File Locks - GNU Emacs Lisp Reference Manual

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. (On file systems that do not support symbolic links, a regular file is used.)

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 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, or if the system does not support locking.

— 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, or if the system does not support locking.

— User Option: create-lockfiles

If this variable is nil, Emacs does not lock files.

— 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.

24.5 File Locks