elisp: Logging Messages
37.4.3 Logging Messages in ‘*Messages*’
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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 ‘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 ‘...’.
Info Catalog
elisp: Progress
elisp: The Echo Area
elisp: Echo Area Customization