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(elisp)File Locks

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File Locks
==========

   When two users edit the same file at the same time, they are likely
to interfere with each other.  Emacs tries to prevent this situation
from arising by recording a "file lock" when a file is being modified.
Emacs can then detect the first attempt to modify a buffer visiting a
file that is locked by another Emacs job, and ask the user what to do.
The file lock is really a file, a symbolic link with a special name,
stored in the same directory as the file you are editing.

   When you access files using NFS, there may be a small probability
that you and another user will both lock the same file "simultaneously".
If this happens, it is possible for the two users to make changes
simultaneously, but Emacs will still warn the user who saves second.
Also, the detection of modification of a buffer visiting a file changed
on disk catches some cases of simultaneous editing; see Note:
Modification Time.

 - Function: file-locked-p filename
     This function returns `nil' if the file FILENAME is not locked.
     It returns `t' if it is locked by this Emacs process, and it
     returns the name of the user who has locked it if it is locked by
     some other job.

          (file-locked-p "foo")
               => nil

 - Function: lock-buffer &optional filename
     This function locks the file FILENAME, if the current buffer is
     modified.  The argument FILENAME defaults to the current buffer's
     visited file.  Nothing is done if the current buffer is not
     visiting a file, or is not modified.

 - Function: unlock-buffer
     This function unlocks the file being visited in the current buffer,
     if the buffer is modified.  If the buffer is not modified, then
     the file should not be locked, so this function does nothing.  It
     also does nothing if the current buffer is not visiting a file.

   File locking is not supported on some systems.  On systems that do
not support it, the functions `lock-buffer', `unlock-buffer' and
`file-locked-p' do nothing and return `nil'.

 - Function: ask-user-about-lock file other-user
     This function is called when the user tries to modify FILE, but it
     is locked by another user named OTHER-USER.  The default
     definition of this function asks the user to say what to do.  The
     value this function returns determines what Emacs does next:

        * A value of `t' says to grab the lock on the file.  Then this
          user may edit the file and OTHER-USER loses the lock.

        * A value of `nil' says to ignore the lock and let this user
          edit the file anyway.

        * This function may instead signal a `file-locked' error, in
          which case the change that the user was about to make does
          not take place.

          The error message for this error looks like this:

               error--> File is locked: FILE OTHER-USER

          where `file' is the name of the file and OTHER-USER is the
          name of the user who has locked the file.

     If you wish, you can replace the `ask-user-about-lock' function
     with your own version that makes the decision in another way.  The
     code for its usual definition is in `userlock.el'.