qt-everywhere-src-5.15.1/qtdoc/doc/src/development/debug.qdoc

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/*!
    \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{Qt Configure Options}{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 \macos 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.

    \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 \li Option \li Description
    \row \li \c -nograb
         \li 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 \li \c -dograb
         \li Ignore any implicit or explicit \c{-nograb}. \c -dograb wins over
            \c -nograb even when \c -nograb is last on the command line.
    \endtable

    \section1 Environment Variables Recognized by Qt

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

    \table
    \header \li Variable \li Description
    \row \li \c QT_DEBUG_PLUGINS
         \li Set to a non-zero value to make Qt print out diagnostic information about the each
             (C++) plugin it tries to load.
    \row \li \c QML_IMPORT_TRACE
         \li Set to a non-zero value to make QML print out diagnostic information from the import
             loading mechanism.
    \row \li \c QT_HASH_SEED
         \li 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.
    \endtable

    \section1 Warning and Debugging Messages

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

    \list
    \li qDebug() is used for writing custom debug output.
    \li qInfo() is used for informational messages.
    \li qWarning() is used to report warnings and recoverable errors in
       your application.
    \li qCritical() is used for writing critical error messages and
       reporting system errors.
    \li qFatal() is used for writing fatal error messages shortly before exiting.
    \endlist

    If you include the <QtDebug> header file, the \c qDebug() macro
    can also be used as an output stream. For example:

    \snippet snippets/code/doc_src_debug.cpp 0

    The Qt implementation of these macros prints to the
    \c 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 \c QT_MESSAGE_PATTERN environment variable. For example:

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

    The format is documented in qSetMessagePattern(). You can also install your
    own message handler using qInstallMessageHandler().

    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.

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

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

    \snippet 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
    \li \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.
    \li \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.
    \li \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 snippets/code/doc_src_debug.cpp 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 snippets/code/doc_src_debug.cpp 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 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.

    \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.
*/