fix tricky regression noticed by Vyacheslav Tokarev on Google Reader.

[kdelibs.git] / DEBUG

Introduction
============

This is a short tutorial on debugging KDE applications. Throughout this
tutorial I will use "kedit" as example application.


Configuring for debugging
=========================

You can use -DCMAKE_BUILD_TYPE=Debug with the configure script, if you want to have
debug code in your KDE libs. If you have the space and can stand code that's
somewhat slower, this is worth it. The extra information really
helps debugging and thus bugfixing.

On the other hand, -DCMAKE_BUILD_TYPE=None removes all debug messages, leading
to a faster and cleaner desktop.


Debugging with GDB
==================

The recommended version of gdb to use is version 4.95 or higher, older 
versions have problems generating proper backtraces.

There are three ways to debug an application with gdb:

1) You can start the application from within gdb.
2) You can attach gdb to an already running application.
3) You can run gdb after an application has crashed using a core file.


Starting applications from within gdb
=====================================

To start an application with gdb you can start gdb as follows:

> gdb kedit
GNU gdb 4.95.0
Copyright 2000 Free Software Foundation, Inc.
GDB is free software, covered by the GNU General Public License, and you are
welcome to change it and/or distribute copies of it under certain conditions.
Type "show copying" to see the conditions.
There is absolutely no warranty for GDB.  Type "show warranty" for details.
This GDB was configured as "i686-pc-linux-gnu"...
(gdb) 

You can now set the command line arguments that you want to pass to kedit with 
the gdb command "set args":

(gdb) set args myfile.txt
(gdb) 

gdb has loaded the kedit executable on startup but it hasn't loaded any of 
the libraries yet. This means that you can set any breakpoints in the 
libraries yet. The easiest way to do that is to set a breakpoint in the
first line of main and then start the program:

(gdb) break main
Breakpoint 1 at 0x8048741: file /d/kde/build/4/kdeutils/kedit/kedit_dummy.cpp, line 3
(gdb) run
Starting program: /opt/kde/bin/kedit myfile.txt
 
Breakpoint 1, main (argc=1, argv=0xbfa798b4) at /d/kde/build/4/kdeutils/kedit/kedit_dummy.cpp:3
3       int main(int argc, char* argv[]) { return kdemain(argc,argv); }
(gdb)  

You can now set breakpoints everywhere. For example lets set a breakpoint 
in the KApplication constructor. Remember to use quotes before C++ method names:
(gdb) break 'KApplication::init()'
Breakpoint 2 at 0xb7a5f86d: file /d/kde/src/4/kdelibs/kdeui/kernel/kapplication.cpp, line 488.
(gdb) 

We can now continue the execution of kedit. Execution will stop when it hits
a breakpoint of when the program exits. In this case execution will stop
in the first line of the KApplication constructor:

(gdb) continue
Continuing.
Qt: gdb: -nograb added to command-line options.
         Use the -dograb option to enforce grabbing.
 
Breakpoint 2, KApplication::init (this=0xbfa797b4) at /d/kde/src/4/kdelibs/kdeui/kernel/kapplication.cpp:488
488       if ((getuid() != geteuid()) ||


Attaching gdb to already running applications
=============================================

Sometimes it is not practical to start an application from within gdb.
E.g. in those cases where you didn't know the application was about to
crash :-) When you get the friendly DrKonqi dialog informing you about 
a crash you are just in time to start your debugger.

First lets attach gdb to an application that hasn't crashed (yet).

You start with finding the process of the application with e.g. "ps -aux":

> ps -aux | grep kedit 
bastian  21570 15.1  6.8 13740 8800 pts/6    S    15:34   0:01 kedit
bastian  21582  0.0  0.3  1132  412 pts/6    R    15:34   0:00 grep kedit

From this you learn that kedit has process id 21570. Now you can start gdb as
follows:

> gdb kedit 21570
GNU gdb 4.95.0
Copyright 2000 Free Software Foundation, Inc.
GDB is free software, covered by the GNU General Public License, and you are
welcome to change it and/or distribute copies of it under certain conditions.
Type "show copying" to see the conditions.
There is absolutely no warranty for GDB.  Type "show warranty" for details.
This GDB was configured as "i686-pc-linux-gnu"...
/home1/bastian/21570: No such file or directory.
Attaching to program: /opt/kde/bin/kedit, Pid 21570
Reading symbols from /opt/kde/lib/kedit.so.0...done.
Loaded symbols for /opt/kde/lib/kedit.so.0
....
Reading symbols from /lib/ld-linux.so.2...done.
Loaded symbols for /lib/ld-linux.so.2
Reading symbols from /lib/libnss_compat.so.2...done.
Loaded symbols for /lib/libnss_compat.so.2
Reading symbols from /lib/libnsl.so.1...done.
Loaded symbols for /lib/libnsl.so.1
0x40c3d88e in __select () from /lib/libc.so.6
(gdb)  

You will usually end up in the middle of a select() call from the event-loop.
This is the place where a KDE application spends most of its time, waiting
for things to happen.

A backtrace will typically look something like this:

(gdb) bt
#0  0x40c3d88e in __select () from /lib/libc.so.6
#1  0x40a22844 in __DTOR_END__ () at fam.c++:356
#2  0x407293bf in QApplication::enter_loop (this=0xbffff6e8)
    at kernel/qapplication.cpp:2552
#3  0x406b1d7b in QApplication::exec (this=0xbffff6e8)
    at kernel/qapplication_x11.cpp:2217
#4  0x4002d500 in main (argc=1, argv=0xbffff854) at kedit.cpp:1662
#5  0x40bbba5e in __libc_start_main (main=0x8048568 <main>, argc=1,
    argv=0xbffff854, init=0x8048514 <_init>, fini=0x80486cc <_fini>,
    rtld_fini=0x4000aa20 <_dl_fini>, stack_end=0xbffff84c)
    at ../sysdeps/generic/libc-start.c:92
(gdb)


Getting core dumps
==================

If you want to have a core dump after your application crashes you need to 
do two things:

1) Disable the KDE crash handler. This can be done either by using the 
--nocrashhandler command line option or by setting the KDE_DEBUG environment
variable to some value e.g. KDE_DEBUG=true.

2) Enable core dump generation by changing the so called 'ulimits' with the
following command: 
        ulimit -c unlimited