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@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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> https://lists.qt-project.org/listinfo/interest
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