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diff --git a/doc/src/development/debug.qdoc b/doc/src/development/debug.qdoc new file mode 100644 index 0000000..c3a3256 --- /dev/null +++ b/doc/src/development/debug.qdoc @@ -0,0 +1,243 @@ +/**************************************************************************** +** +** Copyright (C) 2009 Nokia Corporation and/or its subsidiary(-ies). +** Contact: Nokia Corporation (qt-info@nokia.com) +** +** This file is part of the documentation of the Qt Toolkit. +** +** $QT_BEGIN_LICENSE:LGPL$ +** 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 Lesser General Public License Usage +** Alternatively, this file may be used under the terms of the GNU Lesser +** General Public License version 2.1 as published by the Free Software +** Foundation and appearing in the file LICENSE.LGPL included in the +** packaging of this file. Please review the following information to +** ensure the GNU Lesser General Public License version 2.1 requirements +** will be met: http://www.gnu.org/licenses/old-licenses/lgpl-2.1.html. +** +** In addition, as a special exception, Nokia gives you certain +** additional rights. These rights are described in the Nokia Qt LGPL +** Exception version 1.1, included in the file LGPL_EXCEPTION.txt in this +** package. +** +** 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 Technicle Note TN2124} Qt follows that. + + 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. +*/ |