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| GDB Debugging
| | {{Moved To Community | Guidelines_and_HOWTOs/Debugging/Debugging_with_GDB}} |
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| This is a short tutorial on debugging KDE applications. Throughout this
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| tutorial I will use "kedit" as example application.
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| ==Debugging with GDB==
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| The recommended version of gdb to use is version 4.95 or higher, older
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| versions have problems generating proper backtraces.
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| There are three ways to debug an application with gdb:
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| # You can start the application from within gdb.
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| # You can attach gdb to an already running application.
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| # You can run gdb after an application has crashed using a core file.
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| ==Starting applications from within gdb==
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| To start an application with gdb you can start gdb as follows: | |
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| > gdb kedit
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| GNU gdb 4.95.0
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| Copyright 2000 Free Software Foundation, Inc.
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| GDB is free software, covered by the GNU General Public License, and you are
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| welcome to change it and/or distribute copies of it under certain conditions.
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| Type "show copying" to see the conditions.
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| There is absolutely no warranty for GDB. Type "show warranty" for details.
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| This GDB was configured as "i686-pc-linux-gnu"...
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| (gdb)
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| You can now set the command line arguments that you want to pass to kedit with
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| the gdb command "<tt>set args</tt>":
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| (gdb) set args myfile.txt
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| (gdb)
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| gdb has loaded the kedit executable on startup but it hasn't loaded any of
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| the libraries yet. This means that you can't set any breakpoints in the
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| libraries yet. The easiest way to do that is to set a breakpoint in the
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| first line of main and then start the program:
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| (gdb) break main
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| Breakpoint 1 at 0x804855c
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| (gdb) run
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| Starting program: /ext/kde2.0/bin/kedit myfile.txt
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| Breakpoint 1 at 0x4002cf18: file kedit.cpp, line 1595.
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|
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| Breakpoint 1, main (argc=2, argv=0xbffff814) at kedit.cpp:1595
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| 1595 bool have_top_window = false;
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| Current language: auto; currently c++
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| (gdb)
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| You can now set breakpoints everywhere. For example lets set a breakpoint
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| in the KApplication constructor. Unfortunately gdb is not very good in
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| handling C++ names, so it is not really possible to specify the constructor
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| directly after the break command. Instead we look up a line of source
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| code where we want to place the breakpoint. An external editor is of great
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| use at this point. With the list command we can select the source file we
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| are interested in and verify that we have found the correct source line:
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| (gdb) list kapp.cpp:220
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| 215 parseCommandLine( argc, argv );
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| 216 }
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| 217
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| 218 KApplication::KApplication( bool allowStyles, bool GUIenabled ) :
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| 219 QApplication( *KCmdLineArgs::qt_argc(), *KCmdLineArgs::qt_argv(),
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| 220 GUIenabled ),
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| 221 KInstance( KCmdLineArgs::about),
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| 222 d (new KApplicationPrivate)
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| 223 {
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| 224 if (!GUIenabled)
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| (gdb) break 224
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| Breakpoint 2 at 0x4048aa7e: file kapp.cpp, line 224.
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| (gdb)
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| We can now continue the execution of kedit. Execution will stop when it hits
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| a breakpoint or when the program exits. In this case execution will stop
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| in the first line of the KApplication constructor:
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| (gdb) continue
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| Continuing.
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| Qt: gdb: -nograb added to command-line options.
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| Use the -dograb option to enforce grabbing.
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|
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| Breakpoint 2, KApplication::KApplication (this=0xbffff6a8, allowStyles=true,
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| GUIenabled=true) at kapp.cpp:224
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| 224 if (!GUIenabled)
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| (gdb)
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| ==Attaching gdb to already running applications==
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| Sometimes it is not practical to start an application from within gdb.
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| E.g. in those cases where you didn't know the application was about to
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| crash :-) When you get the friendly DrKonqi dialog informing you about
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| a crash you are just in time to start your debugger.
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| First lets attach gdb to an application that hasn't crashed (yet).
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| You start with finding the process of the application with e.g. "ps -aux":
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| > ps -aux | grep kedit
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| bastian 21570 15.1 6.8 13740 8800 pts/6 S 15:34 0:01 kedit
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| bastian 21582 0.0 0.3 1132 412 pts/6 R 15:34 0:00 grep kedit
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| From this you learn that kedit has process id 21570. Now you can start gdb as
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| follows:
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| > gdb kedit 21570
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| GNU gdb 4.95.0
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| Copyright 2000 Free Software Foundation, Inc.
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| GDB is free software, covered by the GNU General Public License, and you are
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| welcome to change it and/or distribute copies of it under certain conditions.
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| Type "show copying" to see the conditions.
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| There is absolutely no warranty for GDB. Type "show warranty" for details.
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| This GDB was configured as "i686-pc-linux-gnu"...
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| /home1/bastian/21570: No such file or directory.
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| Attaching to program: /ext/kde2.0/bin/kedit, Pid 21570
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| Reading symbols from /ext/kde2.0/lib/kedit.so.0...done.
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| Loaded symbols for /ext/kde2.0/lib/kedit.so.0
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| .
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| Reading symbols from /lib/ld-linux.so.2...done.
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| Loaded symbols for /lib/ld-linux.so.2
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| Reading symbols from /lib/libnss_compat.so.2...done.
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| Loaded symbols for /lib/libnss_compat.so.2
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| Reading symbols from /lib/libnsl.so.1...done.
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| Loaded symbols for /lib/libnsl.so.1
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| 0x40c3d88e in __select () from /lib/libc.so.6
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| (gdb)
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| You will usually end up in the middle of a select() call from the event-loop.
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| This is the place where a KDE application spends most of its time, waiting
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| for things to happen.
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| A backtrace will typically look something like this:
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| (gdb) bt
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| #0 0x40c3d88e in __select () from /lib/libc.so.6
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| #1 0x40a22844 in __DTOR_END__ () at fam.c++:356
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| #2 0x407293bf in QApplication::enter_loop (this=0xbffff6e8)
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| at kernel/qapplication.cpp:2552
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| #3 0x406b1d7b in QApplication::exec (this=0xbffff6e8)
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| at kernel/qapplication_x11.cpp:2217
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| #4 0x4002d500 in main (argc=1, argv=0xbffff854) at kedit.cpp:1662
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| #5 0x40bbba5e in __libc_start_main (main=0x8048568 <main>, argc=1,
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| argv=0xbffff854, init=0x8048514 <_init>, fini=0x80486cc <_fini>,
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| rtld_fini=0x4000aa20 <_dl_fini>, stack_end=0xbffff84c)
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| at ../sysdeps/generic/libc-start.c:92
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| (gdb)
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| ==Improving your gdb experience for KDE/Qt applications==
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| In the SVN path named "kdesdk", you will find the file [http://websvn.kde.org/trunk/KDE/kdesdk/scripts/kde-devel-gdb kdesdk/scripts/kde-devel-gdb].
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| This file contains a few macros that help looking into some Qt objects (for instance QString).
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| See the beginning of the file for instructions on how to use it.
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| If you want to go even further, you can apply those [http://developer.kde.org/documentation/other/gdb-patches patches to the gdb source],
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| to fix a few annoyancies in gdb:
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| * source.c: don't try to open a directory in "." that has the same name as the executable we want
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| to open (not needed for gdb-6.0 and above)
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| * symfile.c: no prompting at end of page while opening shared libraries (not needed for gdb-6.2 and above)
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| * solib.c: less output when opening shared libraries
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| Have fun with gdb! Hmm, ok, the definition of 'fun' is very relative...
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