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/****************************************************************************
**
** Copyright (C) 2010 Nokia Corporation and/or its subsidiary(-ies).
** All rights reserved.
** Contact: Nokia Corporation (qt-info@nokia.com)
**
** This file is part of the documentation of the Qt Toolkit.
**
** $QT_BEGIN_LICENSE:FDL$
** No Commercial Usage
** This file contains pre-release code and may not be distributed.
** You may use this file in accordance with the terms and conditions
** contained in the Technology Preview License Agreement accompanying
** this package.
**
** GNU Free Documentation License
** Alternatively, this file may be used under the terms of the GNU Free
** Documentation License version 1.3 as published by the Free Software
** Foundation and appearing in the file included in the packaging of this
** file.
**
** If you have questions regarding the use of this file, please contact
** Nokia at qt-info@nokia.com.
** $QT_END_LICENSE$
**
****************************************************************************/
/*!
\page debug.html
\title Debugging Techniques
Here we present some useful hints to help you with debugging your
Qt-based software.
\tableofcontents
\section1 Configuring Qt for Debugging
When \l{Installation}{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.
\section2 Debugging in Mac OS X and Xcode
\section3 Debugging With/Without Frameworks
The basic stuff you need to know about debug libraries and
frameworks is found at developer.apple.com in:
\l{http://developer.apple.com/technotes/tn2004/tn2124.html#SECDEBUGLIB}
{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 \c{-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.
\section4 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 \c{DYLD_IMAGE_SUFFIX}.
If you don't set it, you get the release version by default (i.e.,
non _debug). If you set \c{DYLD_IMAGE_SUFFIX=_debug}, you get the
debug version.
\section4 Without Frameworks:
When you tell \e{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.
\section3 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 \c{-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.
\omit
Although it is not necessary to build Qt with debug symbols to use the
other techniques described in this document, certain features are only
available when Qt is configured for debugging.
\endomit
\section1 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.
\table
\header \o Option \o Description
\row \o \c -nograb
\o The application should never grab \link QWidget::grabMouse()
the mouse\endlink or \link QWidget::grabKeyboard() the
keyboard \endlink. This option is set by default when the
program is running in the \c gdb debugger under Linux.
\row \o \c -dograb
\o Ignore any implicit or explicit \c{-nograb}. \c -dograb wins over
\c -nograb even when \c -nograb is last on the command line.
\row \o \c -sync
\o 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 \c -sync
option is only valid for the X11 version of Qt.
\endtable
\section1 Warning and Debugging Messages
Qt includes four global functions for writing out warning and debug
text. You can use them for the following purposes:
\list
\o qDebug() is used for writing custom debug output.
\o qWarning() is used to report warnings and recoverable errors in
your application.
\o qCritical() is used for writing critical error mesages and
reporting system errors.
\o qFatal() is used for writing fatal error messages shortly before exiting.
\endlist
If you include the <QtDebug> header file, the \c qDebug() function
can also be used as an output stream. For example:
\snippet doc/src/snippets/code/doc_src_debug.qdoc 0
The Qt implementation of these functions prints the text to the
\c 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 \c 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 \c QT_NO_DEBUG_OUTPUT and \c
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 \l{QObject::setObjectName()}{object names}
than not, but often useful even without names.
\section1 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 \c QDebug. The functions you need to know about in \c
QDebug are \c space() and \c 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.
\snippet doc/src/snippets/qdebug/qdebugsnippet.cpp 0
Integration of custom types with Qt's meta-object system is covered
in more depth in the \l{Creating Custom Qt Types} document.
\section1 Debugging Macros
The header file \c <QtGlobal> contains some debugging macros and
\c{#define}s.
Three important macros are:
\list
\o \l{Q_ASSERT()}{Q_ASSERT}(cond), where \c cond is a boolean
expression, writes the warning "ASSERT: '\e{cond}' in file xyz.cpp, line
234" and exits if \c cond is false.
\o \l{Q_ASSERT_X()}{Q_ASSERT_X}(cond, where, what), where \c cond is a
boolean expression, \c where a location, and \c what a message,
writes the warning: "ASSERT failure in \c{where}: '\c{what}', file xyz.cpp, line 234"
and exits if \c cond is false.
\o \l{Q_CHECK_PTR()}{Q_CHECK_PTR}(ptr), where \c ptr is a pointer.
Writes the warning "In file xyz.cpp, line 234: Out of memory" and
exits if \c ptr is 0.
\endlist
These macros are useful for detecting program errors, e.g. like this:
\snippet doc/src/snippets/code/doc_src_debug.qdoc 1
Q_ASSERT(), Q_ASSERT_X(), and Q_CHECK_PTR() expand to nothing if
\c 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():
\snippet doc/src/snippets/code/doc_src_debug.qdoc 2
If this code is compiled with \c QT_NO_DEBUG defined, the code in
the Q_CHECK_PTR() expression is not executed and \e 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.
\section1 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 \link moc.html the meta-object compiler\endlink (\c{moc}),
but forget to link the \c{moc}-generated object code into your
executable, you will get very confusing error messages. Any link
error complaining about a lack of \c{vtbl}, \c{_vtbl}, \c{__vtbl}
or similar is likely to be a result of this problem.
*/
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