14.3.2 Protection Against Simultaneous Editing
Simultaneous editing occurs when two users visit the same file, both
make changes, and both save their changes. If no one was informed that
this was happening, and you saved first, you would later find that your
changes were lost. On some systems, Emacs notices immediately when the
second user starts to change a file already being edited, and issues a
warning. When this is not possible, or if the second user has started
to change the file despite the warning, Emacs checks when the file is
saved, and issues a second warning when a user is about to overwrite a
file containing another user's changes. If you are the user editing the
file, you can take corrective action at this point and prevent actual
loss of work.
When you make the first modification in an Emacs buffer that is
visiting a file, Emacs records that you have locked the file. (It does
this by writing another file in a directory reserved for this purpose.)
The lock is removed when you save the changes. The idea is that the
file is locked whenever the buffer is modified. If you begin to modify
the buffer while the visited file is locked by someone else, this
constitutes a collision, and Emacs asks you what to do. It does this
by calling the Lisp function `ask-user-about-lock', which you can
redefine to customize what it does. The standard definition of this
function asks you a question and accepts three possible answers:
Steal the lock. Whoever was already changing the file loses the
lock, and you get the lock.
Proceed. Go ahead and edit the file despite its being locked by
Quit. This causes an error (`file-locked') and the modification
you were trying to make in the buffer does not actually take place.
Note that locking works on the basis of a file name; if a file has
multiple names, Emacs does not realize that the two names are the same
file and cannot prevent two users from editing it simultaneously under
different names. However, basing locking on names means that Emacs can
interlock the editing of new files that do not really exist until they
Some systems are not configured to allow Emacs to make locks. On
these systems, Emacs cannot detect trouble in advance, but it can still
detect it in time to prevent you from overwriting someone else's
Every time Emacs saves a buffer, it first checks the
last-modification date of the existing file on disk to see that it has
not changed since the file was last visited or saved. If the date does
not match, it implies that changes were made in the file in some other
way, and these changes are about to be lost if Emacs actually does
save. To prevent this, Emacs prints a warning message and asks for
confirmation before saving. Occasionally you will know why the file
was changed and know that it does not matter; then you can answer `yes'
and proceed. Otherwise, you should cancel the save with `C-g' and
investigate the situation.
The first thing you should do when notified that simultaneous editing
has already taken place is to list the directory with `C-u C-x C-d'
(Note: Directory Listing.). This will show the file's current
author. You should attempt to contact that person and ask him not to
continue editing. Often the next step is to save the contents of your
Emacs buffer under a different name, and use `diff' to compare the two
Simultaneous editing checks are also made when you visit a file that
is already visited with `C-x C-f' and when you start to modify a file.
This is not strictly necessary, but it is useful to find out about such
a problem as early as possible, when corrective action takes less work.
Another way to protect your file is to set the read, write, and
executable permissions for the file. Use the function
`set-default-file-modes' to set the UNIX `umask' value to the NMASK
argument. The `umask' value is the default protection mode for new
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