path: root/doc/backtrace.txt

===============================================================================
===
=== Collecting Backtrace Information
===
=== Last updated: 2010-04-12
===============================================================================

This document is intended to provide information on how to obtain the
backtraces required on the asterisk bug tracker, available at
https://issues.asterisk.org. 

-------------------------------------------------------------------------------
--- Overview
-------------------------------------------------------------------------------

The backtrace information is required by developers to help fix problem with 
bugs of any kind. Backtraces provide information about what was wrong when a 
program crashed; in our case, Asterisk. 

-------------------------------------------------------------------------------
--- Preparing Asterisk To Produce Core Files On Crash
-------------------------------------------------------------------------------

First of all, when you start Asterisk, you MUST start it with option
-g. This tells Asterisk to produce a core file if it crashes.

If you start Asterisk with the safe_asterisk script, it automatically
starts using the option -g.

If you're not sure if Asterisk is running with the -g option, type the
following command in your shell:

debian:/tmp# ps aux | grep asterisk
root     17832  0.0  1.2   2348   788 pts/1    S    Aug12   0:00 /bin/sh /usr/sbin/safe_asterisk
root     26686  0.0  2.8  15544  1744 pts/1    S    Aug13   0:02 asterisk -vvvg -c
[...]

The interesting information is located in the last column.

Second, your copy of Asterisk must have been built without
optimization or the backtrace will be (nearly) unusable. This can be
done by selecting the 'DONT_OPTIMIZE' option in the Compiler Flags
submenu in the 'make menuselect' tree before building Asterisk.

Running a production server with DONT_OPTIMIZE is generally safe.
You'll notice the binary files may be a bit larger, but in terms of
Asterisk performance, and impact should be negligible.

After Asterisk crashes, a core file will be "dumped" in your /tmp/
directory. To make sure it's really there, you can just type the
following command in your shell:

debian:/tmp# ls -l /tmp/core.*
-rw-------  1 root root 10592256 Aug 12 19:40 /tmp/core.26252
-rw-------  1 root root  9924608 Aug 12 20:12 /tmp/core.26340
-rw-------  1 root root 10862592 Aug 12 20:14 /tmp/core.26374
-rw-------  1 root root  9105408 Aug 12 20:19 /tmp/core.26426
-rw-------  1 root root  9441280 Aug 12 20:20 /tmp/core.26462
-rw-------  1 root root  8331264 Aug 13 00:32 /tmp/core.26647
debian:/tmp#

In the event that there are multiple core files present (as in the
above example), it is important to look at the file timestamps in
order to determine which one you really intend to look at.

-------------------------------------------------------------------------------
--- Getting Information After A Crash
-------------------------------------------------------------------------------

There are two kind of backtraces (aka 'bt') which are useful: bt and bt full.

Now that we've verified the core file has been written to disk, the final part 
is to extract 'bt' from the core file. Core files are pretty big, don't be 
scared, it's normal.

******************************************************************************
*** NOTE: Don't attach core files on the bug tracker as they are only useful *
***       on the machine they were generated on. We only need the output of  *
***       the 'bt' and 'bt full.'                                            *
******************************************************************************

For extraction, we use a really nice tool, called gdb. To verify that
you have gdb installed on your system:

debian:/tmp# gdb -v
GNU gdb 6.3-debian
Copyright 2004 Free Software Foundation, Inc.
GDB is free software, covered by the GNU General Public License, and you are
welcome to change it and/or distribute copies of it under certain conditions.
Type "show copying" to see the conditions.
There is absolutely no warranty for GDB.  Type "show warranty" for details.
This GDB was configured as "i386-linux".
debian:/tmp#

If you don't have gdb installed, go install gdb. You should be able to install
using something like: apt-get install gdb --or-- yum install gdb

Now load the core file in gdb with the following command. This will also save
the output of gdb to the /tmp/backtract.txt file.

# gdb -se "asterisk" -c /tmp/core.26252 | tee /tmp/backtrace.txt

******************************************************************************
*** TIP!
***     Just run the following command to get the output into the
***     backtrace.txt file, ready for uploading to the issue tracker. Be sure
***     to change the name of the core file to your actual core dump file:
***
*** gdb -se "asterisk" -ex "bt full" -ex "thread apply all bt" --batch -c /tmp/core.26252 > /tmp/backtrace.txt
***
******************************************************************************


[...]
(You would see a lot of output here.)
[...]
Reading symbols from /usr/lib/asterisk/modules/app_externalivr.so...done.
Loaded symbols for /usr/lib/asterisk/modules/app_externalivr.so
#0  0x29b45d7e in ?? ()
(gdb)

In order to make extracting the gdb output easier, you may wish to
turn on logging using "set logging on". This command will save all
output to the default file of gdb.txt, which in the end can be 
uploaded as an attachment to the bug tracker.

Now at the gdb prompt, type: bt
You would see output similar to:

(gdb) bt
#0  0x29b45d7e in ?? ()
#1  0x08180bf8 in ?? ()
#2  0xbcdffa58 in ?? ()
#3  0x08180bf8 in ?? ()
#4  0xbcdffa60 in ?? ()
#5  0x08180bf8 in ?? ()
#6  0x180bf894 in ?? ()
#7  0x0bf80008 in ?? ()
#8  0x180b0818 in ?? ()
#9  0x08068008 in ast_stopstream (tmp=0x40758d38) at file.c:180
#10 0x000000a0 in ?? ()
#11 0x000000a0 in ?? ()
#12 0x00000000 in ?? ()
#13 0x407513c3 in confcall_careful_stream (conf=0x8180bf8, filename=0x8181de8 "Zap/pseudo-1324221520") at app_meetme.c:262
#14 0x40751332 in streamconfthread (args=0x8180bf8) at app_meetme.c:1965
#15 0xbcdffbe0 in ?? ()
#16 0x40028e51 in pthread_start_thread () from /lib/libpthread.so.0
#17 0x401ec92a in clone () from /lib/libc.so.6
(gdb)


The bt's output is the information that we need on the bug tracker.

