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authortoma <toma@283d02a7-25f6-0310-bc7c-ecb5cbfe19da>2009-11-25 17:56:58 +0000
committertoma <toma@283d02a7-25f6-0310-bc7c-ecb5cbfe19da>2009-11-25 17:56:58 +0000
commitce4a32fe52ef09d8f5ff1dd22c001110902b60a2 (patch)
tree5ac38a06f3dde268dc7927dc155896926aaf7012 /DEBUG
downloadtdelibs-ce4a32fe52ef09d8f5ff1dd22c001110902b60a2.tar.gz
tdelibs-ce4a32fe52ef09d8f5ff1dd22c001110902b60a2.zip
Copy the KDE 3.5 branch to branches/trinity for new KDE 3.5 features.
BUG:215923 git-svn-id: svn://anonsvn.kde.org/home/kde/branches/trinity/kdelibs@1054174 283d02a7-25f6-0310-bc7c-ecb5cbfe19da
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+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 --enable-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, --disable-debug 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 0x804855c
+(gdb) run
+Starting program: /opt/kde/bin/kedit myfile.txt
+Breakpoint 1 at 0x4002cf18: file kedit.cpp, line 1595.
+
+Breakpoint 1, main (argc=2, argv=0xbffff814) at kedit.cpp:1595
+1595 bool have_top_window = false;
+Current language: auto; currently c++
+(gdb)
+
+You can now set breakpoints everywhere. For example lets set a breakpoint
+in the KApplication constructor. Unfortunately gdb is not very good in
+handling C++ names, so it is not really possible to specify the constructor
+directly after the break command. Instead we look up a line of source
+code where we want to place the breakpoint. An external editor is of great
+use at this point. With the list command we can select the source file we
+are interested in and verify that we have found the correct source line:
+
+(gdb) list kapp.cpp:220
+215 parseCommandLine( argc, argv );
+216 }
+217
+218 KApplication::KApplication( bool allowStyles, bool GUIenabled ) :
+219 QApplication( *KCmdLineArgs::qt_argc(), *KCmdLineArgs::qt_argv(),
+220 GUIenabled ),
+221 KInstance( KCmdLineArgs::about),
+222 d (new KApplicationPrivate)
+223 {
+224 if (!GUIenabled)
+(gdb) break 224
+Breakpoint 2 at 0x4048aa7e: file kapp.cpp, line 224.
+(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::KApplication (this=0xbffff6a8, allowStyles=true,
+ GUIenabled=true) at kapp.cpp:224
+224 if (!GUIenabled)
+(gdb)
+
+
+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
+
+