tqt3/doc/debug.doc

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** Qt Debugging Techniques
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/*!
\page debug.html

\title Debugging Techniques

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

\section1 Command Line Options

When you run a Qt program you can specify several command line options
that can help with debugging.

\table
\header \i Option \i Result
\row \i -nograb
     \i 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 \i -dograb
     \i Ignore any implicit or explicit -nograb. -dograb wins over
	-nograb even when -nograb is last on the command line.
\row \i -sync
     \i 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.
\endtable


\section1 Warning and Debugging Messages

Qt includes three global functions for writing out warning and debug
text.
\list
\i \link ::qDebug() qDebug()\endlink for writing debug output for testing etc.
\i \link ::qWarning() qWarning()\endlink for writing warning output when program
errors occur.
\i \link ::qFatal() qFatal()\endlink for writing fatal error messages
and exiting.
\endlist

The Qt implementation of these functions prints the text to the \c stderr
output under Unix/X11 and to the debugger under Windows. You can
take over these functions by installing a message handler;
\link ::qInstallMsgHandler() qInstallMsgHandler()\endlink.

The debugging functions \l QObject::dumpObjectTree() and \l
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.

\section1 Debugging Macros

The header file \c qglobal.h contains many debugging macros and
\c{#define}s.

Two important macros are:
\list
\i \link ::Q_ASSERT() Q_ASSERT(b)\endlink where b is a boolean
expression, writes the warning: "ASSERT: 'b' in file file.cpp (234)"
if b is FALSE.
\i \link ::Q_CHECK_PTR() Q_CHECK_PTR(p)\endlink where p is a pointer.
Writes the warning "In file file.cpp, line 234: Out of memory" if p is
0.
\endlist

These macros are useful for detecting program errors, e.g. like this:
\code
  char *alloc( int size )
  {
      Q_ASSERT( size > 0 );
      char *p = new char[size];
      Q_CHECK_PTR( p );
      return p;
  }
\endcode

If you define the flag QT_FATAL_ASSERT, Q_ASSERT will call fatal()
instead of warning(), so a failed assertion will cause the program to
exit after printing the error message.

Note that the Q_ASSERT macro is a null expression if \c QT_CHECK_STATE (see
below) is not defined. Any code in it will simply not be
executed. Similarly Q_CHECK_PTR is a null expression if \c QT_CHECK_NULL is
not defined. Here is an example of how you should \e not use Q_ASSERT and
Q_CHECK_PTR:

\code
  char *alloc( int size )
  {
      char *p;
      Q_CHECK_PTR( p = new char[size] ); // WRONG!
      return p;
  }
\endcode

The problem is tricky: \e p is set to a sane value only as long as the
correct checking flags are defined. If this code is compiled without
the QT_CHECK_NULL flag defined, the code in the Q_CHECK_PTR expression is
not executed (correctly, since it's only a debugging aid) and \e alloc
returns a wild pointer.

The Qt library contains hundreds of internal checks that will print
warning messages when some error is detected.

The tests for sanity and the resulting warning messages inside Qt are
conditional, based on the state of various debugging flags:
\table
\header \i Flag		\i Meaning
\row \i QT_CHECK_STATE	\i Check for consistent/expected object state
\row \i QT_CHECK_RANGE	\i Check for variable range errors
\row \i QT_CHECK_NULL	\i Check for dangerous null pointers
\row \i QT_CHECK_MATH	\i Check for dangerous math, e.g. division by 0
\row \i QT_NO_CHECK	\i Turn off all QT_CHECK_... flags
\row \i QT_DEBUG	\i Enable debugging code
\row \i QT_NO_DEBUG	\i Turn off QT_DEBUG flag
\endtable

By default, both QT_DEBUG and all the QT_CHECK flags are on. To turn
off QT_DEBUG, define QT_NO_DEBUG. To turn off the QT_CHECK flags,
define QT_NO_CHECK.

Example:
\code
  void f( char *p, int i )
  {
  #if defined(QT_CHECK_NULL)
      if ( p == 0 )
	  qWarning( "f: Null pointer not allowed" );
  #endif

  #if defined(QT_CHECK_RANGE)
      if ( i < 0 )
	  qWarning( "f: The index cannot be negative" );
  #endif
  }
\endcode

\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 the
\link moc.html moc\endlink, 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 \c{vtbl},
\c{_vtbl}, \c{__vtbl} or similar is likely to be a result of this
problem.

*/