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Protection against Simultaneous Editing

   Simultaneous editing occurs when two users visit the same file, both
make changes, and then both save them.  If nobody were informed that
this was happening, whichever user saved first would later find that his
changes were lost.  On some systems, Emacs notices immediately when the
second user starts to change the file, and issues an immediate warning.

   For the sake of systems where that is not possible, and in case
someone else proceeds to change the file despite the warning, Emacs also
checks when the file is saved, and issues a second warning if you are
about to overwrite a file containing another user's changes.  You can
prevent loss of the other user's work by taking the proper corrective
action at that time.

   When you make the first modification in an Emacs buffer that is
visiting a file, Emacs records that  the file is "locked" by you.  (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 an Emacs buffer visiting it has
unsaved changes.

   If you begin to modify the buffer while the visited file is locked by
someone else, this constitutes a "collision".  When Emacs detects a
collision, it asks you what to do, by calling the Lisp function
`ask-user-about-lock'.  You can redefine this function for the sake of
customization.  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 gain the lock.

     Proceed.  Go ahead and edit the file despite its being locked by
     someone else.

     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 will not really exist until
they are saved.

   Some systems are not configured to allow Emacs to make locks.  On
these systems, Emacs cannot detect trouble in advance, but it still can
detect the collision when you try to save a file and overwrite someone
else's changes.

   Every time Emacs saves a buffer, it first checks the
last-modification date of the existing file on disk to verify 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  shows the file's
current author.  You should attempt to contact him to warn 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 with `C-x
C-f' a file that is already visited and when you start to modify a
file.  This is not strictly necessary, but it can cause you to find out
about the collision earlier, when perhaps correction takes less work.

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