QHash and QMap in threaded application

As you know QMap/QHash are little bit faster than std::map and std::unordered_map … but why? I have no ideas about this before I have used them in high-load muti-thread application. I send data in QHash via queued signal/slot between threads. Anything looks fine, but 1-2 times per day application gets segfault. It was very strange because it crashed after different count iterations but it was millions iterations. GDB shows something like this: #0 0x00007f8421029107 in __GI_raise (sig=sig@entry=6) at ../nptl/sysdeps/unix/sysv/linux/raise.c:56 #1 0x00007f842102a4e8 in __GI_abort () at abort.c:89 #2 0x00007f8421067044 in __libc_message (do_abort=do_abort@entry=1, fmt=fmt@entry=0x7f8421159c40 "*** Error in `%s': %s: 0x%s ***\n") at ../sysdeps/posix/libc_fatal.c:175 #3 0x00007f842106c81e in malloc_printerr (action=1, str=0x7f8421159d90 "double free or corruption (out)", ptr=<optimized out>) at malloc.c:4996 #4 0x00007f842106d526 in _int_free (av=<optimized out>, p=<optimized out>, have_lock=0) at malloc.c:3840 #5 0x00007f8421ede054 in QHashData::free_helper (this=0x7f8418003a80, node_delete=0x40b324 <QHash<unsigned long, logdata>::deleteNode2(QHashData::Node*)>) at tools/qhash.cpp:496 #6 0x000000000040b364 in QHash<unsigned long, logdata>::freeData (this=0x7f84180bb5e0, x=0x7f8418003a80) at /usr/include/x86_64-linux-gnu/qt5/QtCore/qhash.h:600 #7 0x000000000040f208 in QHash<unsigned long, logdata>::~QHash (this=0x7f84180bb5e0, __in_chrg=<optimized out>) at /usr/include/x86_64-linux-gnu/qt5/QtCore/qhash.h:310 after some days of hard debug, Qt source code reading and liters of green tea i have found that different instances of QHash (and also QMap) may share some data structures.

QtCreator 3.3 and Ubuntu (Unity) 14.10 global menu

To compile global menu plugin (as show in previous post) we need also one-line code modification due to using Qt 5.4 in QtCreator: --- appmenuplatformmenubar.cpp.orig 2014-12-12 14:02:46.059940656 +0100 +++ appmenuplatformmenubar.cpp 2014-12-12 14:02:55.943940215 +0100 @@ -301,7 +301,7 @@ KdeAppMenuPlatformTheme::KdeAppMenuPlatformTheme(const QString &kdeHome, int kdeVersion) - : QKdeTheme(kdeHome, kdeVersion) + : QKdeTheme(QStringList(kdeHome), kdeVersion) { }

Qt meta-object system

What is is and why we need it? From the Qt documentation we know that “Qt’s meta-object system provides the signals and slots mechanism for inter-object communication, run-time type information, and the dynamic property system.” [1]. I hope everyone knows what is signal-slot in Qt and why you need Q_OBJECT macro at the beginning of each class declaration. Meta-object is also used for object casting using qobject_cast template function. It’s much more safe then build-in C++ cast functions and often is much faster.

Enabling global menu in Qt Creator in Unity (Ubuntu 14.04)

I have just installed Qt 5.3.1 from qt-project.org and…. Qt Creator doesn’t supports global menu in Unity :-( Goggling doesn’t helps me at all. After a little bit tracing I see that it uses it’s own plugins directory and of course it have no libappmenu-qt5.so So we need just few commands to fix this: sudo apt−get install libgtk2.0−dev libdbusmenu−qt5−dev cd /tmp apt−get source appmenu−qt5 cd appmenu−qt5−0.3.0+14.04.20140415/src ~/Qt/5.3/gcc_64/bin/qmake src.pro make test