MainWindow::event crash when doing textEdit->append

Maarten Verhage

Dear Qt experts,

I appreciate the people who are helping everyone out here with Qt development issues. That's so nice!

I'm using the static Qt 5.3.2 version for Windows 7 SP1, 64 bit. My compiler is GCC 5.1.0.

I have a very small Qt program with just a QTextEdit as the central widget. If I reimplement for example the QMainWindow::enterEvent function I can write something to my textEdit widget. But when I reimplement just the event function. Then the program crashes without even displaying the mainwindow briefly. I got an exception code c0000005 then from Windows.

here are my files:

main.cpp

#include <QApplication>

#include <QtPlugin>
Q_IMPORT_PLUGIN(QWindowsIntegrationPlugin)
#include "mainwindow.h"

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

  MainWindow window;
  window.resize(500, 400);
  window.show();

  return app.exec();
}

mainwindow.h

#ifndef MAINWINDOW_H
#define MAINWINDOW_H

#include <QMainWindow>
#include <QTextEdit>

class MainWindow : public QMainWindow
{
  Q_OBJECT
  public:
    explicit MainWindow();
  protected:
    void enterEvent( QEvent* /* event */ );
    bool event( QEvent* /* event */ );
  private:
    QTextEdit* text;
};

#endif

mainwindow.cpp

#include "mainwindow.h"

MainWindow::MainWindow()
{
  text = new QTextEdit( this );
  text->setReadOnly( true );
  text->setMinimumSize( 400, 300 );
  setCentralWidget( text );

  text->append( tr("Initial textedit contents ") );
}

void MainWindow::enterEvent( QEvent* event )
{
  text->append( tr("enterEvent ") ); // this works fine
  QMainWindow::enterEvent( event );
}

bool MainWindow::event( QEvent* event )
{
  //text->append( tr("Event ") ); // this line crashes the program
  return QMainWindow::event(event);
}

Am I doing anything wrong here? Would it be possible somehow to append something to the text edit from the event function?

Best regards,
Maarten Verhage

dheerendra

event() function is called repeatedly and finally it crashes. You can put debug an d check in your event() function.

J.Hilk

@Maarten-Verhage to extend a bit on what @dheerendra said,

bool MainWindow::event( QEvent* event ) catches all the events, and I believe some actually fire before the initialisation of QMainWindow is finished.

Try to init the text pointer with 0 and check against it:

 private:
    QTextEdit* text = Q_NULLPTR;

bool MainWindow::event( QEvent* event )
{
  if(text)
      text->append( tr("Event ") ); 
  
  return QMainWindow::event(event);
}

jsulm

@Maarten-Verhage Strange. Could be that text is not yet initialized when you get first even (it should actually not be the case). You can try:

// In class MainWindow
QTextEdit* text = nullptr; // Requires C++11
...
bool MainWindow::event( QEvent* event )
{
  if (text != nullptr)
      text->append( tr("Event ") ); // this line crashes the program
  return QMainWindow::event(event);
}

Maarten Verhage

Dear people,

Thanks for the suggestions. I tried suggestion from @J-Hilk and @jsulm, both resulted in a mainwindow that actually displays, but immediately crashes as well but now with exception code c00000fd. A quick google tells me that is a stack overflow.

Putting a virtual keyword in front of my event function in the header like QObject does neither helps.

I might be useful to see my makefile. Am I missing some compiler flag maybe?

