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<title>Debugging Techniques</title>
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<td align="right" valign="center"><img src="logo32.png" align="right" width="64" height="32" border="0"></td></tr></table><h1 align=center>Debugging Techniques</h1>
<p> Here we present some useful hints to debugging your Qt-based software.
<p> <h2> Command Line Options
</h2>
<a name="1"></a><p> When you run a Qt program you can specify several command line options
that can help with debugging.
<p> <center><table cellpadding="4" cellspacing="2" border="0">
<tr bgcolor="#a2c511"> <th valign="top">Option <th valign="top">Result
<tr bgcolor="#f0f0f0"> <td valign="top">-nograb
<td valign="top">The application should never grab <a href="qwidget.html#grabMouse">the mouse</a> or <a href="qwidget.html#grabKeyboard">the
keyboard</a>. This option is set by default when the
program is running in the <tt>gdb</tt> debugger under Linux.
<tr bgcolor="#d0d0d0"> <td valign="top">-dograb
<td valign="top">Ignore any implicit or explicit -nograb. -dograb wins over
-nograb even when -nograb is last on the command line.
<tr bgcolor="#f0f0f0"> <td valign="top">-sync
<td valign="top">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.
</table></center>
<p> <h2> Warning and Debugging Messages
</h2>
<a name="2"></a><p> Qt includes three global functions for writing out warning and debug
text.
<ul>
<li> <a href="qapplication.html#qDebug">qDebug()</a> for writing debug output for testing etc.
<li> <a href="qapplication.html#qWarning">qWarning()</a> for writing warning output when program
errors occur.
<li> <a href="qapplication.html#qFatal">qFatal()</a> for writing fatal error messages
and exiting.
</ul>
<p> The Qt implementation of these functions prints the text to the <tt>stderr</tt>
output under Unix/X11 and to the debugger under Windows. You can
take over these functions by installing a message handler;
<a href="qapplication.html#qInstallMsgHandler">qInstallMsgHandler()</a>.
<p> The debugging functions <a href="qobject.html#dumpObjectTree">QObject::dumpObjectTree</a>() and <a href="qobject.html#dumpObjectInfo">QObject::dumpObjectInfo</a>() 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.
<p> <h2> Debugging Macros
</h2>
<a name="3"></a><p> The header file <a href="qglobal-h.html">qglobal.h</a> contains many debugging macros and
<tt>#define</tt>s.
<p> Two important macros are:
<ul>
<li> <a href="qapplication.html#Q_ASSERT">Q_ASSERT(b)</a> where b is a boolean
expression, writes the warning: "ASSERT: 'b' in file file.cpp (234)"
if b is FALSE.
<li> <a href="qapplication.html#Q_CHECK_PTR">Q_CHECK_PTR(p)</a> where p is a pointer.
Writes the warning "In file file.cpp, line 234: Out of memory" if p is
0.
</ul>
<p> These macros are useful for detecting program errors, e.g. like this:
<pre>
char *alloc( int size )
{
<a href="qapplication.html#Q_ASSERT">Q_ASSERT</a>( size > 0 );
char *p = new char[size];
<a href="qapplication.html#Q_CHECK_PTR">Q_CHECK_PTR</a>( p );
return p;
}
</pre>
<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.
<p> Note that the Q_ASSERT macro is a null expression if <tt>QT_CHECK_STATE</tt> (see
below) is not defined. Any code in it will simply not be
executed. Similarly Q_CHECK_PTR is a null expression if <tt>QT_CHECK_NULL</tt> is
not defined. Here is an example of how you should <em>not</em> use Q_ASSERT and
Q_CHECK_PTR:
<p> <pre>
char *alloc( int size )
{
char *p;
<a href="qapplication.html#Q_CHECK_PTR">Q_CHECK_PTR</a>( p = new char[size] ); // WRONG!
return p;
}
</pre>
<p> The problem is tricky: <em>p</em> 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 <em>alloc</em>
returns a wild pointer.
<p> The Qt library contains hundreds of internal checks that will print
warning messages when some error is detected.
<p> The tests for sanity and the resulting warning messages inside Qt are
conditional, based on the state of various debugging flags:
<center><table cellpadding="4" cellspacing="2" border="0">
<tr bgcolor="#a2c511"> <th valign="top">Flag <th valign="top">Meaning
<tr bgcolor="#d0d0d0"> <td valign="top">QT_CHECK_STATE <td valign="top">Check for consistent/expected object state
<tr bgcolor="#f0f0f0"> <td valign="top">QT_CHECK_RANGE <td valign="top">Check for variable range errors
<tr bgcolor="#d0d0d0"> <td valign="top">QT_CHECK_NULL <td valign="top">Check for dangerous null pointers
<tr bgcolor="#f0f0f0"> <td valign="top">QT_CHECK_MATH <td valign="top">Check for dangerous math, e.g. division by 0
<tr bgcolor="#d0d0d0"> <td valign="top">QT_NO_CHECK <td valign="top">Turn off all QT_CHECK_... flags
<tr bgcolor="#f0f0f0"> <td valign="top">QT_DEBUG <td valign="top">Enable debugging code
<tr bgcolor="#d0d0d0"> <td valign="top">QT_NO_DEBUG <td valign="top">Turn off QT_DEBUG flag
</table></center>
<p> 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.
<p> Example:
<pre>
void f( char *p, int i )
{
#if defined(QT_CHECK_NULL)
if ( p == 0 )
<a href="qapplication.html#qWarning">qWarning</a>( "f: Null pointer not allowed" );
#endif
#if defined(QT_CHECK_RANGE)
if ( i < 0 )
<a href="qapplication.html#qWarning">qWarning</a>( "f: The index cannot be negative" );
#endif
}
</pre>
<p> <h2> Common bugs
</h2>
<a name="4"></a><p> There is one bug that is so common that it deserves mention here: If
you include the <a href="metaobjects.html#Q_OBJECT">Q_OBJECT</a> macro in a class declaration and run the
<a href="moc.html">moc</a>, but forget to link the <a href="moc.html#moc">moc</a>-generated
object code into your executable, you will get very confusing error
messages. Any link error complaining about a lack of <tt>vtbl</tt>,
<tt>_vtbl</tt>, <tt>__vtbl</tt> or similar is likely to be a result of this
problem.
<p>
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