Debug - Getting Traces

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This article will try to give you an idea how to create Arch package. This provides debug and traces information for reporting software bugs (for example after use of Bug-Buddy).

Contents

Discovering name of package(s)

A few facts of debug messages

When looking at debug message, such as (stripped):

...
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
...

you can see ?? at the place where debugging info is missing and also the name of library or executable which called the function. Similarly, when the line (no debugging symbols found) appears in a message, it means that you have to look for a file whose name is stated.

Finding package

Use Pacman to retrieve name of package:

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

We have found that package is called glibc in version 2.5-8. By repeating this step, we are able to create a list of packages which we have to compile ourselves to get full stack trace.

Obtaining PKGBUILD

In order to build a package from source, the PKGBUILD file is required. The location from which you can obtain PKGBUILDs is, in general:

  1. ArchLinux User-community Repository (AUR) or
  2. ABS

Using AUR

Use AUR search page to find the package. If it is not present, the package is stored in one of the official repository trees of Arch Linux. If found, click on its name and download the Tarball. Store it in location of your choice. Use tar to extract it and change directory:

$ tar xvzf name_of_tarball.tar.gz
$ cd name_of_tarball

Using ABS

If the package is a part of official tree, install ABS and use the <copy>abs</copy> command to obtain a copy of the directory structure:

# abs

Use find or slocate locate commands to find the package name:

$ find /var/abs -name "glibc"

or

$ slocate glibc | grep ^/var/abs

In any case, you'll find that package foo is part of extra and multimedia (for example). Copy foo's PKGBUILD file to /var/abs/local/foo:

 $ cp -r /var/abs/extra/multimedia/foo /var/abs/local/foo
 $ cd /var/abs/local/foo

Note that this example assumes that you have created /var/abs/local to build your packages in and that this directory is writeable by your non-root user account. This is merely a convention that a lot of arch users prefer. For alternatives, See ABS#Create_a_Build_Directory.

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.

Global settings

Modify makepkg's configuration file /etc/makepkg.conf to contain following lines:

CFLAGS="-g -march=i686 -O2 -pipe"
CXXFLAGS="-g -march=i686 -O2 -pipe"

and

OPTIONS=(!strip !docs libtool emptydirs)

These settings (in bold) will force compilation with debugging information and will disable stripping of executable.

One package settings only

Modify foo's PKGBUILD file to contain the following lines:

options=(!strip)

Into the build() function, add following lines at the very beginning:

export CFLAGS="$CFLAGS -g"
export CXXFLAGS="$CXXFLAGS -g"

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

Conclusion

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

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