5.8.1.4 Debugging mysqld under gdb

On most systems you can also start mysqld from gdb to get more information if mysqld crashes.

With some older gdb versions on Linux you must use run --one-thread if you want to be able to debug mysqld 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 running mysqld under gdb. Some symptoms are:

• mysqld hangs during startup (before it writes ready for connections).

• mysqld crashes during a pthread_mutex_lock() or pthread_mutex_unlock() call.

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

LD_ASSUME_KERNEL=2.4.1
export LD_ASSUME_KERNEL

When running mysqld under gdb, you should disable the stack trace with --skip-stack-trace to be able to catch segfaults within gdb.

Use the --gdb option to mysqld to install an interrupt handler for SIGINT (needed to stop mysqld with ^C to set breakpoints) and disable stack tracing and core file handling.

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

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

$> gdb mysqld core
gdb>   backtrace full
gdb>   quit

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

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

set print sevenbit off
handle SIGUSR1 nostop noprint
handle SIGUSR2 nostop noprint
handle SIGWAITING nostop noprint
handle SIGLWP nostop noprint
handle SIGPIPE nostop
handle SIGALRM nostop
handle SIGHUP nostop
handle SIGTERM nostop noprint

Here is an example how to debug mysqld:

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

Include the preceding output in a bug report, which you can file using the instructions in Section 1.5, “How to Report Bugs or Problems”.

If mysqld hangs, you can try to use some system tools like strace or /usr/proc/bin/pstack to examine where mysqld has hung.

strace /tmp/log libexec/mysqld

If you are using the Perl DBI interface, you can turn on debugging information by using the trace method or by setting the DBI_TRACE environment variable.