Development/Tutorials/Debugging/Debugging with GDB: Difference between revisions

From KDE TechBase
(Made a Note for the important information about --nofork)
m (Fix syntax highlighting.)
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To start an application with gdb you can start gdb as follows:
To start an application with gdb you can start gdb as follows:


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


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


(gdb) set args myfile.txt
<pre>
(gdb)
(gdb) set args myfile.txt
(gdb)
</pre>


gdb has loaded the kedit executable on startup but it hasn't loaded any of
gdb has loaded the kedit executable on startup but it hasn't loaded any of
Line 39: Line 43:
first line of main and then start the program:
first line of main and then start the program:


(gdb) break main
<pre>
Breakpoint 1 at 0x804855c
(gdb) break main
(gdb) run
Breakpoint 1 at 0x804855c
Starting program: /ext/kde2.0/bin/kedit myfile.txt
(gdb) run
Breakpoint 1 at 0x4002cf18: file kedit.cpp, line 1595.
Starting program: /ext/kde2.0/bin/kedit myfile.txt
Breakpoint 1 at 0x4002cf18: file kedit.cpp, line 1595.
   
   
Breakpoint 1, main (argc=2, argv=0xbffff814) at kedit.cpp:1595
Breakpoint 1, main (argc=2, argv=0xbffff814) at kedit.cpp:1595
1595            bool have_top_window = false;
1595            bool have_top_window = false;
Current language:  auto; currently c++
Current language:  auto; currently c++
(gdb)
(gdb)
</pre>


You can now set breakpoints everywhere. For example lets set a breakpoint
You can now set breakpoints everywhere. For example lets set a breakpoint
Line 58: Line 64:
are interested in and verify that we have found the correct source line:
are interested in and verify that we have found the correct source line:


(gdb) list kapp.cpp:220
<pre>
215    parseCommandLine( argc, argv );
(gdb) list kapp.cpp:220
216 }
215    parseCommandLine( argc, argv );
217
216 }
218 KApplication::KApplication( bool allowStyles, bool GUIenabled ) :
217
219  QApplication( *KCmdLineArgs::qt_argc(), *KCmdLineArgs::qt_argv(),
218 KApplication::KApplication( bool allowStyles, bool GUIenabled ) :
220                GUIenabled ),
219  QApplication( *KCmdLineArgs::qt_argc(), *KCmdLineArgs::qt_argv(),
221  KInstance( KCmdLineArgs::about),
220                GUIenabled ),
222  d (new KApplicationPrivate)
221  KInstance( KCmdLineArgs::about),
223 {
222  d (new KApplicationPrivate)
224    if (!GUIenabled)
223 {
(gdb) break 224
224    if (!GUIenabled)
Breakpoint 2 at 0x4048aa7e: file kapp.cpp, line 224.
(gdb) break 224
(gdb)
Breakpoint 2 at 0x4048aa7e: file kapp.cpp, line 224.
(gdb)
</pre>


We can now continue the execution of kedit. Execution will stop when it hits
We can now continue the execution of kedit. Execution will stop when it hits
Line 77: Line 85:
in the first line of the KApplication constructor:
in the first line of the KApplication constructor:


(gdb) continue
<pre>
Continuing.
(gdb) continue
Qt: gdb: -nograb added to command-line options.
Continuing.
          Use the -dograb option to enforce grabbing.
Qt: gdb: -nograb added to command-line options.
        Use the -dograb option to enforce grabbing.
   
   
Breakpoint 2, KApplication::KApplication (this=0xbffff6a8, allowStyles=true,
Breakpoint 2, KApplication::KApplication (this=0xbffff6a8, allowStyles=true,
    GUIenabled=true) at kapp.cpp:224
    GUIenabled=true) at kapp.cpp:224
224        if (!GUIenabled)
224        if (!GUIenabled)
(gdb)
(gdb)
</pre>


{{Note|
{{Note|
Important: many applications use "KUniqueApplication" to ensure that only one instance can exist at a given time in a given KDE session. This is the case for kwin, kontact, konsole, plasma, etc. To debug those applications, attach to them while they're running (see next session) or use <code>set args --nofork</code>
Important: many applications use "KUniqueApplication" to ensure that only one instance can exist at a given time in a given KDE session. This is the case for kwin, kontact, konsole, plasma, etc. To debug those applications, attach to them while they're running (see next session) or use <tt>set args --nofork</tt>
}}
}}


Line 100: Line 110:
First lets attach gdb to an application that hasn't crashed (yet).
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":
You start with finding the process of the application with e.g. <tt>ps -aux</tt>:


