Memory leak using QThread issue

Cobra91151

Hi! I have made a changelog feature to my application. I have created worker class and connect all signals and slots with QThread. The problem is when I start a thread to update data more then one time it begin to leak RAM memory.

Code:

 appCheckUpdate = new CheckUpdates();
 appCheckUpdatesThread = new QThread();
 appCheckUpdate->moveToThread(appCheckUpdatesThread);
 connect(appCheckUpdatesThread, &QThread::started, appCheckUpdate, &CheckUpdates::checkUpdate);
 connect(appCheckUpdate, &CheckUpdates::updateData, this, &Changelog::getUpdateData);
 connect(appCheckUpdate, &CheckUpdates::networkError, this, &Changelog::changeLogUpdateError);
 connect(appCheckUpdate, &CheckUpdates::localeError, this, &Changelog::updateLocaleError);
 connect(appCheckUpdate, &CheckUpdates::finished, appCheckUpdatesThread, &QThread::quit, Qt::DirectConnection);
 appCheckUpdatesThread->start();

When I create this on the stack:

  appCheckUpdate.moveToThread(&appCheckUpdatesThread);
  connect(&appCheckUpdatesThread, &QThread::started, &appCheckUpdate, &CheckUpdates::checkUpdate);
  connect(&appCheckUpdate, &CheckUpdates::updateData, this, &Changelog::getUpdateData);
  connect(&appCheckUpdate, &CheckUpdates::networkError, this, &Changelog::changeLogUpdateError);
  connect(&appCheckUpdate, &CheckUpdates::localeError, this, &Changelog::updateLocaleError);
  connect(&appCheckUpdate, &CheckUpdates::finished, &appCheckUpdatesThread, &QThread::quit, Qt::DirectConnection);
  appCheckUpdatesThread.start();

It duplicates data and also leak is still present. How to fix this issue? Thanks in advance.

SGaist

Hi,

In the first case, you don't seem to delete appCheckUpdate nor appCheckUpdateThread.

As for the leak, how do you know you are leaking something ?

Are you properly handling all resource in your custom classes ?

Cobra91151

@SGaist

I have checked the memory leak by Task Manager.

I have some questions:

Qt does not delete objects when use connect(appCheckUpdate, &CheckUpdates::finished, appCheckUpdatesThread, &QThread::quit, Qt::DirectConnection);?

I thought when thread has been deleted than the object appCheckUpdate should be also deleted or not?

So where do I have to delete those objects (in destructor)?

Thanks in advance for your help.

SGaist

A thread calling finished doesn't delete itself like that, it's your job to know when it should be deleted. If it's when CheckUpdates::finished then connect the deleteLater function to it.

J.Hilk

@Cobra91151 as an additional note,

Using Qt::DirectConnection instead of Qt::QueuedConnection is asking for trouble!
Signal/Slot Connections are threadsave because they are automatically queued, accpet if you give it Qt::DirectConnection as a 5th parameter.

Cobra91151

@SGaist
@J.Hilk

I will try it. Thanks for the suggestions.

Cobra91151

I use connect(appCheckUpdate, &CheckUpdates::finished, appCheckUpdatesThread, &QThread::deleteLater); but then I get warning: QThread: Destroyed while thread is still running.

So should I connection like this:

connect(appCheckUpdate, &CheckUpdates::finished, appCheckUpdatesThread, &QThread::quit);
connect(appCheckUpdatesThread, &CheckUpdates::destroyed, appCheckUpdatesThread, &QThread::deleteLater);

Or it will not work because the thread is not destroyed?

J.Hilk

@Cobra91151 said in Memory leak using QThread issue:

So should I connection like this:
connect(appCheckUpdate, &CheckUpdates::finished, appCheckUpdatesThread, &QThread::quit);
connect(appCheckUpdatesThread, &CheckUpdates::destroyed, appCheckUpdatesThread, &QThread::deleteLater);
Or it will not work because the thread is not destroyed?

No, this is exactly the correct way to do it.

