Dotaz: QT4 memory leak

[Martin Matějek]

Zdravím, v programu používám zatím samé QT widgety nebo třídy z nich odvozené, takže by se o jejich dealokaci mělo QT postarat. Zkusmo jsem to projel Valgrindem a objevil několik memory leaků při volání funkcí přilinkovaných knihoven. Možná, že jsem někde špatně (ne)přiřadil rodiče widgetů, ale radši jsem si napsal jednoduchý program, který má ty samé memory leaky.

mem.h

#include <QWidget>

class Mem : public QWidget
{
	public:
		Mem	(QWidget * parent = NULL);
};

mem.cpp

#include "mem.h"
#include <QVBoxLayout>
#include <QLabel>

	Mem::Mem	(QWidget * parent)
	: QWidget(parent)
{
	QVBoxLayout * vbox = new QVBoxLayout(this);

	QLabel * one = new QLabel("Hello");
	QLabel * two = new QLabel("world!");

	vbox->addWidget(one);
	vbox->addWidget(two);

	setLayout(vbox);
}

main.cpp

#include <QApplication>
#include "mem.h"

int main (int argc, char ** argv)
{
	QApplication app(argc,argv);

	Mem window;
	window.show();

	return app.exec();
}

A teď výpis z Valgrindu:

==26725== HEAP SUMMARY:
==26725==     in use at exit: 83,285 bytes in 1,696 blocks
==26725==   total heap usage: 17,743 allocs, 16,047 frees, 1,890,066 bytes allocated
==26725== 
==26725== LEAK SUMMARY:
==26725==    definitely lost: 192 bytes in 2 blocks
==26725==    indirectly lost: 912 bytes in 34 blocks
==26725==      possibly lost: 1,720 bytes in 9 blocks
==26725==    still reachable: 80,461 bytes in 1,651 blocks
==26725==         suppressed: 0 bytes in 0 blocks

