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39.4.3 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 …))

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 β€˜question’, and the second is β€˜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 β€˜’, 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 β€˜...’.