FLAGS = -std=c++14 -pedantic -Wextra -Wlogical-op
BINPATHQT = $(QT)\qtbase\bin
INCPATHQT = -I"$(QT)\qtbase\include" -I"$(QT)\qtbase\include\qtcore" -I"$(QT)\qtbase\include\qtgui" -I"$(QT)\qtbase\include\qtwidgets"
LIBPATHQT = -L"$(QT)\qtbase\lib" -L"$(QT)\qtbase\plugins\platforms"
LIBSQT = -l:libqwindows.a -l:libQt5PlatformSupport.a\
	-l:libQt5Widgets.a -l:libQt5Gui.a -l:libQt5Core.a
LIBSWIN = -l:libkernel32.a -l:libuser32.a -l:libshell32.a -l:libadvapi32.a\
	-l:libws2_32.a -l:liboleaut32.a -l:libimm32.a -l:libwinmm.a -l:libole32.a\
	-l:libuuid.a -l:libopengl32.a -l:libole32.a -l:libgdi32.a

OBJFILES = main.o\
	mainwindow.o\
	moc_mainwindow.o

all: test.exe

test.exe : $(OBJFILES)
	g++ -static -mwindows $(LIBPATHQT) $(OBJFILES) $(LIBSQT) $(LIBSWIN) -o test.exe

main.o : main.cpp
	g++ -c $(INCPATHQT) $(FLAGS) main.cpp

mainwindow.o : mainwindow.cpp
	g++ -c $(INCPATHQT) $(FLAGS) mainwindow.cpp

moc_mainwindow.o : moc_mainwindow.cpp
	g++ -c $(INCPATHQT) $(FLAGS) moc_mainwindow.cpp

moc_mainwindow.cpp : mainwindow.h
	$(BINPATHQT)\moc mainwindow.h -o moc_mainwindow.cpp

clean:
	del *.o *.exe

Maarten Verhage

Dear people,

Now I also tried to build it the Qt way. i.e. with a pro file:

QT += widgets

HEADERS	    =	mainwindow.h
SOURCES	    =	mainwindow.cpp \
		main.cpp
CONFIG += release

then commands:

qmake -makefile test.pro
mingw32-make -f makefile

Still the same c00000fd crash. I hope somebody can help me out on this one.

Oh yeah, CONFIG += debug neither works.
Regards, Maarten

ambershark

@Maarten-Verhage What is the stack trace from the crash? This should give us a lot of insight as to what is happening.

It definitely sounds like you are using the text object before it is fully ready.

You can always add a variable that lets you know when the initialization is complete and it's ok to use the text window.

ambershark

@ambershark I ran it for you and put up a backtrace. It is definitely what everyone thought. The QTextEdit is not ready when you try to append to it.

Thread 1 "maincrash" received signal SIGSEGV, Segmentation fault.
0x00007ffff788bad8 in QTextEdit::isReadOnly() const ()
   from /usr/local/Qt/lib/libQt5Widgets.so.5
(gdb) bt
#0  0x00007ffff788bad8 in QTextEdit::isReadOnly() const ()
   from /usr/local/Qt/lib/libQt5Widgets.so.5
#1  0x00007ffff788c1e6 in QTextEdit::append(QString const&) ()
   from /usr/local/Qt/lib/libQt5Widgets.so.5
#2  0x0000555555556980 in MainWindow::event(QEvent*) ()
#3  0x00007ffff76f9dcc in QApplicationPrivate::notify_helper(QObject*, QEvent*)
    () from /usr/local/Qt/lib/libQt5Widgets.so.5
#4  0x00007ffff7701236 in QApplication::notify(QObject*, QEvent*) ()
   from /usr/local/Qt/lib/libQt5Widgets.so.5

I added a debug message to see the event item and here is the output:

[shockwave] ~/tmp/maincrash > ./maincrash 
0x7ffc69b631f0 QEvent::Type(ChildAdded)
Segmentation fault (core dumped)

As you can see it is a ChildAdded event, more than likely the one adding the textedit to the mainwindow. Since it's not ready you can't update it.

Maarten Verhage

Ok, the first version had the issue that event was called before the text object was fully ready. Now with the check for the nullptr or Q_NULLPTR that is resolved. Then I ran into the issue of the exception c00000fd AKA stack overflow. You know what I think, text->append generates an event on it's own leading to repeated calls to the event function and ultimately causing stack overflow.

I will check this later today. Maybe by ignoring the event that is caused by the append function.

Regards, Maarten

SGaist

Hi,

Since it's to show a default text at startup, rather than append, what about using setText ?