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On most systems you can also startmysqld fromgdb to get more information ifmysqld crashes.

With some oldergdb versions on Linux you must userun --one-thread if you want to be able to debugmysqld threads. In this case, you can only have one thread active at a time.

NPTL threads (the new thread library on Linux) may cause problems while runningmysqld undergdb. Some symptoms are:

In this case, you should set the following environment variable in the shell before startinggdb:

LD_ASSUME_KERNEL=2.4.1export LD_ASSUME_KERNEL

When runningmysqld undergdb, you should disable the stack trace with--skip-stack-trace to be able to catch segfaults withingdb.

Use the--gdb option tomysqld to install an interrupt handler forSIGINT (needed to stopmysqld with^C to set breakpoints) and disable stack tracing and core file handling.

It is very hard to debug MySQL undergdb if you do a lot of new connections the whole time asgdb does not free the memory for old threads. You can avoid this problem by startingmysqld withthread_cache_size set to a value equal tomax_connections + 1. In most cases just using--thread_cache_size=5' helps a lot!

If you want to get a core dump on Linux ifmysqld dies with a SIGSEGV signal, you can startmysqld with the--core-file option. This core file can be used to make a backtrace that may help you find out whymysqld died:

$> gdb mysqld coregdb>   backtrace fullgdb>   quit

SeeSection B.3.3.3, “What to Do If MySQL Keeps Crashing”.

If you are usinggdb on Linux, you should install a.gdb file, with the following information, in your current directory:

set print sevenbit offhandle SIGUSR1 nostop noprinthandle SIGUSR2 nostop noprinthandle SIGWAITING nostop noprinthandle SIGLWP nostop noprinthandle SIGPIPE nostophandle SIGALRM nostophandle SIGHUP nostophandle SIGTERM nostop noprint

Here is an example how to debugmysqld:

$> gdb /usr/local/libexec/mysqldgdb> run...backtrace full # Do this when mysqld crashes

Include the preceding output in a bug report, which you can file using the instructions inSection 1.6, “How to Report Bugs or Problems”.

Ifmysqld hangs, you can try to use some system tools likestrace or/usr/proc/bin/pstack to examine wheremysqld has hung.

strace /tmp/log libexec/mysqld

If you are using the PerlDBI interface, you can turn on debugging information by using thetrace method or by setting theDBI_TRACE environment variable.