Debugging TDE applications

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

Configuring for debugging

To build with debugging code, pass -DCMAKE_BUILD_TYPE="Debug" to CMake while configuring. If the application you are building is still using Autotools, you can use --enable-debug with the configure script to get debug code in it.

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="Release" (for CMake) and --disable-debug (for Autotools) 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/trinity/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 TDEApplication 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     TDEApplication::TDEApplication( bool allowStyles, bool GUIenabled ) :
219       QApplication( *TDECmdLineArgs::tqt_argc(), *TDECmdLineArgs::tqt_argv(),
220                     GUIenabled ),
221       TDEInstance( TDECmdLineArgs::about),
222       d (new TDEApplicationPrivate)
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 TDEApplication constructor:

(gdb) continue
Continuing.
Qt: gdb: -nograb added to command-line options.
         Use the -dograb option to enforce grabbing.
 
Breakpoint 2, TDEApplication::TDEApplication (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, let's 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/trinity/bin/kedit, Pid 21570
Reading symbols from /opt/trinity/lib/kedit.so.0...done.
Loaded symbols for /opt/trinity/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 TDE 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 TDE crash handler. This can be done either by using the --nocrashhandler command line option or by setting the TDE_DEBUG environment variable to some value e.g. TDE_DEBUG=true.
  2. Enable core dump generation by changing the so called 'ulimits' with the following command:
$ ulimit -c unlimited

Getting kdDebug() output

When a program is configured with debugging mode on, you also get to see debug messages from your code (generated by kdDebug()). The easiest way to see them is run the program from a Konsole window.

You can configure the messages you see through tdedebugdialog You can also redirect these messages to the syslog or a file. To configure this, launch tdedebugdialog with the --fullmode argument.