Development/FAQs/Debugging FAQ: Difference between revisions

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    ==What is a core file? How do I get a core file?==
    ==What is a core file? How do I get a core file?==
    A core file is an image of the memory when your application crashed. Using the core file, you can now which variables were set and where your application crashed.  
     
    A core file is an image of the memory when your application crashed. Using the core file, you can now which variables were set and where your application crashed.  
     
    Some distributions disable the generation of core files. To re-enable them, use "ulimit -c unlimited".
    Some distributions disable the generation of core files. To re-enable them, use "ulimit -c unlimited".
    Once you have a core file for a crash, you can examine it with gdb appname core . This will open gdb on the core file for the given application. Once at the gdb prompt, the most useful command is "bt" which generates a backtrace of the crash.
    Once you have a core file for a crash, you can examine it with gdb appname core . This will open gdb on the core file for the given application. Once at the gdb prompt, the most useful command is "bt" which generates a backtrace of the crash.
    For more information about how to use gdb, see this page.
    For more information about how to use gdb, see this page.
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      Yes, you must use kdDebug():
      Yes, you must use kdDebug():


    #include <kdebug.h>
    <code cppqt>
    #include <kdebug.h>
    kdDebug() << "KMyApp just started" << endl;  
    kdDebug() << "KMyApp just started" << endl;  
    </code>
    The syntax is much like cout, you can use many native types between the "<<". This will print out a debugging message, which will automatically be turned off at release time (by --disable-debug). In case you want the message to still be there during releases, because it's a warning or an error, use kdWarning() or kdError().
    The syntax is much like cout, you can use many native types between the "<<". This will print out a debugging message, which will automatically be turned off at release time (by --disable-debug). In case you want the message to still be there during releases, because it's a warning or an error, use kdWarning() or kdError().
    Components and libraries are advised to use a debug area number, as in kdDebug(1234). For this, the number must be registered in kdelibs/kdecore/kdebug.areas. Debug areas make it possible to turn off or on the debug output for specific area numbers, using the "kdebugdialog" program, which is part of kdebase. "kdebugdialog --fullmode" also permits to control where to log debug output. It is usually not necessary to register area numbers for standalone applications, unless it's so complex that you want to divide the output into several areas.
    Components and libraries are advised to use a debug area number, as in kdDebug(1234). For this, the number must be registered in kdelibs/kdecore/kdebug.areas. Debug areas make it possible to turn off or on the debug output for specific area numbers, using the "kdebugdialog" program, which is part of kdebase. "kdebugdialog --fullmode" also permits to control where to log debug output. It is usually not necessary to register area numbers for standalone applications, unless it's so complex that you want to divide the output into several areas.
    To make it clear: do NOT use qDebug(), this one doesn't get disabled at releases. Also avoid using assert() or kdFatal() which lead to a crash when something goes wrong, never nice for the user. Better detect the error, output a kdWarning or kdError, and recover if possible.
    To make it clear: do NOT use qDebug(), this one doesn't get disabled at releases. Also avoid using assert() or kdFatal() which lead to a crash when something goes wrong, never nice for the user. Better detect the error, output a kdWarning or kdError, and recover if possible.


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    Memory leak tracer : See kdesdk/kmtrace. The README explains it all.
    Memory leak tracer : See kdesdk/kmtrace. The README explains it all.
    kdcop and dcop allow to browse the dcop interface and to easily make dcop calls.
    kdcop and dcop allow to browse the dcop interface and to easily make dcop calls.
    Check this page and kdesdk, there are a bunch of useful scripts there.  
    Check this page and kdesdk, there are a bunch of useful scripts there.  


    ==How do I print a QString in gdb?==
    ==How do I print a QString in gdb?==
    Check out kdesdk, and add this line to your ~/.gdbinit :
    Check out kdesdk, and add this line to your ~/.gdbinit :
      source /path/to/kde/sources/kdesdk/scripts/kde-devel-gdb  
      source /path/to/kde/sources/kdesdk/scripts/kde-devel-gdb  
    Then in gdb you can do printqstring myqstring to see its contents.
    Then in gdb you can do printqstring myqstring to see its contents.
    For instance, QString myqstring = QString::fromLatin1("contents"); can be examined using
    For instance, QString myqstring = QString::fromLatin1("contents"); can be examined using
       
     
    (gdb) printqstring myqstring
      (gdb) printqstring myqstring
      $1 = "content"   
      $1 = "content"   
    See the kde-devel-gdb file for the other macros it defines.
    See the kde-devel-gdb file for the other macros it defines.


