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41.4.3 Logging Messages in *Messages*

Almost all the messages displayed in the echo area are also recorded in the *Messages* buffer so that the user can refer back to them. This includes all the messages that are output with message. By default, this buffer is read-only and uses the major mode messages-buffer-mode. Nothing prevents the user from killing the *Messages* buffer, but the next display of a message recreates it. Any Lisp code that needs to access the *Messages* buffer directly and wants to ensure that it exists should use the function messages-buffer.

Function: messages-buffer ¶

This function returns the *Messages* buffer. If it does not exist, it creates it, and switches it to messages-buffer-mode.

User Option: message-log-max ¶

This variable specifies how many lines to keep in the *Messages* buffer. The value t means there is no limit on how many lines to keep. The value nil disables message logging entirely. Here’s how to display a message and prevent it from being logged:

(let (message-log-max)
  (message …))

Variable: messages-buffer-name ¶

This variable has the name of the buffer where messages should be logged to, and defaults to *Messages*. Some packages may find it useful to temporarily redirect the output to a different buffer (perhaps to write the buffer out to a log file later), and they can bind this variable to a different buffer name. (Note that this buffer (if it doesn’t exist already), will be created and put into messages-buffer-mode.)

To make *Messages* more convenient for the user, the logging facility combines successive identical messages. It also combines successive related messages for the sake of two cases: question followed by answer, and a series of progress messages.

A question followed by an answer has two messages like the ones produced by y-or-n-p: the first is ‘question’, and the second is ‘question...answer’. The first message conveys no additional information beyond what’s in the second, so logging the second message discards the first from the log.

A series of progress messages has successive messages like those produced by make-progress-reporter. They have the form ‘base...how-far’, where base is the same each time, while how-far varies. Logging each message in the series discards the previous one, provided they are consecutive.

The functions make-progress-reporter and y-or-n-p don’t have to do anything special to activate the message log combination feature. It operates whenever two consecutive messages are logged that share a common prefix ending in ‘...’.