFinished (QNetworkReply* pNetReply)
{
if (pNetReply)
{
if(pNetReply->error())
{
qDebug() << "ERROR!";
qDebug() << pNetReply->errorString();
}
else
{
QString qsDownloadPath = "downloaded.txt";
QFile *pOutFile = new QFile(qsDownloadPath);
if(pOutFile->open(QFile::WriteOnly))
{
pOutFile->write(pNetReply->readAll());
pOutFile->flush();
pOutFile->close();
}
delete pOutFile;
qDebug() << "Download finished";
qDebug() << "Content written to";
qDebug() << qsDownloadPath;
}
pNetReply->deleteLater();
}
}

I'm getting mad: it runs fine in Release, but when i launch the debugger, I get the message

"The inferior stopped because it received a signal from the operating system
Signal name: SIGSEGV
Signal meaning: Segmentation Fault"

I'm not new to Qt, but I'm new to everything QNetwork. :-(

Thanks a lot!

Fernando

[SGaist]

Hi and welcome to devnet,

connect(mpNetManager, SIGNAL(finished(QNetworkReply*)), this, SLOT(downloadFinished()));

You are missing the parameter of downloadFinished

[Fernando]

You're right, sorry.

Corrected .cpp source (still the same problem):

#include "mainwindow.h"
#include "ui_mainwindow.h"

MainWindow::MainWindow(QWidget *parent) : QMainWindow(parent), ui(new Ui::MainWindow)
{
    ui->setupUi(this);
    initializeNetwork();
    testDownload();
}


MainWindow::~MainWindow()
{
    delete ui;
    mpNetManager->deleteLater();
}


void MainWindow::initializeNetwork()
{
    mpNetManager = new QNetworkAccessManager;
}


void MainWindow::testDownload()
{
    if (mpNetManager)
    {
        QNetworkRequest netRequest;
        QUrl downloadURL(TEST_URL);
        netRequest.setUrl(downloadURL);
        connect(mpNetManager, SIGNAL(finished(QNetworkReply * )), this, SLOT(downloadFinished(QNetworkReply * )));
        mpNetManager->get(netRequest);
    }
}


void MainWindow::downloadFinished (QNetworkReply* pNetReply)
{
    if (pNetReply)
    {
        if(pNetReply->error())
        {
                qDebug() << "ERROR!";
                qDebug() << pNetReply->errorString();
        }
        else
        {
            QString    qsDownloadPath = "downloaded.txt";
            QFile *pOutFile = new QFile(qsDownloadPath);
            if(pOutFile->open(QFile::WriteOnly))
            {
                pOutFile->write(pNetReply->readAll());
                pOutFile->flush();
                pOutFile->close();
            }
            delete pOutFile;
                qDebug() << "Download finished";
                qDebug() << "Content written to";
                qDebug() << qsDownloadPath;
        }
        pNetReply->deleteLater();
    }
}

[edit: Added missing coding tags SGaist]

[SGaist]

What do you get if you run your application through a debugger ?

[Fernando]

As I stated, when i launch the debugger, as soon as the "finished" signal fires I get the message

"The inferior stopped because it received a signal from the operating system
Signal name: SIGSEGV
Signal meaning: Segmentation Fault"

This is from C:\Windows\SysWOW64\ntdll.dll, instruction "rtlmovmemory"

This only happens while debugging: the program runs fine in Release.

[SGaist]

Can you check with a tool like Dependency Walker the libraries you are currently linking to ?

[Fernando]

Sure:

LIBGCC_S_DW2-1.DLL
QT5CORED.DLL
QT5NETWORKD.DLL
QT5WIDGETSD.DLL
API-MS-WIN-APPMODEL-RUNTIME-L1-1-0.DLL
API-MS-WIN-CORE-WINRT-ERROR-L1-1-0.DLL
API-MS-WIN-CORE-WINRT-L1-1-0.DLL
API-MS-WIN-CORE-WINRT-ROBUFFER-L1-1-0.DLL
API-MS-WIN-CORE-WINRT-STRING-L1-1-0.DLL
API-MS-WIN-SHCORE-SCALING-L1-1-1.DLL
DCOMP.DLL
IESHIMS.DLL

[SGaist]

That part looks good

Then let's get step by step. Does it also fail if you comment out the network stuff ?

[Fernando]

It debugs nicely without

mpNetManager->get(netRequest);

And it runs nicely (even with mpNetManager->get(netRequest)) when not debugging (even if the build was in debug mode).
To me, it's a bug in the debugger :-) , at least under Windows7-64.

[SGaist]

Using Qt's MinGW package ?

[Fernando]

Sorry, I was out of town.

Yes, using Qt's MinGW package (I don't have and don't like Visual Studio :-) ).

[SGaist]

So you have MinGW 4.9 ?

[Fernando]

Yes, MinGW 4.9.1.
Do you think I should file a bug report?