Výpis s --leak-check=full

==27097== Memcheck, a memory error detector
==27097== Copyright (C) 2002-2010, and GNU GPL'd, by Julian Seward et al.
==27097== Using Valgrind-3.6.1 and LibVEX; rerun with -h for copyright info
==27097== Command: ./memory_leak
==27097== Parent PID: 24134
==27097== 
==27097== 
==27097== HEAP SUMMARY:
==27097==     in use at exit: 83,285 bytes in 1,696 blocks
==27097==   total heap usage: 17,438 allocs, 15,742 frees, 1,884,214 bytes allocated
==27097== 
==27097== 120 bytes in 1 blocks are possibly lost in loss record 116 of 170
==27097==    at 0x4024150: memalign (in /usr/lib/valgrind/vgpreload_memcheck-x86-linux.so)
==27097==    by 0x402420E: posix_memalign (in /usr/lib/valgrind/vgpreload_memcheck-x86-linux.so)
==27097==    by 0x509FFA9: ??? (in /usr/lib/libglib-2.0.so.0.2800.6)
==27097==    by 0x50A14C8: g_slice_alloc (in /usr/lib/libglib-2.0.so.0.2800.6)
==27097==    by 0x50A219D: g_slist_prepend (in /usr/lib/libglib-2.0.so.0.2800.6)
==27097==    by 0x50A516C: g_strsplit (in /usr/lib/libglib-2.0.so.0.2800.6)
==27097==    by 0x50BACB8: g_get_language_names (in /usr/lib/libglib-2.0.so.0.2800.6)
==27097==    by 0x50BB216: ??? (in /usr/lib/libglib-2.0.so.0.2800.6)
==27097==    by 0x50AD289: g_thread_init_glib (in /usr/lib/libglib-2.0.so.0.2800.6)
==27097==    by 0x50364B5: g_thread_init (in /usr/lib/libgthread-2.0.so.0.2800.6)
==27097==    by 0x4C61941: QEventDispatcherGlibPrivate::QEventDispatcherGlibPrivate(_GMainContext*) (in /usr/lib/qt/lib/libQtCore.so.4.7.0)
==27097==    by 0x4227BB5: ??? (in /usr/lib/qt/lib/libQtGui.so.4.7.0)
==27097== 
==27097== 124 bytes in 1 blocks are definitely lost in loss record 117 of 170
==27097==    at 0x4025BB8: malloc (in /usr/lib/valgrind/vgpreload_memcheck-x86-linux.so)
==27097==    by 0x53CCE52: ??? (in /usr/lib/libxcb.so.1.1.0)
==27097==    by 0x53CCF54: ??? (in /usr/lib/libxcb.so.1.1.0)
==27097==    by 0x53CC813: xcb_connect_to_display_with_auth_info (in /usr/lib/libxcb.so.1.1.0)
==27097==    by 0x53CCB5B: xcb_connect (in /usr/lib/libxcb.so.1.1.0)
==27097==    by 0x52D5652: _XConnectXCB (in /usr/lib/libX11.so.6.3.0)
==27097==    by 0x52C5476: XOpenDisplay (in /usr/lib/libX11.so.6.3.0)
==27097==    by 0x41FBC5C: ??? (in /usr/lib/qt/lib/libQtGui.so.4.7.0)
==27097==    by 0x417DAD7: QApplicationPrivate::construct(_XDisplay*, unsigned long, unsigned long) (in /usr/lib/qt/lib/libQtGui.so.4.7.0)
==27097==    by 0x417E342: QApplication::QApplication(int&, char**, int) (in /usr/lib/qt/lib/libQtGui.so.4.7.0)
==27097==    by 0x8049E7A: main (in /home/yenn/qt/memory_leak/memory_leak)
==27097== 
==27097== 360 bytes in 3 blocks are possibly lost in loss record 132 of 170
==27097==    at 0x4024150: memalign (in /usr/lib/valgrind/vgpreload_memcheck-x86-linux.so)
==27097==    by 0x402420E: posix_memalign (in /usr/lib/valgrind/vgpreload_memcheck-x86-linux.so)
==27097==    by 0x509FFA9: ??? (in /usr/lib/libglib-2.0.so.0.2800.6)
==27097==    by 0x50A14E5: g_slice_alloc (in /usr/lib/libglib-2.0.so.0.2800.6)
==27097==    by 0x50A219D: g_slist_prepend (in /usr/lib/libglib-2.0.so.0.2800.6)
==27097==    by 0x50A516C: g_strsplit (in /usr/lib/libglib-2.0.so.0.2800.6)
==27097==    by 0x50BACB8: g_get_language_names (in /usr/lib/libglib-2.0.so.0.2800.6)
==27097==    by 0x50BB216: ??? (in /usr/lib/libglib-2.0.so.0.2800.6)
==27097==    by 0x50AD289: g_thread_init_glib (in /usr/lib/libglib-2.0.so.0.2800.6)
==27097==    by 0x50364B5: g_thread_init (in /usr/lib/libgthread-2.0.so.0.2800.6)
==27097==    by 0x4C61941: QEventDispatcherGlibPrivate::QEventDispatcherGlibPrivate(_GMainContext*) (in /usr/lib/qt/lib/libQtCore.so.4.7.0)
==27097==    by 0x4227BB5: ??? (in /usr/lib/qt/lib/libQtGui.so.4.7.0)
==27097== 
==27097== 980 (68 direct, 912 indirect) bytes in 1 blocks are definitely lost in loss record 150 of 170
==27097==    at 0x4025C9E: realloc (in /usr/lib/valgrind/vgpreload_memcheck-x86-linux.so)
==27097==    by 0x52E9A57: ??? (in /usr/lib/libX11.so.6.3.0)
==27097==    by 0x52E9FD0: ??? (in /usr/lib/libX11.so.6.3.0)
==27097==    by 0x52EBC91: ??? (in /usr/lib/libX11.so.6.3.0)
==27097==    by 0x52EC4B4: _XlcCreateLC (in /usr/lib/libX11.so.6.3.0)
==27097==    by 0x530F509: _XlcUtf8Loader (in /usr/lib/libX11.so.6.3.0)
==27097==    by 0x52F43BB: _XOpenLC (in /usr/lib/libX11.so.6.3.0)
==27097==    by 0x52F467D: _XrmInitParseInfo (in /usr/lib/libX11.so.6.3.0)
==27097==    by 0x52DAA10: ??? (in /usr/lib/libX11.so.6.3.0)
==27097==    by 0x52DE1B7: XrmGetStringDatabase (in /usr/lib/libX11.so.6.3.0)
==27097==    by 0x52B9ADE: ??? (in /usr/lib/libX11.so.6.3.0)
==27097==    by 0x52B9D0E: XGetDefault (in /usr/lib/libX11.so.6.3.0)
==27097== 
==27097== 1,240 bytes in 5 blocks are possibly lost in loss record 156 of 170
==27097==    at 0x4024150: memalign (in /usr/lib/valgrind/vgpreload_memcheck-x86-linux.so)
==27097==    by 0x402420E: posix_memalign (in /usr/lib/valgrind/vgpreload_memcheck-x86-linux.so)
==27097==    by 0x509FFA9: ??? (in /usr/lib/libglib-2.0.so.0.2800.6)
==27097==    by 0x50A14E5: g_slice_alloc (in /usr/lib/libglib-2.0.so.0.2800.6)
==27097==    by 0x50545F8: g_array_sized_new (in /usr/lib/libglib-2.0.so.0.2800.6)
==27097==    by 0x50546D2: g_array_new (in /usr/lib/libglib-2.0.so.0.2800.6)
==27097==    by 0x50AD16E: g_static_private_set (in /usr/lib/libglib-2.0.so.0.2800.6)
==27097==    by 0x5064846: g_get_filename_charsets (in /usr/lib/libglib-2.0.so.0.2800.6)
==27097==    by 0x50648CC: ??? (in /usr/lib/libglib-2.0.so.0.2800.6)
==27097==    by 0x50AD279: g_thread_init_glib (in /usr/lib/libglib-2.0.so.0.2800.6)
==27097==    by 0x50364B5: g_thread_init (in /usr/lib/libgthread-2.0.so.0.2800.6)
==27097==    by 0x4C61941: QEventDispatcherGlibPrivate::QEventDispatcherGlibPrivate(_GMainContext*) (in /usr/lib/qt/lib/libQtCore.so.4.7.0)
==27097== 
==27097== LEAK SUMMARY:
==27097==    definitely lost: 192 bytes in 2 blocks
==27097==    indirectly lost: 912 bytes in 34 blocks
==27097==      possibly lost: 1,720 bytes in 9 blocks
==27097==    still reachable: 80,461 bytes in 1,651 blocks
==27097==         suppressed: 0 bytes in 0 blocks
==27097== Reachable blocks (those to which a pointer was found) are not shown.
==27097== To see them, rerun with: --leak-check=full --show-reachable=yes
==27097== 
==27097== For counts of detected and suppressed errors, rerun with: -v
==27097== ERROR SUMMARY: 5 errors from 5 contexts (suppressed: 75 from 9)