Cobra91151

@J.Hilk

I have changed code to:

 appCheckUpdate = new CheckUpdates();
 appCheckUpdatesThread = new QThread();
 appCheckUpdate->moveToThread(appCheckUpdatesThread);
 connect(appCheckUpdatesThread, &QThread::started, appCheckUpdate, &CheckUpdates::checkUpdate);
 connect(appCheckUpdate, &CheckUpdates::updateData, this, &Changelog::getUpdateData);
 connect(appCheckUpdate, &CheckUpdates::networkError, this, &Changelog::changeLogUpdateError);
 connect(appCheckUpdate, &CheckUpdates::localeError, this, &Changelog::updateLocaleError);
 connect(appCheckUpdate, &CheckUpdates::finished, appCheckUpdatesThread, &QThread::quit);
 connect(appCheckUpdatesThread, &CheckUpdates::destroyed, appCheckUpdatesThread, &QThread::deleteLater);
 appCheckUpdatesThread->start();

But the problem still exists, it leaks memory. So this - connect(appCheckUpdatesThread, &CheckUpdates::destroyed, appCheckUpdatesThread, &QThread::deleteLater); - deletes only appCheckUpdatesThread (QThread) object.

So should I also delete appCheckUpdate by calling delete appCheckUpdate?

J.Hilk

@Cobra91151
yes of course,
at the end of your threaded operation, you'll have to delete your QThread object and your Worker object, seperately.

In case you haven't seen it, heres an wiki-example

Cobra91151

@J.Hilk

Thanks for the link. I will try it.

jsulm

@Cobra91151 It should be:

connect(appCheckUpdatesThread, &QThread::finished, appCheckUpdatesThread, &QThread::deleteLater);

In http://doc.qt.io/qt-5.8/qthread.html#finished
"This signal can be connected to QObject::deleteLater(), to free objects in that thread."

Cobra91151

@J.Hilk

Thank you. I have fixed the memory leak issue with changelog, but I also have the memory leak when I reload some WMI data during language change of my app. I will try to fix it and reply.

Cobra91151

Hi! I have some WMI memory leak issue.

Code:

//Initialization
IWbemLocator *pLocator = 0;
IWbemServices *pService = 0;
IEnumWbemClassObject* pEnumerator = NULL;
IWbemClassObject *pclsObj = NULL;

while (pEnumerator)
{
     hres = pEnumerator->Next(WBEM_INFINITE, 1, &pclsObj, &uReturn);

      VARIANT processName;
      pclsObj->Get(L"Name", 0, &processName, 0, 0);
      QString userProcessName;
      userProcessName = QString::fromWCharArray(processName.bstrVal);
       
      emit testData(userProcessName);
      VariantClear(&processName);
}

//Cleanup
 pService->Release();
 pLocator->Release();
 //pEnumerator->Release(); - Clang Static Analyzer displays issue - called C++ object pointer is null
 //pclsObj->Release(); - Clang Static Analyzer displays issue - called C++ object pointer is null

Every time I change language in my app it takes 1 - 5 MB RAM. How to fix this issue? Thanks in advance.

ambershark

@Cobra91151 This isn't Qt code.. You should probably post this question on a win32 API message board.

Also are you sure it's actually leaking? Just because the size of memory for your running application grows does not mean it's leaking. You need something like valgrind to test for leaks on exit.

Cobra91151

@ambershark

I have checked it in Task Manager. For example, my app takes 25 MB RAM, when changing languages it grows to 30 MB and 35, 40... and never release it. So should I consider this as not a leak?

SGaist

What do you mean by changing languages ? What do you do to make that happen ?

Cobra91151

@SGaist

I have all WMI data in QTreeWidgetItems (QTreeWidget). I have two columns:
1 - Property
2 - Data

Property and data I get from a worker class which connected with signals and slots to thread. So to change property to another language I call clear function on QTreeWidget to delete all property and data, then I call function which connects worker to thread and get this data as QTreeWidgetItems in chosen (translated) language.

SGaist

Are you loading any language file in order to provide translations ?

Cobra91151

@SGaist

Yes, but when QComboBox index changes I call this function to load appropriate translation file:

Some code:

appTranslator = new QTranslator(this);
qApp->removeTranslator(appTranslator);
appTranslator->load(":Localization/Test_ru.qm");
 qApp->installTranslator(appTranslator);

and when apply button is clicked I retranslate string and reload WMI data using my loadTraslatedText function