Hi all. I’m starting a new thread since the focal question has completely changed. I’ve tracked down the cause of the crash that I discussed in the previous thread, to a specific bit of code:
————————
static QFont *tickFont = nullptr;
if (!tickFont)
{
tickFont = new QFont();
tickFont->setPointSize(9);
}
painter.setFont(*tickFont);
————————
I do this inside a QOpenGLWidget subclass, in my paintGL() override. If I comment this code out, no more crash on quit. I figured this out because the backtrace for my crash seemed to involve private font cache teardown in QApplication, so I figured maybe I was doing something naughty involved QFonts, so I commented out every usage of QFont in my app, and then started uncommenting them one by one; when I uncommented this one, it crashes on quit. (I got AddressSanitizer working, but it didn’t provide any additional info; I’m still working on getting Valgrind working on macOS 10.15.)
So. What is wrong with the pattern above? I’m surprised that this crashes. My intent is simply to keep this QFont instance permanently allocated, so that whatever overhead is associated with making a new one doesn’t occur each time my view repaints (which happens at ~60Hz, ideally). That is perhaps premature optimization, which is the root of all evil, I realize. But I want to understand *why* this pattern is bad, and what I might do instead, if the font creation were to indeed be a bottleneck. Obviously there’s something about memory management in Qt that I don’t understand, because I don’t see anything wrong with this code! :-> Thanks for any help.
Cheers,
-B.
Benjamin C. Haller
Messer Lab
Cornell University
> On Sep 23, 2020, at 7:50 AM, Ben Haller via Interest <interest at qt-project.org> wrote:
>
> Hi all. I’m trying to figure out what appears to be a double dealloc problem of some sort. I’m sure the actual bug is in my code (I’m trying to get a window with complex subsidiary windows, etc., to disassemble and free the things it owns, without freeing itself), but the backtrace at the point that the crash happens doesn’t have any of my code in it at all. I’ve appended the full backtrace below my signature, in case it is informative; maybe somebody has seen this pattern before and knows what I’m likely to be doing wrong. My code does not use QFontCache or QScopedPointer at all, so it’s hard to guess where my bug might be.
>
> My question, though, is bigger: how would folks here recommend debugging a double dealloc in a Qt-based app, running under Qt Creator? I’m coming from the Cocoa/Xcode world, and in that world there’s a very nice tool called NSZombie: you set a flag, and then whenever an object would be deallocated, it instead turns into an NSZombie object that produces nice console logs if anybody ever tries to use that memory as an object again. But that’s for NSObject. Is there something similar in the Qt world, such that I could get a console log or a break in the debugger at the point where a C++/Qt object that has been dealloced gets touched again? Or if not, how would you recommend approaching this? Thanks for any help.
>
> Cheers,
> -B.
>
> Benjamin C. Haller
> Messer Lab
> Cornell University
>
>
> 1 QScopedPointer<QObjectData, QScopedPointerDeleter<QObjectData>>::operator->() const qscopedpointer.h 118 0x10201b594
> 2 decltype(fp.operator->()) qGetPtrHelper<QScopedPointer<QObjectData, QScopedPointerDeleter<QObjectData>> const>(QScopedPointer<QObjectData, QScopedPointerDeleter<QObjectData>> const&) qglobal.h 1133 0x10201b594
> 3 QObject::d_func() const qobject.h 132 0x10201b594
> 4 QObject::thread() const qobject.cpp 1512 0x10201b594
> 5 QOpenGLVertexArrayObjectPrivate::destroy() qopenglvertexarrayobject.cpp 212 0x101b21481
> 6 QOpenGLVertexArrayObject::destroy() qopenglvertexarrayobject.cpp 424 0x101b217cc
> 7 QOpenGLVertexArrayObject::~QOpenGLVertexArrayObject() qopenglvertexarrayobject.cpp 392 0x101b217c3
> 8 QOpenGLVertexArrayObject::~QOpenGLVertexArrayObject() qopenglvertexarrayobject.cpp 391 0x101b217b9
