Valgrind on Basic Qt Application: Reports Masive Leaks

Question

So I wanted to see what would happen to a basic Qt4 Application compiled from a simple Makefile when run through Valgrind. What would be causing these leaks? I am deleting any dynamically allocated objects on my end. Also when exiting the program I only click on the close (X) up in the upper right corner. Do GUI libs just not "free()" any of the memory they malloc()'d?

The Makefile reduced from qmake command is:

####### Compiler, tools and options

CXX           = g++
CXXFLAGS      = -pipe -O2 -Wall -W
LIBS_INCLUDE  = ${HOME}
INCPATH       = -I$(LIBS_INCLUDE)/QtSDK/Desktop/Qt/473/gcc/mkspecs/default \
                -I. \
                -I$(LIBS_INCLUDE)/QtSDK/Desktop/Qt/473/gcc/include/QtCore \
                -I$(LIBS_INCLUDE)/QtSDK/Desktop/Qt/473/gcc/include/QtGui \
                -I$(LIBS_INCLUDE)/QtSDK/Desktop/Qt/473/gcc/include 
LINK          = g++
LFLAGS        = -Wl,-O1 -Wl,-rpath,$(LIBS_INCLUDE)/QtSDK/Desktop/Qt/473/gcc/lib
LIBS          = $(SUBLIBS)  -L$(LIBS_INCLUDE)/QtSDK/Desktop/Qt/473/gcc/lib -lQtGui -lQtCore -lpthread
QMAKE         = $(LIBS_INCLUDE)/QtSDK/Desktop/Qt/473/gcc/bin/qmake
OBJECTS_DIR   = ./
SOURCES       = main.cpp
OBJECTS       = main.o
QMAKE_TARGET  = Main
TARGET        = Main

####### Build rules
first: all

all: $(TARGET)

$(TARGET):  $(OBJECTS)
        $(LINK) $(LFLAGS) -o $(TARGET) $(OBJECTS) $(OBJCOMP) $(LIBS)

####### Compile

main.o: main.cpp
        $(CXX) -c $(CXXFLAGS) $(INCPATH) -o main.o main.cpp

The only source code is:

int main(int argc, char *argv[])
{
    QApplication app(argc, argv);
    QWidget window;
    window.resize( 320,240 );
    window.setWindowTitle( 
        QApplication::translate( "toplevel", "Top-level Widget" ) );
    window.show( );

    QPushButton *button = new QPushButton(
        QApplication::translate( "childwidget", "Press me"), &window );
    button->move( 100, 100 );
    button->show( );
    delete button;
    return app.exec();
}

When run through valgrind I get the following (Removed Large Portion In Middle):

