[Warning] QObject::disconnect: Unexpected null parameter in QNetworkAccessManager

Zlatko Karakas

Hello everyone,

I'm trying to find the source of this warning in the program I'm working on:
[Warning] QObject::disconnect: Unexpected null parameter

It happens sometimes as I switch Wifi on/off. In order to trace it I've modified Qobject to break into debugger when it happens, but it seems to be coming from the Qt itself. I'm not really sure what's going on here, and would be grateful if someone wiser could take a look and point me to right direction. Is this a bug in Qt, or just some invalid usage of the class?

Qt version is 5.7.1, and it happens on OS X El Capitan 10.11.4.

Here is the trace:

[14:15:29.368] [140735132925952] [Warning] QObject::disconnect: Unexpected null parameter

Process 83482 stopped
* thread #1: tid = 0x1e533, 0x0000000101d94f1c QtCore`QObject::disconnect(sender=<unavailable>, signal="2stateChanged(QNetworkSession::State)", receiver=0x00000001036df1e0, method="1_q_networkSessionStateChanged(QNetworkSession::State)") + 3308 at qobject.cpp:2910, queue = 'com.apple.main-thread', stop reason = EXC_BAD_INSTRUCTION (code=EXC_I386_INVOP, subcode=0x0)
    frame #0: 0x0000000101d94f1c QtCore`QObject::disconnect(sender=<unavailable>, signal="2stateChanged(QNetworkSession::State)", receiver=0x00000001036df1e0, method="1_q_networkSessionStateChanged(QNetworkSession::State)") + 3308 at qobject.cpp:2910
   2907	{
   2908	    if (sender == 0 || (receiver == 0 && method != 0)) {
   2909	        qWarning("QObject::disconnect: Unexpected null parameter");
-> 2910	__builtin_trap();
   2911	        return false;
   2912	    }
   2913
(lldb) bt
* thread #1: tid = 0x1e533, 0x0000000101d94f1c QtCore`QObject::disconnect(sender=<unavailable>, signal="2stateChanged(QNetworkSession::State)", receiver=0x00000001036df1e0, method="1_q_networkSessionStateChanged(QNetworkSession::State)") + 3308 at qobject.cpp:2910, queue = 'com.apple.main-thread', stop reason = EXC_BAD_INSTRUCTION (code=EXC_I386_INVOP, subcode=0x0)
  * frame #0: 0x0000000101d94f1c QtCore`QObject::disconnect(sender=<unavailable>, signal="2stateChanged(QNetworkSession::State)", receiver=0x00000001036df1e0, method="1_q_networkSessionStateChanged(QNetworkSession::State)") + 3308 at qobject.cpp:2910
    frame #1: 0x00000001028c2f14 QtNetwork`QNetworkAccessManagerPrivate::_q_networkSessionClosed(this=0x00000001036df5a0) + 196 at qnetworkaccessmanager.cpp:1608
    frame #2: 0x0000000101d8f9f2 QtCore`QObject::event(this=<unavailable>, e=<unavailable>) + 146 at qobject.cpp:1263
    frame #3: 0x0000000102c2552a QtWidgets`QApplicationPrivate::notify_helper(this=<unavailable>, receiver=0x00000001036df1e0, e=0x0000000115f104a0) + 314 at qapplication.cpp:3799
    frame #4: 0x0000000102c2695e QtWidgets`QApplication::notify(this=<unavailable>, receiver=<unavailable>, e=<unavailable>) + 510 at qapplication.cpp:3159
    frame #5: 0x0000000101d64164 QtCore`QCoreApplication::notifyInternal2(receiver=0x00000001036df1e0, event=0x0000000115f104a0) + 164 at qcoreapplication.cpp:988
    frame #6: 0x0000000101d64ecb QtCore`QCoreApplicationPrivate::sendPostedEvents(QObject*, int, QThreadData*) [inlined] QCoreApplication::sendEvent(receiver=0x00000001036df1e0, event=<unavailable>) + 891 at qcoreapplication.h:231
