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172 lines
7.7 KiB
172 lines
7.7 KiB
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
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<!-- /home/espenr/tmp/qt-3.3.8-espenr-2499/qt-x11-free-3.3.8/doc/debug.doc:36 -->
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<html>
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<head>
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<meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1">
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<title>Debugging Techniques</title>
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<a href="index.html">
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<font color="#004faf">Home</font></a>
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<font color="#004faf">All Classes</font></a>
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<font color="#004faf">Main Classes</font></a>
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<font color="#004faf">Functions</font></a>
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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>
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<p> Here we present some useful hints to debugging your TQt-based software.
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<p> <h2> Command Line Options
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</h2>
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<a name="1"></a><p> When you run a TQt program you can specify several command line options
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that can help with debugging.
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<p> <center><table cellpadding="4" cellspacing="2" border="0">
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<tr bgcolor="#a2c511"> <th valign="top">Option <th valign="top">Result
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<tr bgcolor="#f0f0f0"> <td valign="top">-nograb
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<td valign="top">The application should never grab <a href="ntqwidget.html#grabMouse">the mouse</a> or <a href="ntqwidget.html#grabKeyboard">the
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keyboard</a>. This option is set by default when the
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program is running in the <tt>gdb</tt> debugger under Linux.
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<tr bgcolor="#d0d0d0"> <td valign="top">-dograb
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<td valign="top">Ignore any implicit or explicit -nograb. -dograb wins over
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-nograb even when -nograb is last on the command line.
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<tr bgcolor="#f0f0f0"> <td valign="top">-sync
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<td valign="top">Runs the application in X synchronous mode. Synchronous mode
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forces the X server to perform each X client request
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immediately and not use buffer optimization. It makes the
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program easier to debug and often much slower. The -sync
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option is only valid for the X11 version of TQt.
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</table></center>
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<p> <h2> Warning and Debugging Messages
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</h2>
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<a name="2"></a><p> TQt includes three global functions for writing out warning and debug
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text.
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<ul>
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<li> <a href="ntqapplication.html#qDebug">qDebug()</a> for writing debug output for testing etc.
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<li> <a href="ntqapplication.html#qWarning">qWarning()</a> for writing warning output when program
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errors occur.
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<li> <a href="ntqapplication.html#qFatal">qFatal()</a> for writing fatal error messages
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and exiting.
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</ul>
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<p> The TQt implementation of these functions prints the text to the <tt>stderr</tt>
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output under Unix/X11 and to the debugger under Windows. You can
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take over these functions by installing a message handler;
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<a href="ntqapplication.html#qInstallMsgHandler">qInstallMsgHandler()</a>.
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<p> The debugging functions <a href="ntqobject.html#dumpObjectTree">TQObject::dumpObjectTree</a>() and <a href="ntqobject.html#dumpObjectInfo">TQObject::dumpObjectInfo</a>() are often useful when an application looks
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or acts strangely. More useful if you use object names than not, but
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often useful even without names.
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<p> <h2> Debugging Macros
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</h2>
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<a name="3"></a><p> The header file <a href="qglobal-h.html">ntqglobal.h</a> contains many debugging macros and
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<tt>#define</tt>s.
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<p> Two important macros are:
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<ul>
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<li> <a href="ntqapplication.html#Q_ASSERT">Q_ASSERT(b)</a> where b is a boolean
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expression, writes the warning: "ASSERT: 'b' in file file.cpp (234)"
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if b is FALSE.
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<li> <a href="ntqapplication.html#TQ_CHECK_PTR">TQ_CHECK_PTR(p)</a> where p is a pointer.
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Writes the warning "In file file.cpp, line 234: Out of memory" if p is
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0.
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</ul>
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<p> These macros are useful for detecting program errors, e.g. like this:
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<pre>
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char *alloc( int size )
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{
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<a href="ntqapplication.html#Q_ASSERT">Q_ASSERT</a>( size > 0 );
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char *p = new char[size];
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<a href="ntqapplication.html#TQ_CHECK_PTR">TQ_CHECK_PTR</a>( p );
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return p;
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}
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</pre>
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<p> If you define the flag QT_FATAL_ASSERT, Q_ASSERT will call fatal()
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instead of warning(), so a failed assertion will cause the program to
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exit after printing the error message.
