Backtrace

Debugging Central

This page is part of the debugging series — pages with debugging details for a variety of Ubuntu packages.

A backtrace shows a listing of which program functions are still active. Since functions are nested when they are called, the program must record where it left one function, to jump into an inner one. It does this on the stack, which we dump for the backtrace.

By getting a backtrace at the point of a bug, a developer may be able to isolate where that bug is, because it will narrow down to the function, or even the line, that caused the erroneous behaviour.

When using KDE the KDE Crash Handler will intercept debugging information. However, it is possible to disable the KDE Crash Handler when you are launching an application by passing the --nocrashhandler argument to the application. For example, 'kget --nocrashhandler' instead of 'kget'.

Generation

  1. Please ensure you have packages with debug symbols installed. You can do this by following the instructions at DebuggingProgramCrash.

  2. Make sure the GNU Debugger is installed.

    sudo apt-get install gdb
  3. Start the program under control of gdb:

    gdb <program> 2>&1 | tee ~/gdb-<program>.txt
    (gdb) handle SIG33 pass nostop noprint
    (gdb) set pagination 0
    (gdb) run <arguments, if any>

    (If the program must run as root, use sudo gdb instead of just gdb above.)

  4. The program will start. Perform any actions necessary to reproduce the crash. If the program hangs but doesn't crash you can press ctrl+c in gdb while the program is frozen and then continue with the next step.
  5. Retrieve a backtrace:

    (gdb) backtrace full
    (gdb) info registers
    (gdb) x/16i $pc
    (gdb) thread apply all backtrace
    (gdb) quit
  6. Attach the complete output from GDB, contained in gdb-<program>.txt, in your bug report. You will find the file in your home directory /home/<username>/.

Applications using Mono

To obtain a backtrace from a Mono application such as Beagle or F-Spot, use Mono's --debug option, e.g.

mono --debug /usr/lib/f-spot/f-spot.exe

Already running programs

You can ask GDB to attach to a program that's already running. This is useful for debugging things that start up, but crash when you perform a particular task.

  1. Make sure the GNU Debugger is installed.

    sudo apt-get install gdb
  2. Find the process ID of <program>:

    pidof <program>
  3. Start gdb:

    gdb 2>&1 | tee gdb-<program>.txt
    (gdb) handle SIG33 pass nostop noprint
    (gdb) set pagination 0
    (gdb) attach <PID>

    (If the program is running as root, use sudo gdb instead of just gdb above.)

  4. Continue the <program>:

    (gdb) continue
  5. The program will continue running. Perform any actions necessary to reproduce the crash. If the program hangs but doesn't crash you can press ctrl+c in gdb while the program is frozen and then continue with the next step.
  6. Retrieve a backtrace:

    (gdb) backtrace full
    (gdb) info registers
    (gdb) x/16i $pc
    (gdb) thread apply all backtrace
    (gdb) quit
  7. Attach the complete output from GDB, contained in gdb-<program>.txt, in your bug report.

Note that you can also set logging to a file like this:

(gdb) set logging file gdb-<program>.txt
(gdb) set logging on

Core Files

You are also able to retrace a core file if you have one produced. (I believe these are disabled by default)

  1. Load the core file into the debugger

     gdb -c <corefile> 2>&1 | tee gdb-<program>.txt
  2. Retrieve a backtrace of the crash:

    (gdb) backtrace full
    (gdb) info registers
    (gdb) x/16i $pc
    (gdb) thread apply all backtrace
    (gdb) quit
  3. Attach the complete output from GDB, contained in gdb-<program>.txt, in your bug report. You will find the file in your home directory /home/<username>/.

Other resources

Summary in script form

You can automate backtrace collection as described above using this Bourne shell script:

  • save it somewhere in your $PATH
  • make it executable

#---------------------------------------------------------------------
usage() {
    cat<<EOF
Usage: ${0} program_name [program_args]

Trace a given program using gdb.

EOF
}

log() {
    echo "${*}" 1>&2
}

die() {
    usage
    log 'error:' ${*}'.'
    exit 1
}


#---------------------------------------------------------------------
test "x${*}" = "x" && die 'no process given'

LOG="/tmp/gdb-`basename ${1}`.txt"
log "outputting trace to '${LOG}'"

exec gdb -batch-silent \
    -ex 'set logging overwrite on' \
    -ex "set logging file ${LOG}" \
    -ex 'set logging on' \
    -ex 'handle SIG33 pass nostop noprint' \
    -ex 'set pagination 0' \
    -ex 'run' \
    -ex 'backtrace full' \
    -ex 'info registers' \
    -ex 'x/16i $pc' \
    -ex 'thread apply all backtrace' \
    -ex 'quit' \
    --args ${*} \
    < /dev/null


CategoryBugSquad CategoryDebugging

Backtrace (last edited 2010-02-02 17:51:45 by sense)