The error log 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 exits abnormally. The trace can be used to determine where mysqld exited. See Section 24.5, “Debugging and Porting MySQL”.

If mysqld_safe is used to start mysqld and mysqld exits abnormally, mysqld_safe notices this, restarts mysqld, and writes a mysqld restarted message to the error log.

In the following discussion, “console” means stderr, the standard error output; this is your terminal or console window unless the standard error output has been redirected. (For example, if invoked with the --syslog option, mysqld_safe arranges for the server's stderr to be sent to syslog, as described later.)

On Windows, the --log-error, --pid-file, and --console options affect error logging:

In addition, on Windows, the server writes events and error messages to the Windows Event Log within the Application log. Entries marked as Warning and Note are written to the Event Log, but not informational messages such as information statements from individual storage engines. These log entries have a source of MySQL. You cannot disable writing information to the Windows Event Log.

On Unix and Unix-like systems, mysqld writes error log messages as follows:

The log_error system variable indicates the error log file name if error output is written to a file.

If you specify --log-error in an option file in a [mysqld], [server], or [mysqld_safe] section, mysqld_safe finds and uses the option.

If you flush the logs using FLUSH LOGS or mysqladmin flush-logs and mysqld is writing the error log to a file (for example, if it was started with the --log-error option), the server closes and reopens the log file. To rename the file, do so manually before flushing. Flushing the logs then reopens a new file with the original file name. For example, to rename the file and create a new one, use the following commands (assuming a log file name of host_name.err):

shell> mv host_name.err host_name.err-old
shell> mysqladmin flush-logs
shell> mv host_name.err-old backup-directory

On Windows, use rename rather than mv.

If the server is not writing to a named file, no error log renaming occurs when the logs are flushed.

If you use mysqld_safe to start mysqld, mysqld_safe arranges for mysqld to write error messages to a log file or to syslog. mysqld_safe has three error-logging options, --syslog, --skip-syslog, and --log-error. The default with no logging options or with --skip-syslog is to use the default log file. To explicitly specify use of an error log file, specify --log-error=file_name to mysqld_safe, and mysqld_safe will arrange for mysqld to write messages to a log file. To use syslog instead, specify the --syslog option.

The --log-warnings option or log_warnings system variable controls warning logging to the error log. The default value is enabled (1). To disable warning logging, set --log-warnings or log_warnings to 0. If the value is greater than 1, aborted connections are written to the error log, and access-denied errors for new connection attempts are written. See Section B.5.2.11, “Communication Errors and Aborted Connections”.