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How can I make Emacs retain its undo history for my buffer after doing revert-buffer or using auto-revert-mode?
In Vim, if a file that is open in a buffer is changed on disc, Vim prompts me to reload the file. I can then simply click 'u' to undo the reload if I so wish and even go back further from then. Emacs seems to trash all the undo information the moment I revert the buffer.
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The number of buffers you can have really has no limit. Most of the time, only one or two buffers are displayed, but even if you can't see them, all the buffers you create in an Emacs session are still active. You can think of them as a stack of pages, with the one being displayed as the top page.
When Emacs has multiple windows, each window has a chosen buffer which is displayed there, but at any time only one of the windows is selected and its chosen buffer is the selected buffer. Each window's mode line displays the name of the buffer that the window is displaying (see section Multiple Windows).
This is because Emacs tracks buffer positions using that data type. For typical 64-bit machines, this maximum buffer size is 2^{61} - 2 bytes, or about 2 EiB. For typical 32-bit machines, the maximum is usually 2^{29} - 2 bytes, or about 512 MiB.
Refresh the Dired buffer using switches as dired-listing-switches . Type g ( revert-buffer ) to update the contents of the Dired buffer, based on changes in the files and directories listed. This preserves all marks except for those on files that have vanished. Hidden subdirectories are updated but remain hidden.
Emacs allows you to set revert-buffer-function to override the behaviour. Here's a revert-buffer implementation that keeps the history.
;; emacs doesn't actually save undo history with revert-buffer
;; see http://lists.gnu.org/archive/html/bug-gnu-emacs/2011-04/msg00151.html
;; fix that.
(defun revert-buffer-keep-history (&optional IGNORE-AUTO NOCONFIRM PRESERVE-MODES)
(interactive)
;; tell Emacs the modtime is fine, so we can edit the buffer
(clear-visited-file-modtime)
;; insert the current contents of the file on disk
(widen)
(delete-region (point-min) (point-max))
(insert-file-contents (buffer-file-name))
;; mark the buffer as not modified
(not-modified)
(set-visited-file-modtime))
(setq revert-buffer-function 'revert-buffer-keep-history)
The upcoming Emacs-24.4 does what you want by default.
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