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Debugging Techniques

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

Command Line Options

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

• -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.
• -sync 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.

Warning and Debugging Messages

Qt includes three global functions for writing out warning and debug text.

• qDebug() for writing debug output for testing etc.
• qWarning() for writing warning output when program errors occur.
• qFatal() for writing fatal error messages and exit.

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

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.

Debugging Macros

The header file qglobal.h contains many debugging macros and #defines.

Two important macros are:

• Q_ASSERT(b) where b is a boolean expression, writes the warning: "ASSERT: 'b' in file file.cpp (234)" if b is FALSE.
• Q_CHECK_PTR(p) where p is a pointer. Writes the warning "In file file.cpp, line 234: Out of memory" if p is null.

These macros are useful for detecting program errors, e.g. like this:

  char *alloc( int size )
  {
      Q_ASSERT( size > 0 );
      char *p = new char[size];
      Q_CHECK_PTR( p );
      return p;
  }

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 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 QT_CHECK_NULL is not defined. Here is an example of how you should not use Q_ASSERT and Q_CHECK_PTR:

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

The problem is tricky: 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 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:

• QT_CHECK_STATE: Check for consistent/expected object state
• QT_CHECK_RANGE: Check for variable range errors
• QT_CHECK_NULL: Check for dangerous null pointers
• QT_CHECK_MATH: Check for dangerous math, e.g. division by 0
• QT_NO_CHECK: Turn off all QT_CHECK_... flags
• QT_DEBUG: Enable debugging code
• QT_NO_DEBUG: Turn off QT_DEBUG flag

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:

  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
  }

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 moc, but forget to link the moc-generated object code into your executable, you will get very confusing error message.

Any link error complaining about a lack of vtbl, _vtbl, __vtbl or similar is likely to be this problem.