# Debug - Getting Traces

Related articles

- General troubleshooting
- Reporting bug guidelines
- Step-by-step debugging guide

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

This article or section needs language, wiki syntax or style improvements.

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)

Note: `debug` in addition to DEBUG_* conterparts, would also add flags from CFLAGS and CXXFLAGS, which might not be what you want, as these flags usually contains optimisations. One way to mitigate this is by putting

    CFLAGS=""
    CXXFLAGS=""

at the beginning of the `PKGBUILD` file. This way isn't documented, and may break at some point though.

Another one is to use a separate config file instead of the `/etc/makepkg.conf`, by pointing at it like `makepkg --config my-other-config`.

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.

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

This article or section needs expansion.

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

- Gentoo Wiki - Backtraces with Gentoo
- Fedora - StackTraces
- GNOME - Getting Stack Traces
- gdb mini intro

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- Package development

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