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# Debugging Techniques

Here we present some useful hints to help you with debugging your Qt-based software.

## 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.
`-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, and to the debugger under Windows. 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.

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.

The Object Inspector, is also an option for Qt Commercial Edition licensees.

## Debugging Macros

The header file `<QtGlobal>` contains some debugging macros and `#define`s.

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 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.

## 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.