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# Debugging/Getting traces

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Related articles

- General troubleshooting
- Reporting bug guidelines
- Debuginfod
- Core dump

This article aims to help debugging software by providing traces and debug information. This information can then be used for the bug report to the (upstream) software developers or package maintainers.

## Introduction

Usually, executable files are stripped of human readable context to make them smaller. Obtaining traces without debugging information available drastically reduces their usefulness. For example, a trace from a gdb session where debugging information is not available may look as follows:

    [...]
    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.

To obtain a proper trace that is useful to the program developers, follow the next sections. Separate debug files are available for most official Arch packages and can be downloaded with Debuginfod (see #Getting the trace). When enhanced debugging information was not added to the executable in the first place, one has to rebuild the package with debugging symbols enabled.

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.

## Getting the trace

The actual backtrace (or stack trace) can be obtained via gdb, the GNU Debugger. It can be used in several ways, depending on whether it should start a new instance of a program, attach to an existing process, or examine a previous crash.

### Starting a new instance of a program

Start gdb with an executable program that can be found in `$PATH` (or a full path):

    $ gdb application

gdb automatically tries to download debug information and symbols for packages in the official repositories. When gdb asks whether Debuginfod should be enabled in the debugging session, answer `y`:

    This GDB supports auto-downloading debuginfo from the following URLs:
      <https://debuginfod.archlinux.org>
    Enable debuginfod for this session? (y or [n]) y
    Debuginfod has been enabled.
    To make this setting permanent, add 'set debuginfod enabled on' to .gdbinit.
    Downloading separate debug info for /usr/bin/application
    Reading symbols from /home/user/.cache/debuginfod_client/fbaee841e2ed2c11ecbbda26f39eeec1da23d6c3/debuginfo...

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

    (gdb) run arguments

Tip Alternatively, you can pass arguments when starting gdb using `gdb --args application arguments...` and then use only `run` without arguments within gdb. For example, to debug an application written in Python, run `gdb --args /usr/bin/python /path/to/application`.

Now do whatever is necessary to evoke the bug. gdb will automatically halt the application when it crashes and prompt for commands. In case of freezes or similar issues, press `Ctrl+c` and you will be returned to the command prompt, too.

Then enable logging to a file:

    (gdb) set logging enabled on

Tip The default file name is `gdb.txt`. An alternate file name can be specified with `set logging file trace.log` within gdb.

And finally have the backtrace written to the specified file in the current working directory:

    (gdb) thread apply all backtrace full

### Attaching to an existing process

If the program you want to debug is already running, you need to first find its process ID. Tools such as pidof(1) or pgrep(1) are available. For example:

    $ pidof python3

    907171 491909

When the output does not give a unique ID, you can try more filtering or look at the output of `ps aux` or `pstree --show-pids`.

Attaching as regular user does not work by default due to restricted ptrace scope. The restriction can be lowered temporarily with `echo 0 > /proc/sys/kernel/yama/ptrace_scope` or you can run gdb as a privileged user, e.g. using sudo.

Start gdb attaching it to the process:

    $ gdb --pid=PID

gdb will ask if Debuginfod should be enabled in this debugging session, to which you should answer `y`.

Note that attaching to a process has stopped it and it needs to be explicitly continued. This replaces the `run` command from the workflow in the #Starting a new instance of a program section:

    (gdb) continue

Now do whatever is necessary to evoke the bug in the attached process. Then proceed with enabling logging and obtaining the trace same as in #Starting a new instance of a program.

### Examining a previous crash

To debug an application that has already crashed, you will want to invoke gdb on its core dump. See Core dump#Analyzing a core dump for details.

If debugging information for the crashed program is not available and a proper backtrace was not obtained, you may need to rebuild the package and reproduce the crash again.

## Manually getting debug info

This article or section is out of date.

Reason: In the Debuginfod era, installing separate debug packages with pacman is not needed for official packages. (Discuss in Talk:Debugging/Getting traces)

The first thing to do is to obtain the names of the packages which require rebuilding or the install of a debug package.

    [...]
    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)
    [...]

For example for the above extract from a trace, the package name for the associated package can be obtained 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 information.

### Installing debug packages

This article or section needs expansion.

Reason: Explain the different scenarios properly. Either you install a debug package or you let debuginfod fetch the things it needs? This also states "A few mirrors" but lists only one. (Discuss in Talk:Debugging/Getting traces#mirrors distributing debug packages)
Note Debug packages are not archived by Arch Linux on Arch Linux Archive.