OS: Slackware-current, g++ 4.5.3, QT 4.7

Je to chyba v QT nebo dělám něco špatně já?

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Odpovědi

[Martin Tůma]

Nijak zvlášť jsem to nestudoval, ale IMHO minimálně neuklízíš vbox, one a two

[Martin Matějek]

Neuklízím, ale to bych snad ani neměl, teda pokud to chápu správně. Viz:

addWidget()
[...]
Note: The ownership of item is transferred to the layout, and it's the layout's responsibility to delete it

Teď jsem zkusil přidat destruktor, který smaže vbox (a tím pádem i objekty v layoutu), ale žádná změna. Tím jsem, hádám, akorát udělal totéž, co by udělal interní „garbage collector“, protože vbox „vlastní“ objekty v layoutu a instance třídy je rodič (čili vlastní) layout. Na konci běhu programu se smaže staticky naalokovaný widget (v main() ) a smaže všechno pod sebou. Nebo ne?

[Martin Tůma]

Pokuď layout manager uklízí sám své widgety a widget svůj layout manager, pak je všechno v pořádku.

[David Watzke]

Dokonce ani není třeba psát "(this)" při vytváření layoutu, protože setLayout() se o tohle postará.

[Martin Matějek]

Vypadá to v pořádku. Přelož tohle do angličtiny a hoď to do Qt bugzilly.

[Martin Matějek]

Ok, napíšu bugreport.

[Martin Matějek]

Možná tam žádný memory leak nemáš. Když použiješ například glib, Xlib, a další knihovny, tak některé z nich si alokují paměť, kterou už nikdy neuvolní. Nevím teď přesně, jak se tomu říká. Tuto paměť vidí valgrind jako memory leak, ale ve skutečnosti to leak není (tato paměť je alokovaná jen 1x).

Pohledej na netu, valgrind se dá nakonfigurovat tak, aby "leaky" z takových knihoven ignoroval a jeho výstup byl užitečnější.

[:::]

presne tak, pokud je nejaka pamet potreba celou dobu behu programu, tak uvolnovat ji tesne pred koncem by byla ztrata casu...

[Martin Matějek]

Je to tak jak píšete, díky za vysvětlení. Bugreport jsem ještě neposlal, protože jsem čekal na další komentáře a teď ho ani posílat nebudu.

[chochi]

Napr. memory leak v xcb_connect_to_display_with_auth_info je chyba opravena v libxcb 1.7:

Release 1.7 (2010-08-13)
========================
- Always wake up readers after writing
- Get rid of PATH_MAX and MAXPATHLEN
- Add ~ operator support in code generator
- xcb_open: Improve protocol/host parsing
- xcb_connect_to_display_with_auth_info: Fix memory leak
- Report which extensions are being built

Dost z tech glib leaku je staticky alokovana pamet, ktera se alokoje jen jednou pri startu - nemela by mit vliv na program.