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.

File locks do not work properly when multiple machines can share file systems, such as with NFS. Perhaps a better file locking system will be implemented in the future. When file locks do not work, it is possible for two users to make changes simultaneously, but Emacs can 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 section Comparison of Modification Time.

Function: file-locked-p filename

This function returns nil if the file filename is not locked by this Emacs process. It returns t if it is locked by this Emacs, and it returns the name of the user who has locked it if it is locked by someone else.

(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.

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 value it returns determines what happens 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.

The default definition of this function asks the user to choose what to do. If you wish, you can replace the ask-user-about-lock function with your own version that decides in another way. The code for its usual definition is in `userlock.el'.

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