4 This is a short tutorial on debugging KDE applications. Throughout this
5 tutorial I will use "kedit" as example application.
8 Configuring for debugging
9 =========================
11 You can use --enable-debug with the configure script, if you want to have
12 debug code in your KDE libs. If you have the space and can stand code that's
13 somewhat slower, this is worth it. The extra information really
14 helps debugging and thus bugfixing.
16 On the other hand, --disable-debug removes all debug messages, leading
17 to a faster and cleaner desktop.
23 The recommended version of gdb to use is version 4.95 or higher, older
24 versions have problems generating proper backtraces.
26 There are three ways to debug an application with gdb:
28 1) You can start the application from within gdb.
29 2) You can attach gdb to an already running application.
30 3) You can run gdb after an application has crashed using a core file.
33 Starting applications from within gdb
34 =====================================
36 To start an application with gdb you can start gdb as follows:
40 Copyright 2000 Free Software Foundation, Inc.
41 GDB is free software, covered by the GNU General Public License, and you are
42 welcome to change it and/or distribute copies of it under certain conditions.
43 Type "show copying" to see the conditions.
44 There is absolutely no warranty for GDB. Type "show warranty" for details.
45 This GDB was configured as "i686-pc-linux-gnu"...
48 You can now set the command line arguments that you want to pass to kedit with
49 the gdb command "set args":
51 (gdb) set args myfile.txt
54 gdb has loaded the kedit executable on startup but it hasn't loaded any of
55 the libraries yet. This means that you can set any breakpoints in the
56 libraries yet. The easiest way to do that is to set a breakpoint in the
57 first line of main and then start the program:
60 Breakpoint 1 at 0x804855c
62 Starting program: /opt/kde/bin/kedit myfile.txt
63 Breakpoint 1 at 0x4002cf18: file kedit.cpp, line 1595.
65 Breakpoint 1, main (argc=2, argv=0xbffff814) at kedit.cpp:1595
66 1595 bool have_top_window = false;
67 Current language: auto; currently c++
70 You can now set breakpoints everywhere. For example lets set a breakpoint
71 in the KApplication constructor. Unfortunately gdb is not very good in
72 handling C++ names, so it is not really possible to specify the constructor
73 directly after the break command. Instead we look up a line of source
74 code where we want to place the breakpoint. An external editor is of great
75 use at this point. With the list command we can select the source file we
76 are interested in and verify that we have found the correct source line:
78 (gdb) list kapp.cpp:220
79 215 parseCommandLine( argc, argv );
82 218 KApplication::KApplication( bool allowStyles, bool GUIenabled ) :
83 219 QApplication( *KCmdLineArgs::qt_argc(), *KCmdLineArgs::qt_argv(),
85 221 KInstance( KCmdLineArgs::about),
86 222 d (new KApplicationPrivate)
90 Breakpoint 2 at 0x4048aa7e: file kapp.cpp, line 224.
93 We can now continue the execution of kedit. Execution will stop when it hits
94 a breakpoint of when the program exits. In this case execution will stop
95 in the first line of the KApplication constructor:
99 Qt: gdb: -nograb added to command-line options.
100 Use the -dograb option to enforce grabbing.
102 Breakpoint 2, KApplication::KApplication (this=0xbffff6a8, allowStyles=true,
103 GUIenabled=true) at kapp.cpp:224
108 Attaching gdb to already running applications
109 =============================================
111 Sometimes it is not practical to start an application from within gdb.
112 E.g. in those cases where you didn't know the application was about to
113 crash :-) When you get the friendly DrKonqi dialog informing you about
114 a crash you are just in time to start your debugger.
116 First lets attach gdb to an application that hasn't crashed (yet).
118 You start with finding the process of the application with e.g. "ps -aux":
120 > ps -aux | grep kedit
121 bastian 21570 15.1 6.8 13740 8800 pts/6 S 15:34 0:01 kedit
122 bastian 21582 0.0 0.3 1132 412 pts/6 R 15:34 0:00 grep kedit
124 From this you learn that kedit has process id 21570. Now you can start gdb as
129 Copyright 2000 Free Software Foundation, Inc.
130 GDB is free software, covered by the GNU General Public License, and you are
131 welcome to change it and/or distribute copies of it under certain conditions.
132 Type "show copying" to see the conditions.
133 There is absolutely no warranty for GDB. Type "show warranty" for details.
134 This GDB was configured as "i686-pc-linux-gnu"...
135 /home1/bastian/21570: No such file or directory.
136 Attaching to program: /opt/kde/bin/kedit, Pid 21570
137 Reading symbols from /opt/kde/lib/kedit.so.0...done.
138 Loaded symbols for /opt/kde/lib/kedit.so.0
140 Reading symbols from /lib/ld-linux.so.2...done.
141 Loaded symbols for /lib/ld-linux.so.2
142 Reading symbols from /lib/libnss_compat.so.2...done.
143 Loaded symbols for /lib/libnss_compat.so.2
144 Reading symbols from /lib/libnsl.so.1...done.
145 Loaded symbols for /lib/libnsl.so.1
146 0x40c3d88e in __select () from /lib/libc.so.6
149 You will usually end up in the middle of a select() call from the event-loop.
150 This is the place where a KDE application spends most of its time, waiting
151 for things to happen.
153 A backtrace will typically look something like this:
156 #0 0x40c3d88e in __select () from /lib/libc.so.6
157 #1 0x40a22844 in __DTOR_END__ () at fam.c++:356
158 #2 0x407293bf in QApplication::enter_loop (this=0xbffff6e8)
159 at kernel/qapplication.cpp:2552
160 #3 0x406b1d7b in QApplication::exec (this=0xbffff6e8)
161 at kernel/qapplication_x11.cpp:2217
162 #4 0x4002d500 in main (argc=1, argv=0xbffff854) at kedit.cpp:1662
163 #5 0x40bbba5e in __libc_start_main (main=0x8048568 <main>, argc=1,
164 argv=0xbffff854, init=0x8048514 <_init>, fini=0x80486cc <_fini>,
165 rtld_fini=0x4000aa20 <_dl_fini>, stack_end=0xbffff84c)
166 at ../sysdeps/generic/libc-start.c:92
173 If you want to have a core dump after your application crashes you need to
176 1) Disable the KDE crash handler. This can be done either by using the
177 --nocrashhandler command line option or by setting the KDE_DEBUG environment
178 variable to some value e.g. KDE_DEBUG=true.
180 2) Enable core dump generation by changing the so called 'ulimits' with the