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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN"><html><head><meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1"><title>Qt Toolkit - Debugging Techniques</title><style type="text/css"><!--
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<h1 align=center> Debugging Techniques</h1><br clear="all">
Here we present some useful hints to debugging your Qt-based software.
<p>
<h2>Command Line Options</h2>
<p>
When you run a Qt program you can specify several command line options
that can help with debugging.
<p>
<ul>
<li>-nograb The application should never grab <a href="qwidget.html#5bdd5f">the
mouse</a> or <a href="qwidget.html#6830ee">the keyboard</a>.
This option is set by default when the program is running in the <code>gdb</code> debugger under Linux.
<li>-dograb Ignore any implicit or explicit -nograb. -dograb wins
over -nograb even when -nograb is last on the command line.
<li>-sync Runs the application in X synchronous mode. Synchronous
mode forces the X server perform each X client request immediately and
not use a 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.
</ul>
<p>
<hr>
<h2>Warning and Debugging Messages</h2>
<p>
Qt includes three global functions for writing out warning and debug
text.
<ul>
<li><a href="qapplication.html#72e78c">qDebug()</a> for writing debug output for testing etc.
<li><a href="qapplication.html#290ef4">qWarning()</a> for writing warning output when program
errors occur.
<li><a href="qapplication.html#0e1d68">qFatal()</a> for writing fatal error messages and exit.
</ul>
<p>
The Qt implementation of these functions prints the text to the <code>stderr</code>
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#a8a31b">qInstallMsgHandler()</a>.
<p>
The debugging functions <a href="qobject.html#c21cea">QObject::dumpObjectTree()</a> and <a href="qobject.html#d459b3">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>
<hr>
<h2>Debugging Macros</h2>
<p>
The header file qglobal.h contains many debugging macros and #defines.
<p>
Two important macros are:
<ul>
<li><a href="qapplication.html#6c7a8c">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#7ed4c6">CHECK_PTR(p)</a> where p is a pointer.
Writes the warning "In file file.cpp, line 234: Out of memory" if p is null.
</ul>
<p>
These macros are useful for detecting program errors, e.g. like this:
<pre> char *alloc( int size )
{
ASSERT( size > 0 );
char *p = new char[size];
CHECK_PTR( p );
return p;
}
</pre>
<p>
If you define the flag QT_FATAL_ASSERT, 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 ASSERT macro is a null expression if <code>CHECK_STATE</code> (see
below) is not defined. Any code in it will simply not be
executed. Similarly CHECK_PTR is a null expression if <code>CHECK_NULL</code> is
not defined. Here is an example of how you should NOT use ASSERT and
CHECK_PTR:
<p>
<pre> char *alloc( int size )
{
char *p;
CHECK_PTR( p = new char[size] ); // never do this!
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 CHECK_NULL flag defined, the code in the 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:
<ul>
<li> <code>CHECK_STATE:</code> Check for consistent/expected object state
<li> <code>CHECK_RANGE:</code> Check for variables range errors
<li> <code>CHECK_NULL:</code> Check for dangerous null pointer
<li> <code>CHECK_MATH:</code> Check for dangerous math, e.g. division by 0.
<li> <code>NO_CHECK:</code> Turn off all CHECK_... flags
<li> <code>DEBUG:</code> Enable debugging code
<li> <code>NO_DEBUG:</code> Turn off DEBUG flag
</ul>
<p>
By default, both DEBUG and all the CHECK flags are on. To turn off
DEBUG, define NO_DEBUG. To turn off the CHECK flags, define NO_CHECK.
<p>
Example:
<pre> void f( char *p, int i )
{
#if defined(CHECK_NULL)
if ( p == 0 )
<a href="qapplication.html#290ef4">qWarning</a>( "f: Null pointer not allowed" );
#endif
#if defined(CHECK_RANGE)
if ( i < 0 )
<a href="qapplication.html#290ef4">qWarning</a>( "f: The index cannot be negative" );
#endif
}
</pre>
<p>
<hr>
<h2>Common bugs</h2>
<p>
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.
<p>
Any link error complaining about a lack of <code>vtbl</code>,
<code>_vtbl</code>, <code>__vtbl</code> or similar is likely to be
this problem.
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<td>Copyright 2001 Trolltech<td><a href="http://www.trolltech.com/trademarks.html">Trademarks</a>
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