    frame #7: 0x0000000101d64eb3 QtCore`QCoreApplicationPrivate::sendPostedEvents(receiver=0x0000000000000000, event_type=0, data=0x00000001046ed040) + 867 at qcoreapplication.cpp:1649
    frame #8: 0x000000010893919e libqcocoa.dylib`QCocoaEventDispatcherPrivate::processPostedEvents(this=0x0000000108f008c0) + 190 at qcocoaeventdispatcher.mm:889
    frame #9: 0x0000000108939a51 libqcocoa.dylib`QCocoaEventDispatcherPrivate::postedEventsSourceCallback(info=0x0000000108f008c0) + 33 at qcocoaeventdispatcher.mm:926
    frame #10: 0x00007fff98c0f881 CoreFoundation`__CFRUNLOOP_IS_CALLING_OUT_TO_A_SOURCE0_PERFORM_FUNCTION__ + 17
    frame #11: 0x00007fff98beefbc CoreFoundation`__CFRunLoopDoSources0 + 556
    frame #12: 0x00007fff98bee4df CoreFoundation`__CFRunLoopRun + 927
    frame #13: 0x00007fff98beded8 CoreFoundation`CFRunLoopRunSpecific + 296
    frame #14: 0x00007fff933a8935 HIToolbox`RunCurrentEventLoopInMode + 235
    frame #15: 0x00007fff933a876f HIToolbox`ReceiveNextEventCommon + 432
    frame #16: 0x00007fff933a85af HIToolbox`_BlockUntilNextEventMatchingListInModeWithFilter + 71
    frame #17: 0x00007fff8df3cefa AppKit`_DPSNextEvent + 1067
    frame #18: 0x00007fff8df3c32a AppKit`-[NSApplication _nextEventMatchingEventMask:untilDate:inMode:dequeue:] + 454
    frame #19: 0x00007fff8df30e84 AppKit`-[NSApplication run] + 682
    frame #20: 0x000000010893896f libqcocoa.dylib`QCocoaEventDispatcher::processEvents(this=0x00000001046fbea0, flags=<unavailable>) + 2191 at qcocoaeventdispatcher.mm:422
    frame #21: 0x0000000101d60341 QtCore`QEventLoop::exec(QFlags<QEventLoop::ProcessEventsFlag>) [inlined] QEventLoop::processEvents(flags=<unavailable>) + 417 at qeventloop.cpp:134
    frame #22: 0x0000000101d60311 QtCore`QEventLoop::exec(this=0x00007fff5fbff700, flags=<unavailable>) + 369 at qeventloop.cpp:212
    frame #23: 0x0000000101d647d5 QtCore`QCoreApplication::exec() + 341 at qcoreapplication.cpp:1261
    frame #24: 0x0000000100008a05 TopTracker`main(argc=1, argv=0x00007fff5fbffa88) + 1893 at main.cpp:288
    frame #25: 0x00007fff8f4a15ad libdyld.dylib`start + 1
    frame #26: 0x00007fff8f4a15ad libdyld.dylib`start + 1

Thank you.

SGaist

HI and welcome to devnet,

Looks like a "bad timing" situation. The disconnect likely happens after the originally connected object has been deleted. One place to start looking at is the corewlan bearer plugin.

Zlatko Karakas

Thanks for the pointer. It seems QNetworkManager's shared pointer to QNetworkSession gets deleted somewhere along the way. Looks more like an internal Qt issue to me.

CanardMoussant

I confirm I have the very same stack here. The same code was working fine with Qt 5.6, I highly suspect a regression in Qt.

mrjj

@CanardMoussant
Hi
Is it possible to reproduce this in a small sample?
That is needed to open a bug report and increases the changes
of a fix.

CanardMoussant

I will try to work on that. Unfortunately for the moment I see this in a program with a significant amount of code, and I would need to narrow it down to a small reproducer.

Yurik147

I can reproduce this issue. I have created relative bug QTBUG-57110