> 9 QOpenGLTextureGlyphCache::~QOpenGLTextureGlyphCache() qopengltextureglyphcache.cpp 93 0x101b1cacd
> 10 QOpenGLTextureGlyphCache::~QOpenGLTextureGlyphCache() qopengltextureglyphcache.cpp 88 0x101b1cbce
> 11 QOpenGLTextureGlyphCache::~QOpenGLTextureGlyphCache() qopengltextureglyphcache.cpp 88 0x101b1cbc9
> 12 QExplicitlySharedDataPointer<QFontEngineGlyphCache>::~QExplicitlySharedDataPointer() qshareddata.h 184 0x101883c92
> 13 QExplicitlySharedDataPointer<QFontEngineGlyphCache>::~QExplicitlySharedDataPointer() qshareddata.h 184 0x101883c74
> 14 QFontEngine::GlyphCacheEntry::~GlyphCacheEntry() qfontengine.cpp 1570 0x101883c74
> 15 QFontEngine::GlyphCacheEntry::~GlyphCacheEntry() qfontengine.cpp 1569 0x101883c74
> 16 void std::allocator_traits<std::allocator<std::__list_node<QFontEngine::GlyphCacheEntry, void *>>>::__destroy<QFontEngine::GlyphCacheEntry>(std::integral_constant<bool, false>, std::allocator<std::__list_node<QFontEngine::GlyphCacheEntry, void *>>&, QFontEngine::GlyphCacheEntry *) memory 1732 0x101883c74
> 17 void std::allocator_traits<std::allocator<std::__list_node<QFontEngine::GlyphCacheEntry, void *>>>::destroy<QFontEngine::GlyphCacheEntry>(std::allocator<std::__list_node<QFontEngine::GlyphCacheEntry, void *>>&, QFontEngine::GlyphCacheEntry *) memory 1595 0x101883c74
> 18 std::__list_imp<QFontEngine::GlyphCacheEntry, std::allocator<QFontEngine::GlyphCacheEntry>>::clear() list 733 0x101883c74
> 19 std::__list_imp<QFontEngine::GlyphCacheEntry, std::allocator<QFontEngine::GlyphCacheEntry>>::~__list_imp() list 712 0x101883c24
> 20 std::list<QFontEngine::GlyphCacheEntry>::~list() list 805 0x101883c24
> 21 std::list<QFontEngine::GlyphCacheEntry>::~list() list 805 0x101883c24
> 22 QHashNode<void const *, std::list<QFontEngine::GlyphCacheEntry>>::~QHashNode() qhash.h 147 0x101883c24
> 23 QHashNode<void const *, std::list<QFontEngine::GlyphCacheEntry>>::~QHashNode() qhash.h 147 0x101883c24
> 24 QHash<void const *, std::list<QFontEngine::GlyphCacheEntry>>::deleteNode2(QHashData::Node *) qhash.h 563 0x101883c24
> 25 QHashData::free_helper(void ( *)(QHashData::Node *)) qhash.cpp 572 0x101e5db39
> 26 QHash<void const *, std::list<QFontEngine::GlyphCacheEntry>>::freeData(QHashData *) qhash.h 603 0x10187a47b
> 27 QHash<void const *, std::list<QFontEngine::GlyphCacheEntry>>::~QHash() qhash.h 250 0x10187a46f
> 28 QHash<void const *, std::list<QFontEngine::GlyphCacheEntry>>::~QHash() qhash.h 250 0x10187a44f
> 29 QFontEngine::~QFontEngine() qfontengine.cpp 277 0x10187a44f
> 30 QCoreTextFontEngine::~QCoreTextFontEngine() qfontengine_coretext.mm 215 0x105b7cf8e
> 31 QCoreTextFontEngine::~QCoreTextFontEngine() qfontengine_coretext.mm 215 0x105b7cf89
> 32 QFontEngineMulti::~QFontEngineMulti() qfontengine.cpp 1782 0x101880039
> 33 QFontEngineMulti::~QFontEngineMulti() qfontengine.cpp 1778 0x10188012e
> 34 QFontEngineMulti::~QFontEngineMulti() qfontengine.cpp 1778 0x101880129
> 35 QFontCache::clear() qfont.cpp 2861 0x1018762eb
> 36 QFontCache::~QFontCache() qfont.cpp 2817 0x10187603c
> 37 QFontCache::~QFontCache() qfont.cpp 2816 0x1018763ee
> 38 QFontCache::~QFontCache() qfont.cpp 2816 0x1018763e9
> 39 QThreadStorageData::set(void *) qthreadstorage.cpp 162 0x101e25777
> 40 QGuiApplicationPrivate::~QGuiApplicationPrivate() qguiapplication.cpp 1694 0x1017ca490
> 41 QApplicationPrivate::~QApplicationPrivate() qapplication.cpp 184 0x10116f81e
> 42 QApplicationPrivate::~QApplicationPrivate() qapplication.cpp 184 0x10116f819
> 43 QScopedPointerDeleter<QObjectData>::cleanup(QObjectData *) qscopedpointer.h 60 0x10201c06c
> 44 QScopedPointer<QObjectData, QScopedPointerDeleter<QObjectData>>::~QScopedPointer() qscopedpointer.h 107 0x10201c061
> 45 QScopedPointer<QObjectData, QScopedPointerDeleter<QObjectData>>::~QScopedPointer() qscopedpointer.h 105 0x10201c05d
> 46 QObject::~QObject() qobject.cpp 1119 0x10201c05d
> 47 QApplication::~QApplication() qapplication.cpp 837 0x101171b3d
> 48 main main.cpp 246 0x100012256
> 49 start (x86_64) /usr/lib/system/libdyld.dylib 0x7fff6b461cc9
>
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