Debugging Techniques | Qt 5.15

- Qt 5.15
- Debugging Techniques
- Qt 5.15.1 Reference Documentation

# 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 macOS 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 global macros for writing out warning and debug text. You can use them for the following purposes:

- qDebug() is used for writing custom debug output.
- qInfo() is used for informational messages.
- 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()` macro can also be used as an output stream. For example:

     qDebug() << "Widget" << widget << "at position" << widget->pos();

The Qt implementation of these macros prints to the `stderr` output under Unix/X11 and macOS. 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}"

The format is documented in qSetMessagePattern(). 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.

qDebug(), qInfo(), and qWarning() are debugging tools. They can be compiled away by defining `QT_NO_DEBUG_OUTPUT`, `QT_NO_INFO_OUTPUT`, or `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`. Use `QDebugStateSaver` to temporarily save the formatting options of the stream. Use nospace() and QTextStream manipulators to further customize the formatting.

Here is an example for a class that represents a 2D coordinate.

     QDebug operator<<(QDebug dbg, const Coordinate &c)
     {
         QDebugStateSaver saver(dbg);
         dbg.nospace() << "(" << c.x() << ", " << c.y() << ")";

         return dbg;
     }

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 `#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 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.