Debugging Techniques

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.

With Frameworks:

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.

Without Frameworks:

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.

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.

Option	Description
-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.

Environment Variables Recognized by Qt

At runtime, a Qt application recognizes many environment variables, some of which can be helpful for debugging:

Variable	Description
QT_DEBUG_PLUGINS	Set to a non-zero value to make Qt print out diagnostic information about the each (C++) plugin it tries to load.
QML_IMPORT_TRACE	Set to a non-zero value to make QML print out diagnostic information from the import loading mechanism.
QT_HASH_SEED	Set to an integer value to disable QHash and QSet using a new random ordering for each application run, which in some cases might make testing and debugging difficult.

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 messages 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 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.

By default, only the message is printed. You can include additional information by setting the QT_MESSAGE_PATTERN environment variable. For example:

QT_MESSAGE_PATTERN="[%{type}] %{appname} (%{file}:%{line}) - %{message}"

Placeholder	Description
%{appname}	QCoreApplication::applicationName()
%{file}	Path to source file
%{function}	Function
%{line}	Line in source file
%{message}	The actual message
%{pid}	QCoreApplication::applicationPid()
%{threadid}	ID of current thread
%{type}	"debug", "warning", "critical" or "fatal"

You can also install your own message handler using qInstallMessageHandler().

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.

Both qDebug() and qWarning() are debugging tools. They can be compiled away by defining QT_NO_DEBUG_OUTPUT and QT_NO_WARNING_OUTPUT during compilation.

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.

QDebug operator<<(QDebug dbg, const Coordinate &c)
{
    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.

Debugging Macros

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];
    Q_CHECK_PTR(ptr);
    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 uninitialized 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.

Common Bugs

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.