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


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


> gdb kedit 21570
<pre>
GNU gdb 4.95.0
> gdb kedit 21570
Copyright 2000 Free Software Foundation, Inc.
GNU gdb 4.95.0
GDB is free software, covered by the GNU General Public License, and you are
Copyright 2000 Free Software Foundation, Inc.
welcome to change it and/or distribute copies of it under certain conditions.
GDB is free software, covered by the GNU General Public License, and you are
Type "show copying" to see the conditions.
welcome to change it and/or distribute copies of it under certain conditions.
There is absolutely no warranty for GDB.  Type "show warranty" for details.
Type "show copying" to see the conditions.
This GDB was configured as "i686-pc-linux-gnu"...
There is absolutely no warranty for GDB.  Type "show warranty" for details.
/home1/bastian/21570: No such file or directory.
This GDB was configured as "i686-pc-linux-gnu"...
Attaching to program: /ext/kde2.0/bin/kedit, Pid 21570
/home1/bastian/21570: No such file or directory.
Reading symbols from /ext/kde2.0/lib/kedit.so.0...done.
Attaching to program: /ext/kde2.0/bin/kedit, Pid 21570
Loaded symbols for /ext/kde2.0/lib/kedit.so.0
Reading symbols from /ext/kde2.0/lib/kedit.so.0...done.
...
Loaded symbols for /ext/kde2.0/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/ld-linux.so.2...done.
Reading symbols from /lib/libnss_compat.so.2...done.
Loaded symbols for /lib/ld-linux.so.2
Loaded symbols for /lib/libnss_compat.so.2
Reading symbols from /lib/libnss_compat.so.2...done.
Reading symbols from /lib/libnsl.so.1...done.
Loaded symbols for /lib/libnss_compat.so.2
Loaded symbols for /lib/libnsl.so.1
Reading symbols from /lib/libnsl.so.1...done.
0x40c3d88e in __select () from /lib/libc.so.6
Loaded symbols for /lib/libnsl.so.1
(gdb)
0x40c3d88e in __select () from /lib/libc.so.6
(gdb)
</pre>


You will usually end up in the middle of a select() call from the event-loop.
You will usually end up in the middle of a select() call from the event-loop.
Line 137: Line 151:
A backtrace will typically look something like this:
A backtrace will typically look something like this:


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


==Debugging core files with GDB==
==Debugging core files with GDB==


Debugging process requires much memory. If you have to inspect crash, you can debug core files. It's much faster and requires less memory. First limit the maximum size of core files and run the application:
Debugging process requires much memory. If you have to inspect crash, you can debug core files. It's much faster and requires less memory. First limit the maximum size of core files and run the application:
<pre>
  ulimit -c 100000
  ulimit -c 100000
  kedit --nocrashhandler
  kedit --nocrashhandler
Don't forget to use '--nocrashhandler' option. Core file would be created if the application crashed, so you can use gdb with created core file:
</pre>
gdb kedit ./core-file #in my system it is core.PID
Do not forget to use <tt>--nocrashhandler</tt> option. Core file would be created if the application crashed, so you can use gdb with created core file:
<pre>
gdb kedit ./core-file #in my system it is core.PID
</pre>


==Improving your gdb experience for KDE/Qt applications==
==Improving your gdb experience for KDE/Qt applications==

Revision as of 15:59, 27 June 2011


Development/Tutorials/Debugging/Debugging with GDB

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

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't 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: /ext/kde2.0/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 or 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)
Note
Important: many applications use "KUniqueApplication" to ensure that only one instance can exist at a given time in a given KDE session. This is the case for kwin, kontact, konsole, plasma, etc. To debug those applications, attach to them while they're running (see next session) or use set args --nofork


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: /ext/kde2.0/bin/kedit, Pid 21570
Reading symbols from /ext/kde2.0/lib/kedit.so.0...done.
Loaded symbols for /ext/kde2.0/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)

Debugging core files with GDB

Debugging process requires much memory. If you have to inspect crash, you can debug core files. It's much faster and requires less memory. First limit the maximum size of core files and run the application:

 ulimit -c 100000
 kedit --nocrashhandler

Do not forget to use --nocrashhandler option. Core file would be created if the application crashed, so you can use gdb with created core file:

gdb kedit ./core-file #in my system it is core.PID

Improving your gdb experience for KDE/Qt applications

In the SVN path named "kdesdk", you will find the file kdesdk/scripts/kde-devel-gdb. This file contains a few macros that help looking into some Qt objects (for instance QString). See the beginning of the file for instructions on how to use it.

If you want to go even further, you can apply those patches to the gdb source, to fix a few annoyancies in gdb:

  • source.c: don't try to open a directory in "." that has the same name as the executable we want to open (not needed for gdb-6.0 and above)
  • symfile.c: no prompting at end of page while opening shared libraries (not needed for gdb-6.2 and above)
  • solib.c: less output when opening shared libraries

Those patches are maintained by David Faure.

Have fun with gdb! Hmm, ok, the definition of 'fun' is very relative...