5.2.2 The Error Log

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 dies. The trace can be used to determine where mysqld died. See Section 22.4, “Debugging and Porting MySQL”.

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 the syslog facility, as described later.)

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

In addition, on Windows, events and error messages are written 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:

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

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 effect is version dependent:

No error log renaming occurs when the logs are flushed in any case if the server is not writing to a named file.

If you use mysqld_safe to start mysqld, mysqld_safe arranges for mysqld to write error messages to a log file or (as of MySQL 5.1.20) to syslog:

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

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.

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. If the value is greater than 1, aborted connections are written to the error log. See Section C.5.2.11, “Communication Errors and Aborted Connections”.