On some operating systems, the error log will contain a stack trace if mysqld dies unexpectedly. You can use this to find out where (and maybe why) mysqld died. See Error log. To get a stack trace, you must not compile mysqld with the -fomit-frame-pointer option to gcc. See Compiling for debugging.
If the error file contains something like the following:
mysqld got signal 11; The manual section 'Debugging a MySQL server' tells you how to use a stack trace and/or the core file to produce a readable backtrace that may help in finding out why mysqld died Attemping backtrace. You can use the following information to find out where mysqld died. If you see no messages after this, something went terribly wrong stack range sanity check, ok, backtrace follows 0x40077552 0x81281a0 0x8128f47 0x8127be0 0x8127995 0x8104947 0x80ff28f 0x810131b 0x80ee4bc 0x80c3c91 0x80c6b43 0x80c1fd9 0x80c1686
you can find where mysqld died by doing the following:
Copy the above numbers to a file, for example mysqld.stack.
Make a symbol file for the mysqld server:
nm -n libexec/mysqld > /tmp/mysqld.sym
Note that most MySQL binary distributions (except for the "debug" packages, where this information is included inside of the binaries themselves) already ship with the above file, named mysqld.sym.gz. In this case you can simply unpack it by doing:
gunzip < bin/mysqld.sym.gz > /tmp/mysqld.sym
Execute resolve_stack_dump -s /tmp/mysqld.sym -n mysqld.stack.
This will print out where mysqld died. If this doesn't help you find out why mysqld died, you should make a bug report and include the output from the above command with the bug report.
Note however that in most cases it will not help us to just have a stack trace to find the reason for the problem. To be able to locate the bug or provide a workaround, we would in most cases need to know the query that killed mysqld and preferable a test case so that we can repeat the problem! See Bug reports.