A few mirrors currently distribute debug packages in accessible repositories. These are sponsored mirrors controlled by Arch Linux and are given access to the debug repositories.

- https://geo.mirror.pkgbuild.com (GeoDNS mirror)

To install a package you can install it directly from the repository. For example:

    # pacman -U https://geo.mirror.pkgbuild.com/core-debug/os/x86_64/zstd-debug-1.5.2-2-x86_64.pkg.tar.zst

Warning Debug packages from one mirror are not compatible with regular packages from another mirror, if both mirrors are not in sync, and thus have mismatching builds. In this case, avoid mixing packages from different mirrors (this would result in a partial upgrade), but point all repositories to the `debug` mirror.

This article or section is a candidate for merging with Official repositories.

Notes: Official repositories have a dedicated page (Discuss in Talk:Debugging/Getting traces)

Another option is to add the repositories to your pacman configuration.

    /etc/pacman.conf

    # Testing Repositories

    [core-testing-debug]
    Include = /etc/pacman.d/mirrorlist

    [extra-testing-debug]
    Include = /etc/pacman.d/mirrorlist

    [multilib-testing-debug]
    Include = /etc/pacman.d/mirrorlist

    # Stable repositories

    [core-debug]
    Include = /etc/pacman.d/mirrorlist

    [extra-debug]
    Include = /etc/pacman.d/mirrorlist

    [multilib-debug]
    Include = /etc/pacman.d/mirrorlist

Place a mirror with debug packages as the first one in the mirrorlist file:

    /etc/pacman.d/mirrorlist

    Server = https://geo.mirror.pkgbuild.com/$repo/os/$arch
    ...

### Rebuilding packages

If debug information is not exposed through debuginfod (for example, when the package originates from the AUR), then it can be rebuilt from source. See ABS for packages in the official repositories, or AUR#Acquire build files for packages in the AUR.

To set the required #Compilation options, you can modify the makepkg configuration if you will only use makepkg for debug purposes. In other cases, you should modify package's `PKGBUILD` file only for each package you would like to rebuild.

#### Compilation options

As of pacman 4.1, makepkg.conf(5) has debug compilation flags in `DEBUG_CFLAGS` and `DEBUG_CXXFLAGS`. To use them, enable the `debug` makepkg option, and disable `strip`.

These settings will force compilation with debug symbols and will disable their stripping from executables.

    /etc/makepkg.conf

    OPTIONS+=(debug !strip)

To apply this setting to a single package, modify the `PKGBUILD`:

    PKGBUILD

    options=(debug !strip)

Alternatively you can put the debug information in a separate package by enabling both `debug` and `strip`, debug symbols will then be stripped from the main package and placed, together with source files to aid in stepping through the debugger, in a separate `pkgbase-debug` package. This is advantageous if the package contains very large binaries (e.g. over a GB with debug symbols included) as it might cause freezing and other strange, unwanted behavior occurring.

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 symbol files are installed under `/usr/lib/debug/`, and source files are installed under `/usr/src/debug`. See the GDB documentation for more information about debug packages.

##### glibc

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,makedb} \
                          usr/bin/{pcprofiledump,pldd,rpcgen,sln,sprof} \
                          usr/lib/getconf/*

    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.

##### Clang

This article or section is out of date.

Reason: The package that served as a reference is no longer in the repositories, do we have a more recent example? (Discuss in Talk:Debugging/Getting traces)

Packages using Clang as the compiler will not build with the `debug` option due to the debug flag `-fvar-tracking-assignments'` not being handled (e.g. the previous js78 PKGBUILD).

Add the following at the top of the `build()` function to only remove the flag for the affected package:

    build() {
      CFLAGS=${CFLAGS/-fvar-tracking-assignments}
      CXXFLAGS=${CXXFLAGS/-fvar-tracking-assignments}
    [...]

##### LTO

Using Link-time optimization (LTO) will, both during compiling and in a debugger, use more memory[1][2]. Depending on the application, especially if it is a large one like Firefox or Qt, it might exceed the available memory. Build the application without LTO if this happens.

All packages in the official repositories are generally built with LTO.

#### 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-2.26-1-x86_64.pkg.tar.gz

## See also

- Debian:HowToGetABacktrace
- Gentoo:Project:Quality Assurance/Backtraces
- Fedora:StackTraces
- GNOME - Getting Stack Traces
- gdb mini intro

Categories:

- Package development
- Development

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