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File README of Package segv_handler

segv_handler
============

Andrew Tridgell
junkcode@tridgell.net

This tool allows you to catch segmemtation faults globally on your
system, giving a full backtrace for any program that dies with a
segmemtation fault. This is done with zero overhead in the running
program, so you can install it on production systems without
interference.

Building
--------

Just type 'make'. If it fails to build then consult a systems
programmer or local Linux oracle.

Installing
----------

You need to install two files. The small shared library
segv_handler.so can be put anywhere on the system, but somewhere like
/usr/local/lib probably makes most sense.

The shell script 'backtrace' needs to be installed somewhere in the
main system PATH, where all programs can see it. This usually means
/usr/bin. It must be made executable using 'chmod +x backtrace'.

You may wish to edit the script to place the resulting backtraces in
some other directory than /tmp, although you have to make sure the
directory is writable by all programs that you wish to monitor.

Testing
-------

After installing try 'make test'. You should see something like this
at the end of the output:

-rw-rw-r--    1 tridge   tridge        479 Sep 16 20:38 /tmp/segv_testprog.11817.out

look in that file. Notice that it contains a full backtrace of the
program 'testprog' when it died.

Usage Method 1 - per program
----------------------------

You can use segv_handler.so on a per program basis by setting the
LD_PRELOAD environment variable before starting the program. For
example you might start the 'winbindd' daemon from Samba in a script
like this:

#!/bin/sh

export LD_PRELOAD=/usr/local/lib/segv_handler.so
   /usr/local/samba/sbin/winbindd

If you use the above script to start winbindd then if winbindd dies it
will leave behind a file called /tmp/segv_winbindd.XXXX.out. That is
the file that you would show to a programmer to find the bug.

Usage Method 2 - entire system
------------------------------

Note! This method can be dangerous if you don't get it right. It is
the way I usually use segv_handler as it provides a useful way to
catch segmemtation faults across the entire system, but if you make a
typo when installing then your system may become unbootable. Have a
rescue disk handy.

To install globally you need to edit or create a file called
   /etc/ld.so.preload
That file should contain the full path to where you installed 
segv_handler.so. It must be exactly the full path and nothing
more. You must also be careful to remove /etc/ld.so.preload before
removing or renaming segv_handler.so. If you leave a file in
/etc/ld.so.preload that does not exist or is not an appropriate
shared library then all programs on your system will fail to start.

That's it! After you create /etc/ld.so.preload any program that starts
from then on will have the segmentation handler installed. If it dies
with a segmentation fault then you will get a full backtrace in /tmp

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