The error log file contains information indicating when mysqld was started and stopped and also any critical errors that occur while the server is running. If mysqld notices a table that needs to be automatically checked or repaired, it writes a message to the error log.

On some operating systems, the error log contains a stack trace if mysqld dies. The trace can be used to determine where mysqld died. See Using a Stack Trace.

If mysqld_safe is used to start mysqld and mysqld dies unexpectedly, mysqld_safe notices that it needs to restart mysqld and writes a restarted mysqld message to the error log.

You can specify where mysqld stores the error log file with the --log-error[=file_name] option. If no file_name value is given, mysqld uses the name host_name.err and writes the file in the data directory. If you execute FLUSH LOGS, the error log is renamed with the suffix -old and mysqld creates a new empty log file. (No renaming occurs if the --log-error option was not given.)

If you do not specify --log-error, or (on Windows) if you use the --console option, errors are written to stderr, the standard error output. Usually this is your terminal.

On Windows, error output is always written to the .err file if --console is not given.

The --log-warnings option or log_warnings system variable can be used to control warning logging to the error log. The default value is enabled (1). Warning logging can be disabled using a value of 0. Aborted connections are not logged to the error log unless the value is greater than 1. See Section B.1.2.10, “Communication Errors and Aborted Connections”.