Debug - Getting Traces

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This article aims to help in creating a debugging Arch package and using it to provide trace and debug information for reporting software bugs to developers.

Package names

Tango-edit-clear.pngThis article or section needs language, wiki syntax or style improvements.Tango-edit-clear.png

Reason: "call the function", "stack trace", gdb is only mentioned later in the article -> unclear introduction (Discuss in Talk:Debug - Getting Traces#)

When looking at debug messages, such as:

[...]
Backtrace was generated from '/usr/bin/epiphany'

(no debugging symbols found)
Using host libthread_db library "/lib/libthread_db.so.1".
(no debugging symbols found)
[Thread debugging using libthread_db enabled]
[New Thread -1241265952 (LWP 12630)]
(no debugging symbols found)
0xb7f25410 in __kernel_vsyscall ()
#0  0xb7f25410 in __kernel_vsyscall ()
#1  0xb741b45b in ?? () from /lib/libpthread.so.0
[...]

?? shows where debugging info is missing, as well as the name of library or executable which called the function. Similarly, when (no debugging symbols found) appears, you should look for the stated file names. For example, with pacman:

# pacman -Qo /lib/libthread_db.so.1
/lib/libthread_db.so.1 is owned by glibc 2.5-8

The package is called glibc in version 2.5-8. Repeat this step for every package that needs debugging.

PKGBUILD

In order to build a package from source, the PKGBUILD file is required. See ABS for packages in the official repositories, and AUR#Acquire build files for packages in the AUR.

Compilation settings

At this stage, you can modify the global configuration file of makepkg if you will be using it only for debug purposes. In other cases, you should modify package's PKGBUILD file only for each package you would like to rebuild.

General

As of pacman 4.1, /etc/makepkg.conf has debug compilation flags in DEBUG_CFLAGS and DEBUG_CXXFLAGS. To use them, enable the debug makepkg option, and disable strip.

OPTIONS+=(debug !strip)

These settings will force compilation with debugging information and will disable the stripping of debug symbols from executables. To apply this setting to a single package, modify the PKGBUILD:

options=(debug !strip)

Alternatively you can put the debug information in a separate package by enabling both debug and strip, debugging information will then be stripped from the main package and placed in a separate foo-debug package.

It is possible to have a separate config for your debug builds that will load the default config and alter some variables. This way you can create debug builds only when you need to and without editing PKGBUILD or configuration files each time.

. /etc/makepkg.conf

cmake() {
  command cmake -DCMAKE_BUILD_TYPE=Debug "$@"
}

_remove_opt() {
local newarr=()
for el in "${OPTIONS[@]}"; do
case $el in
"$1") ;;
*) newarr+=( "$el" );;
esac
done
OPTIONS=( "${newarr[@]}" )
}

_remove_opt strip
_remove_opt !strip
_remove_opt debug 
_remove_opt !debug
OPTIONS=( "${OPTIONS[@]}" !strip debug )

Save it into /etc/makepkg-debug.conf and use the it like this:

 makepkg --config /etc/makepkg-debug.conf ...


Note: It is insufficient to simply install the newly compiled debug package, because the debugger will check that the file containing the debug symbols is from the same build as the associated library and executable. You must install both of the recompiled packages. In Arch, the debug symbols files are installed under /usr/lib/debug. See the GDB documentation for more information about debug packages.

Note that certain packages such as glibc are stripped regardless. Check the PKGBUILD for sections such as:

strip $STRIP_BINARIES usr/bin/{gencat,getconf,getent,iconv,iconvconfig} \
                      usr/bin/{ldconfig,locale,localedef,nscd,makedb} \
                      usr/bin/{pcprofiledump,pldd,rpcgen,sln,sprof} \
                      usr/lib/getconf/*
[[ $CARCH = "i686" ]] && strip $STRIP_BINARIES usr/bin/lddlibc4

strip $STRIP_STATIC usr/lib/*.a

strip $STRIP_SHARED usr/lib/{libanl,libBrokenLocale,libcidn,libcrypt}-*.so \
                    usr/lib/libnss_{compat,db,dns,files,hesiod,nis,nisplus}-*.so \
                    usr/lib/{libdl,libm,libnsl,libresolv,librt,libutil}-*.so \
                    usr/lib/{libmemusage,libpcprofile,libSegFault}.so \
                    usr/lib/{audit,gconv}/*.so

And remove them where appropriate.

Qt4

In addition to the previous general settings, pass -developer-build option to the configure script in the PKGBUILD. By default, -developer-build passes -Werror to the compiler, which may cause the compilation to fail. To avoid compilation errors, you may need pass -no-warnings-are-errors, too.

Qt5

The qt-debug repository contains pre-built Qt/PyQt packages with debug symbols. See also upstream instructions.

CMAKE (KDE) applications

KDE and related programs typically use cmake. To enable debug information and disable optimisations, change -DCMAKE_BUILD_TYPE to Debug. To enable debug information while keeping optimisations enabled, change -DCMAKE_BUILD_TYPE to RelWithDebInfo.

Building and installing the package

Build the package from source using makepkg while in the PKGBUILD's directory. This could take some time:

# makepkg

Then install the built package:

# pacman -U glibc-2.5-8-i686.pkg.tar.gz

Getting the trace

Tango-view-fullscreen.pngThis article or section needs expansion.Tango-view-fullscreen.png

Reason: Mention core dumps (Discuss in Talk:Debug - Getting Traces#)

The actual backtrace (or stack trace) can now be obtained via e.g. gdb, the GNU Debugger. Run it either via:

# gdb /path/to/file

or:

# gdb
(gdb) exec /path/to/file

The path is optional, if already set in the $PATH variable.

Then, within gdb, type run followed by any arguments you wish the program to start with, e.g.:

(gdb) run --no-daemon --verbose

to start execution of the file. Do whatever necessary to evoke the bug. For the actual log, type the lines:

(gdb) set logging file trace.log
(gdb) set logging on

and then:

(gdb) thread apply all bt full

to output the trace to trace.log into the directory gdb was started in. To exit, enter:

(gdb) set logging off
(gdb) quit
Tip: To debug an application written in python:
# gdb /usr/bin/python
(gdb) run <python application>

You can also debug an already running application, e.g.:

 # gdb --pid=$(pidof firefox)
 (gdb) continue

Conclusion

Use the complete stack trace to inform developers of a bug you have discovered before. This will be highly appreciated by them and will help to improve your favorite program.

See also