<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN"> <!-- /home/espenr/tmp/qt-3.3.8-espenr-2499/qt-x11-free-3.3.8/doc/debug.doc:36 --> <html> <head> <meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1"> <title>Debugging Techniques</title> <style type="text/css"><!-- fn { margin-left: 1cm; text-indent: -1cm; } a:link { color: #004faf; text-decoration: none } a:visited { color: #672967; text-decoration: none } body { background: #ffffff; color: black; } --></style> </head> <body> <table border="0" cellpadding="0" cellspacing="0" width="100%"> <tr bgcolor="#E5E5E5"> <td valign=center> <a href="index.html"> <font color="#004faf">Home</font></a> | <a href="classes.html"> <font color="#004faf">All Classes</font></a> | <a href="mainclasses.html"> <font color="#004faf">Main Classes</font></a> | <a href="annotated.html"> <font color="#004faf">Annotated</font></a> | <a href="groups.html"> <font color="#004faf">Grouped Classes</font></a> | <a href="functions.html"> <font color="#004faf">Functions</font></a> </td> <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 TQt-based software. <p> <h2> Command Line Options </h2> <a name="1"></a><p> When you run a TQt 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="ntqwidget.html#grabMouse">the mouse</a> or <a href="ntqwidget.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 TQt. </table></center> <p> <h2> Warning and Debugging Messages </h2> <a name="2"></a><p> TQt includes three global functions for writing out warning and debug text. <ul> <li> <a href="ntqapplication.html#qDebug">tqDebug()</a> for writing debug output for testing etc. <li> <a href="ntqapplication.html#qWarning">tqWarning()</a> for writing warning output when program errors occur. <li> <a href="ntqapplication.html#qFatal">tqFatal()</a> for writing fatal error messages and exiting. </ul> <p> The TQt 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="ntqapplication.html#qInstallMsgHandler">qInstallMsgHandler()</a>. <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 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">ntqglobal.h</a> contains many debugging macros and <tt>#define</tt>s. <p> Two important macros are: <ul> <li> <a href="ntqapplication.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="ntqapplication.html#TQ_CHECK_PTR">TQ_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="ntqapplication.html#Q_ASSERT">Q_ASSERT</a>( size > 0 ); char *p = new char[size]; <a href="ntqapplication.html#TQ_CHECK_PTR">TQ_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 TQ_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 TQ_CHECK_PTR: <p> <pre> char *alloc( int size ) { char *p; <a href="ntqapplication.html#TQ_CHECK_PTR">TQ_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 TQ_CHECK_PTR expression is not executed (correctly, since it's only a debugging aid) and <em>alloc</em> returns a wild pointer. <p> The TQt 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 TQt 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">TQT_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">TQT_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 TQT_NO_DEBUG. To turn off the QT_CHECK flags, define TQT_NO_CHECK. <p> Example: <pre> void f( char *p, int i ) { #if defined(QT_CHECK_NULL) if ( p == 0 ) <a href="ntqapplication.html#qWarning">tqWarning</a>( "f: Null pointer not allowed" ); #endif #if defined(QT_CHECK_RANGE) if ( i < 0 ) <a href="ntqapplication.html#qWarning">tqWarning</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#TQ_OBJECT">TQ_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> <!-- eof --> <p><address><hr><div align=center> <table width=100% cellspacing=0 border=0><tr> <td>Copyright © 2007 <a href="troll.html">Trolltech</a><td align=center><a href="trademarks.html">Trademarks</a> <td align=right><div align=right>TQt 3.3.8</div> </table></div></address></body> </html>