Here we present some useful hints to help you with debugging your Qt-based software.
Configuring Qt for Debugging
When configuring Qt for installation, it is possible to ensure that it is built to include debug symbols that can make it easier to track bugs in applications and libraries. However, on some platforms, building Qt in debug mode will cause applications to be larger than desirable.
Debugging in Mac OS X and Xcode
Debugging With/Without Frameworks
The basic stuff you need to know about debug libraries and frameworks is found at developer.apple.com in: Apple Technical Note TN2124.
When you build Qt, frameworks are built by default, and inside the framework you will find both a release and a debug version (e.g., QtCore and QtCore_debug). If you pass the -no-framework flag when you build Qt, two dylibs are built for each Qt library (e.g., libQtCore.4.dylib and libQtCore_debug.4.dylib).
What happens when you link depends on whether you use frameworks or not. We don't see a compelling reason to recommend one over the other.
Since the release and debug libraries are inside the framework, the app is simply linked against the framework. Then when you run in the debugger, you will get either the release version or the debug version, depending on whether you set DYLD_IMAGE_SUFFIX. If you don't set it, you get the release version by default (i.e., non _debug). If you set DYLD_IMAGE_SUFFIX=_debug, you get the debug version.
When you tell qmake to generate a Makefile with the debug config, it will link against the _debug version of the libraries and generate debug symbols for the app. Running this program in GDB will then work like running GDB on other platforms, and you will be able to trace inside Qt.
Debug Symbols and Size
The amount of space taken up by debug symbols generated by GCC can be excessively large. However, with the release of Xcode 2.3 it is now possible to use Dwarf symbols which take up a significantly smaller amount of space. To enable this feature when configuring Qt, pass the -dwarf-2 option to the configure script.
This is not enabled by default because previous versions of Xcode will not work with the compiler flag used to implement this feature. Mac OS X 10.5 will use dwarf-2 symbols by default.
dwarf-2 symbols contain references to source code, so the size of the final debug application should compare favorably to a release build.
Command Line Options Recognized by Qt
When you run a Qt application, you can specify several command-line options that can help with debugging. These are recognized by QApplication.
|-nograb||The application should never grab the mouse or the keyboard. This option is set by default when the program is running in the gdb debugger under Linux.|
|-dograb||Ignore any implicit or explicit -nograb. -dograb wins over -nograb even when -nograb is last on the command line.|
|-sync||Runs the application in X synchronous mode. Synchronous mode forces the X server to perform each X client request immediately and not use buffer optimization. It makes the program easier to debug and often much slower. The -sync option is only valid for the X11 version of Qt.|
Warning and Debugging Messages
Qt includes four global functions for writing out warning and debug text. You can use them for the following purposes:
- qDebug() is used for writing custom debug output.
- qWarning() is used to report warnings and recoverable errors in your application.
- qCritical() is used for writing critical error mesages and reporting system errors.
- qFatal() is used for writing fatal error messages shortly before exiting.
If you include the <QtDebug> header file, the qDebug() function can also be used as an output stream. For example:
- qDebug() << "Widget" << widget << "at position" << widget->pos();
The Qt implementation of these functions prints the text to the stderr output under Unix/X11 and Mac OS X. With Windows, if it is a console application, the text is sent to console; otherwise, it is sent to the debugger. You can take over these functions by installing a message handler using qInstallMsgHandler().
If the QT_FATAL_WARNINGS environment variable is set, qWarning() exits after printing the warning message. This makes it easy to obtain a backtrace in the debugger.
The debugging functions QObject::dumpObjectTree() and QObject::dumpObjectInfo() are often useful when an application looks or acts strangely. More useful if you use object names than not, but often useful even without names.
Providing Support for the qDebug() Stream Operator
You can implement the stream operator used by qDebug() to provide debugging support for your classes. The class that implements the stream is QDebug . The functions you need to know about in QDebug are space() and nospace(). They both return a debug stream; the difference between them is whether a space is inserted between each item. Here is an example for a class that represents a 2D coordinate.
- dbg.nospace() << "(" << c.x() << ", " << c.y() << ")";
- return dbg.space();
Integration of custom types with Qt's meta-object system is covered in more depth in the Creating Custom Qt Types document.
The header file <QtGlobal> contains some debugging macros and #defines.
Three important macros are:
- Q_ASSERT(cond), where cond is a boolean expression, writes the warning "ASSERT: 'cond' in file xyz.cpp, line 234" and exits if cond is false.
- Q_ASSERT_X(cond, where, what), where cond is a boolean expression, where a location, and what a message, writes the warning: "ASSERT failure in where: 'what', file xyz.cpp, line 234" and exits if cond is false.
- Q_CHECK_PTR(ptr), where ptr is a pointer. Writes the warning "In file xyz.cpp, line 234: Out of memory" and exits if ptr is 0.
These macros are useful for detecting program errors, e.g. like this:
- char *alloc(int size)
- Q_ASSERT(size > 0);
- char *ptr = new char[size];
- return ptr;
Q_ASSERT(), Q_ASSERT_X(), and Q_CHECK_PTR() expand to nothing if QT_NO_DEBUG is defined during compilation. For this reason, the arguments to these macro should not have any side-effects. Here is an incorrect usage of Q_CHECK_PTR():
- char *alloc(int size)
- char *ptr;
- Q_CHECK_PTR(ptr = new char[size]); // WRONG
- return ptr;
If this code is compiled with QT_NO_DEBUG defined, the code in the Q_CHECK_PTR() expression is not executed and alloc returns an unitialized pointer.
The Qt library contains hundreds of internal checks that will print warning messages when a programming error is detected. We therefore recommend that you use a debug version of Qt when developing Qt-based software.
There is one bug that is so common that it deserves mention here: If you include the Q_OBJECT macro in a class declaration and run the meta-object compiler (moc), but forget to link the moc-generated object code into your executable, you will get very confusing error messages. Any link error complaining about a lack of vtbl, _vtbl, __vtbl or similar is likely to be a result of this problem.