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<p> Note that the Q_ASSERT macro is a null expression if <tt>QT_CHECK_STATE</tt> (see
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below) is not defined. Any code in it will simply not be
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executed. Similarly TQ_CHECK_PTR is a null expression if <tt>QT_CHECK_NULL</tt> is
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not defined. Here is an example of how you should <em>not</em> use Q_ASSERT and
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TQ_CHECK_PTR:
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<p> <pre>
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char *alloc( int size )
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{
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char *p;
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<a href="ntqapplication.html#TQ_CHECK_PTR">TQ_CHECK_PTR</a>( p = new char[size] ); // WRONG!
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return p;
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}
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</pre>
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<p> The problem is tricky: <em>p</em> is set to a sane value only as long as the
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correct checking flags are defined. If this code is compiled without
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the QT_CHECK_NULL flag defined, the code in the TQ_CHECK_PTR expression is
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not executed (correctly, since it's only a debugging aid) and <em>alloc</em>
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returns a wild pointer.
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<p> The TQt library contains hundreds of internal checks that will print
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warning messages when some error is detected.
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<p> The tests for sanity and the resulting warning messages inside TQt are
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conditional, based on the state of various debugging flags:
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<center><table cellpadding="4" cellspacing="2" border="0">
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<tr bgcolor="#a2c511"> <th valign="top">Flag <th valign="top">Meaning
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<tr bgcolor="#d0d0d0"> <td valign="top">QT_CHECK_STATE <td valign="top">Check for consistent/expected object state
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<tr bgcolor="#f0f0f0"> <td valign="top">QT_CHECK_RANGE <td valign="top">Check for variable range errors
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<tr bgcolor="#d0d0d0"> <td valign="top">QT_CHECK_NULL <td valign="top">Check for dangerous null pointers
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<tr bgcolor="#f0f0f0"> <td valign="top">QT_CHECK_MATH <td valign="top">Check for dangerous math, e.g. division by 0
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<tr bgcolor="#d0d0d0"> <td valign="top">QT_NO_CHECK <td valign="top">Turn off all QT_CHECK_... flags
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<tr bgcolor="#f0f0f0"> <td valign="top">QT_DEBUG <td valign="top">Enable debugging code
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<tr bgcolor="#d0d0d0"> <td valign="top">QT_NO_DEBUG <td valign="top">Turn off QT_DEBUG flag
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</table></center>
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<p> By default, both QT_DEBUG and all the QT_CHECK flags are on. To turn
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off QT_DEBUG, define QT_NO_DEBUG. To turn off the QT_CHECK flags,
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define QT_NO_CHECK.
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<p> Example:
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<pre>
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void f( char *p, int i )
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{
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#if defined(QT_CHECK_NULL)
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if ( p == 0 )
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<a href="ntqapplication.html#qWarning">qWarning</a>( "f: Null pointer not allowed" );
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#endif
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#if defined(QT_CHECK_RANGE)
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if ( i < 0 )
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<a href="ntqapplication.html#qWarning">qWarning</a>( "f: The index cannot be negative" );
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#endif
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}
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</pre>
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<p> <h2> Common bugs
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</h2>
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<a name="4"></a><p> There is one bug that is so common that it deserves mention here: If
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you include the <a href="metaobjects.html#TQ_OBJECT">TQ_OBJECT</a> macro in a class declaration and run the
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<a href="moc.html">moc</a>, but forget to link the <a href="moc.html#moc">moc</a>-generated
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object code into your executable, you will get very confusing error
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messages. Any link error complaining about a lack of <tt>vtbl</tt>,
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<tt>_vtbl</tt>, <tt>__vtbl</tt> or similar is likely to be a result of this
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problem.
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<p>
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<!-- eof -->
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<p><address><hr><div align=center>
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<table width=100% cellspacing=0 border=0><tr>
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<td>Copyright © 2007
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<a href="troll.html">Trolltech</a><td align=center><a href="trademarks.html">Trademarks</a>
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<td align=right><div align=right>TQt 3.3.8</div>
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</table></div></address></body>
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</html>
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