==3836== Memcheck, a memory error detector
==3836== Copyright (C) 2002-2009, and GNU GPL'd, by Julian Seward et al.
==3836== Using Valgrind-3.5.0 and LibVEX; rerun with -h for copyright info
==3836== Command: ./Main
==3836== 
Xlib:  extension "RANDR" missing on display ":0.0".
==3836== 
==3836== HEAP SUMMARY:
==3836==     in use at exit: 924,383 bytes in 8,679 blocks
==3836==   total heap usage: 37,234 allocs, 28,555 frees, 4,314,180 bytes allocated
==3836== 
==3836== 1 bytes in 1 blocks are possibly lost in loss record 1 of 4,534
==3836==    at 0x400677E: malloc (vg_replace_malloc.c:195)
==3836==    by 0xA1DFA4: g_malloc (in /lib/libglib-2.0.so.0.2600.0)
==3836==    by 0xA37F29: g_strdup (in /lib/libglib-2.0.so.0.2600.0)
==3836==    by 0xB2A6FA: g_param_spec_string (in /lib/libgobject-2.0.so.0.2600.0)
==3836==    by 0x41F36473: ??? (in /usr/lib/libgtk-x11-2.0.so.0.2200.0)
==3836==    by 0xB3D237: g_type_class_ref (in /lib/libgobject-2.0.so.0.2600.0)
==3836==    by 0xB20B38: g_object_newv (in /lib/libgobject-2.0.so.0.2600.0)
==3836==    by 0xB212EF: g_object_new (in /lib/libgobject-2.0.so.0.2600.0)
==3836==    by 0x41F34857: gtk_settings_get_for_screen (in /usr/lib/libgtk-x11-2.0.so.0.2200.0)
==3836==    by 0x41ED0CB6: ??? (in /usr/lib/libgtk-x11-2.0.so.0.2200.0)
==3836==    by 0xB377C7: g_cclosure_marshal_VOID__OBJECT (in /lib/libgobject-2.0.so.0.2600.0)
==3836==    by 0xB1ABE2: g_closure_invoke (in /lib/libgobject-2.0.so.0.2600.0)
==3836== 
...
==3836== 
==3836== 23,048 bytes in 1 blocks are possibly lost in loss record 4,531 of 4,534
==3836==    at 0x400677E: malloc (vg_replace_malloc.c:195)
==3836==    by 0x16F42D: ??? (in /usr/lib/libfreetype.so.6.6.0)
==3836==    by 0x17400B: ft_mem_qalloc (in /usr/lib/libfreetype.so.6.6.0)
==3836==    by 0x174063: ft_mem_alloc (in /usr/lib/libfreetype.so.6.6.0)
==3836==    by 0x174508: ft_mem_qrealloc (in /usr/lib/libfreetype.so.6.6.0)
==3836==    by 0x17457F: ft_mem_realloc (in /usr/lib/libfreetype.so.6.6.0)
==3836==    by 0x1A2E52: ??? (in /usr/lib/libfreetype.so.6.6.0)
==3836==    by 0x1A709B: ??? (in /usr/lib/libfreetype.so.6.6.0)
==3836==    by 0x180338: ??? (in /usr/lib/libfreetype.so.6.6.0)
==3836==    by 0x175B5D: ??? (in /usr/lib/libfreetype.so.6.6.0)
==3836==    by 0x1780B0: FT_Open_Face (in /usr/lib/libfreetype.so.6.6.0)
==3836==    by 0x1791FF: FT_New_Face (in /usr/lib/libfreetype.so.6.6.0)
==3836== 
==3836== 53,244 bytes in 29 blocks are possibly lost in loss record 4,534 of 4,534
==3836==    at 0x400677E: malloc (vg_replace_malloc.c:195)
==3836==    by 0xA1DFA4: g_malloc (in /lib/libglib-2.0.so.0.2600.0)
==3836==    by 0xA36050: g_slice_alloc (in /lib/libglib-2.0.so.0.2600.0)
==3836==    by 0xA36315: g_slice_alloc0 (in /lib/libglib-2.0.so.0.2600.0)
==3836==    by 0xB40077: g_type_create_instance (in /lib/libgobject-2.0.so.0.2600.0)
==3836==    by 0xB1CE35: ??? (in /lib/libgobject-2.0.so.0.2600.0)
==3836==    by 0xB205C6: g_object_newv (in /lib/libgobject-2.0.so.0.2600.0)
==3836==    by 0xB212EF: g_object_new (in /lib/libgobject-2.0.so.0.2600.0)
==3836==    by 0x54B8FA3: ??? (in /usr/lib/gtk-2.0/2.10.0/engines/libclearlooks.so)
==3836==    by 0x41F0CDDD: ??? (in /usr/lib/libgtk-x11-2.0.so.0.2200.0)
==3836==    by 0x41F11C24: gtk_rc_get_style (in /usr/lib/libgtk-x11-2.0.so.0.2200.0)
==3836==    by 0x4200A81F: ??? (in /usr/lib/libgtk-x11-2.0.so.0.2200.0)
==3836== 
==3836== LEAK SUMMARY:
==3836==    definitely lost: 1,912 bytes in 7 blocks
==3836==    indirectly lost: 5,060 bytes in 250 blocks
==3836==      possibly lost: 491,358 bytes in 2,893 blocks
==3836==    still reachable: 426,053 bytes in 5,529 blocks
==3836==         suppressed: 0 bytes in 0 blocks
==3836== Reachable blocks (those to which a pointer was found) are not shown.
==3836== To see them, rerun with: --leak-check=full --show-reachable=yes
==3836== 
==3836== For counts of detected and suppressed errors, rerun with: -v
==3836== ERROR SUMMARY: 1336 errors from 1336 contexts (suppressed: 114 from 11)

Answer 1

I compiled your short application and (using the visual studio crt memoryleak detector) could not find any memory leaks. So whatever valgrind reports is not directly related to your code.

However, you are deleting the QPushButton before the application is actually running. Usually ownership of QWindows is transfered to the parent window, so you don't need to cleanup the objects by yourself, the parent window handles that for you (if this is not the case, it's stated in the documentation).

So, to correct your code, just delete the delete.