    ==I have no symbol when I debug an app that uses kpart, what should I do?==
    ==I have no symbol when I debug an app that uses kpart, what should I do?==
    You must stop just after the main to load the debugging symbols of the shared library. After that, you can debug normally.  
    You must stop just after the main to load the debugging symbols of the shared library. After that, you can debug normally.  
    One can go as far as creating a gdb macro, to stop right after the part was loaded. For kword, by example, I use :
    One can go as far as creating a gdb macro, to stop right after the part was loaded. For kword, by example, I use :
      define startkword
      define startkword
      break main
      break main
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    ==How do I debug an ioslave?==
    ==How do I debug an ioslave?==
    See kdebase/kioslave/DEBUG.howto
     
    See kdebase/kioslave/DEBUG.howto

    Revision as of 17:31, 23 December 2006

    How do I avoid Dr Konqi?

    You must set the environment variable KDE_DEBUG (to 1 or whatever you want in fact).

    What is a core file? How do I get a core file?

    A core file is an image of the memory when your application crashed. Using the core file, you can now which variables were set and where your application crashed.

    Some distributions disable the generation of core files. To re-enable them, use "ulimit -c unlimited".

    Once you have a core file for a crash, you can examine it with gdb appname core . This will open gdb on the core file for the given application. Once at the gdb prompt, the most useful command is "bt" which generates a backtrace of the crash. For more information about how to use gdb, see this page.

    Is there a preferred way to print debug output on stderr?

    Yes, you must use kdDebug():
    

    1. include <kdebug.h>

    kdDebug() << "KMyApp just started" << endl;

    The syntax is much like cout, you can use many native types between the "<<". This will print out a debugging message, which will automatically be turned off at release time (by --disable-debug). In case you want the message to still be there during releases, because it's a warning or an error, use kdWarning() or kdError().

    Components and libraries are advised to use a debug area number, as in kdDebug(1234). For this, the number must be registered in kdelibs/kdecore/kdebug.areas. Debug areas make it possible to turn off or on the debug output for specific area numbers, using the "kdebugdialog" program, which is part of kdebase. "kdebugdialog --fullmode" also permits to control where to log debug output. It is usually not necessary to register area numbers for standalone applications, unless it's so complex that you want to divide the output into several areas.

    To make it clear: do NOT use qDebug(), this one doesn't get disabled at releases. Also avoid using assert() or kdFatal() which lead to a crash when something goes wrong, never nice for the user. Better detect the error, output a kdWarning or kdError, and recover if possible.

    What tools are available to debug my application?

    kdDebug() calls are a simple but efficient way to debug an application. gdb, the GNU debugger, is the quickest way to execute step-by-step and investigate variables (prefer the 5.0 version, it is really better than the 4.1.x). Valgrind kdbg is a nice graphical frontend to gdb with a KDE GUI. It has support for many Qt types (including QString). Memory leak tracer : See kdesdk/kmtrace. The README explains it all. kdcop and dcop allow to browse the dcop interface and to easily make dcop calls. Check this page and kdesdk, there are a bunch of useful scripts there.

    How do I print a QString in gdb?

    Check out kdesdk, and add this line to your ~/.gdbinit :

    source /path/to/kde/sources/kdesdk/scripts/kde-devel-gdb 
    

    Then in gdb you can do printqstring myqstring to see its contents. For instance, QString myqstring = QString::fromLatin1("contents"); can be examined using

    (gdb) printqstring myqstring
    $1 = "content"  
    

    See the kde-devel-gdb file for the other macros it defines.

    I have no symbol when I debug an app that uses kpart, what should I do?

    You must stop just after the main to load the debugging symbols of the shared library. After that, you can debug normally. One can go as far as creating a gdb macro, to stop right after the part was loaded. For kword, by example, I use :

    define startkword
    break main
    run
    break 'KoDocument::KoDocument(int, QWidget *, char const *, QObject *, char const *, bool)' cont
    

    How do I debug an ioslave?

    See kdebase/kioslave/DEBUG.howto