Valgrind Memory Leak on QThread::exec()

NickJT

Hello,

I am using valgrind to detect and resolve memory leaks in our Qt C++ app.

We are getting many duplicate reports of a memory leak that does not include any of our code on the stacktrace:

==5739== 38 bytes in 1 blocks are definitely lost in loss record 108 of 238
==5739==    at 0x402C4F8: malloc (vg_replace_malloc.c:309)
==5739==    by 0x62265D0: ???
==5739==    by 0x62245BC: ???
==5739==    by 0x6203CFC: ???
==5739==    by 0x5EF8AB8: ??? (in /opt/Qt5.9/lib/libQt5DBus.so.5.9.7)
==5739==    by 0x5EF97FD: ??? (in /opt/Qt5.9/lib/libQt5DBus.so.5.9.7)
==5739==    by 0x51D3E51: QMetaCallEvent::placeMetaCall(QObject*) (in /opt/Qt5.9/lib/libQt5Core.so.5.9.7)
==5739==    by 0x51D836C: QObject::event(QEvent*) (in /opt/Qt5.9/lib/libQt5Core.so.5.9.7)
==5739==    by 0x4FD17B9: QThread::event(QEvent*) (in /opt/Qt5.9/lib/libQt5Core.so.5.9.7)
==5739==    by 0x51A84C3: QCoreApplicationPrivate::notify_helper(QObject*, QEvent*) (in /opt/Qt5.9/lib/libQt5Core.so.5.9.7)
==5739==    by 0x51A85C1: QCoreApplication::notifyInternal2(QObject*, QEvent*) (in /opt/Qt5.9/lib/libQt5Core.so.5.9.7)
==5739==    by 0x51AADDB: QCoreApplicationPrivate::sendPostedEvents(QObject*, int, QThreadData*) (in /opt/Qt5.9/lib/libQt5Core.so.5.9.7)
==5739==    by 0x51FE72F: QEventDispatcherUNIX::processEvents(QFlags<QEventLoop::ProcessEventsFlag>) (in /opt/Qt5.9/lib/libQt5Core.so.5.9.7)
==5739==    by 0x51A5E78: QEventLoop::processEvents(QFlags<QEventLoop::ProcessEventsFlag>) (in /opt/Qt5.9/lib/libQt5Core.so.5.9.7)
==5739==    by 0x51A62D3: QEventLoop::exec(QFlags<QEventLoop::ProcessEventsFlag>) (in /opt/Qt5.9/lib/libQt5Core.so.5.9.7)
==5739==    by 0x4FD16FA: QThread::exec() (in /opt/Qt5.9/lib/libQt5Core.so.5.9.7)
==5739==    by 0x5EF5D5D: ??? (in /opt/Qt5.9/lib/libQt5DBus.so.5.9.7)
==5739==    by 0x4FD6791: ??? (in /opt/Qt5.9/lib/libQt5Core.so.5.9.7)
==5739==    by 0x54F5BBB: start_thread (in /usr/lib/libpthread-2.17.so)
==5739==    by 0x57520ED: clone (in /usr/lib/libc-2.17.so)

This is from a release build using Qt 5.9.7 32 bit on RHEL 7.

Here is the valgrind options:

valgrind --child-silent-after-fork=yes --smc-check=stack --tool=memcheck --gen-suppressions=all --leak-check=full --show-leak-kinds=definite,indirect,possible --log-file=/tmp/valgrindresults_`date +%s`.txt --num-callers=35 <path to exec>

There are a bunch of duplicates of the above report as well as some variations where calloc or realloc are at the top of the stack. They always mention QThread::exec and fucntions related to 'events'.

I was wondering if maybe we are doing something wrong to cause these leaks? Or maybe these are false positives? Or known issues?

Any ideas would be appreciated.

Thanks,
Nick

JoeCFD

are you sure you made a debug build of your own app?

NickJT

@JoeCFD I had been doing the valgrind scans with a debug build of my project and was getting these reports. Then on the forum I actually read that the debug build mode can introduce false positives so I tried scanning a release build. Either way it made no difference in frequency or content of these aforementioned reports.

JonB

@NickJT
If you are to get anywhere I suspect you will need to write a minimal, standalone example exhibiting this behaviour. You can post it here for others to make sure your code is as it should be.

JoeCFD

@NickJT You are using 5.9 and 38 bytes leak can be ignored. Valgrind gives all kinds of info and a lot can be ignored. A lot of issues with the libs can not be handled. You simply live with it or upgrade your libs.

NickJT

@JoeCFD Thanks that was I was thinking. Just wanted to check if there was something obvious I was doing wrong.