Now do a bt full as follows:

(gdb) bt full
#0  0x29b45d7e in ?? ()
No symbol table info available.
#1  0x08180bf8 in ?? ()
No symbol table info available.
#2  0xbcdffa58 in ?? ()
No symbol table info available.
#3  0x08180bf8 in ?? ()
No symbol table info available.
#4  0xbcdffa60 in ?? ()
No symbol table info available.
#5  0x08180bf8 in ?? ()
No symbol table info available.
#6  0x180bf894 in ?? ()
No symbol table info available.
#7  0x0bf80008 in ?? ()
No symbol table info available.
#8  0x180b0818 in ?? ()
No symbol table info available.
#9  0x08068008 in ast_stopstream (tmp=0x40758d38) at file.c:180
No locals.
#10 0x000000a0 in ?? ()
No symbol table info available.
#11 0x000000a0 in ?? ()
No symbol table info available.
#12 0x00000000 in ?? ()
No symbol table info available.
#13 0x407513c3 in confcall_careful_stream (conf=0x8180bf8, filename=0x8181de8 "Zap/pseudo-1324221520") at app_meetme.c:262
        f = (struct ast_frame *) 0x8180bf8
        trans = (struct ast_trans_pvt *) 0x0
#14 0x40751332 in streamconfthread (args=0x8180bf8) at app_meetme.c:1965
No locals.
#15 0xbcdffbe0 in ?? ()
No symbol table info available.
#16 0x40028e51 in pthread_start_thread () from /lib/libpthread.so.0
No symbol table info available.
#17 0x401ec92a in clone () from /lib/libc.so.6
No symbol table info available.
(gdb)

The final "extraction" would be to know all traces by all threads. Even if 
Asterisk runs on the same thread for each call, it could have created some new
threads.

To make sure we have the correct information, just do:
(gdb) thread apply all bt

Thread 1 (process 26252):
#0  0x29b45d7e in ?? ()
#1  0x08180bf8 in ?? ()
#2  0xbcdffa58 in ?? ()
#3  0x08180bf8 in ?? ()
#4  0xbcdffa60 in ?? ()
#5  0x08180bf8 in ?? ()
#6  0x180bf894 in ?? ()
#7  0x0bf80008 in ?? ()
#8  0x180b0818 in ?? ()
#9  0x08068008 in ast_stopstream (tmp=0x40758d38) at file.c:180
#10 0x000000a0 in ?? ()
#11 0x000000a0 in ?? ()
#12 0x00000000 in ?? ()
#13 0x407513c3 in confcall_careful_stream (conf=0x8180bf8, filename=0x8181de8 "Zap/pseudo-1324221520") at app_meetme.c:262
#14 0x40751332 in streamconfthread (args=0x8180bf8) at app_meetme.c:1965
#15 0xbcdffbe0 in ?? ()
#16 0x40028e51 in pthread_start_thread () from /lib/libpthread.so.0
#17 0x401ec92a in clone () from /lib/libc.so.6
(gdb)


That output tells us crucial information about each thread.

-------------------------------------------------------------------------------
--- Getting Information For A Deadlock
-------------------------------------------------------------------------------

Whenever supplying information about a deadlock (i.e. when you run the
'core show locks' command on the Asterisk console), it is useful to also have
additional information about the threads. We can generate this information by
attaching to a running Asterisk process and gathering that information.

You can easily attach to a running Asterisk process, gather the output required
and then detach from the process all in a single step. Execute the following
command and upload the resulting backtrace-threads.txt file to the Asterisk
issue tracker:

   gdb -ex "thread apply all bt" --batch /usr/sbin/asterisk `pidof asterisk` > /tmp/backtrace-threads.txt

Note that this gathers information from the running Asterisk process, so you
want to make sure you run this command immediately before or after gathering
the output of 'core show locks'. You can gather that information by running the
following command:

   asterisk -rx "core show locks" > /tmp/core-show-locks.txt

-------------------------------------------------------------------------------
--- Verify Your Backtraces
-------------------------------------------------------------------------------

Before uploading your backtraces to the issue tracker, you should double check
to make sure the data you have is of use to the developers. Check your
backtrace files to make sure you're not seeing several of the following:

   <value optimized out>

If you are, then you likely haven't compiled with DONT_OPTIMIZE. The impact of
DONT_OPTIMIZE is negligible on most systems. Be sure you've enabled the
DONT_OPTIMIZE flag within the Compiler Flags section of menuselect. After
doing so, be sure to run 'make install' and restart Asterisk.

-------------------------------------------------------------------------------
--- Uploading Your Information To The Issue Tracker
-------------------------------------------------------------------------------

You're now ready to upload your files to the Asterisk issue tracker (located at
https://issues.asterisk.org).

******************************************************************************
*** NOTE: Please ATTACH your output! DO NOT paste it as a note!              *
******************************************************************************

If you have questions or comments regarding this documentation, feel free to
pass by the #asterisk-bugs channel on irc.freenode.net.