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. The name is constructed by prepending
.# to the filename of the buffer. The target of the symbolic link will be of the form
user is replaced with the current username (from
host with the name of the host where Emacs is running (from
pid with Emacs’s process id, and
boot with the time since the last reboot.
:boot is omitted if the boot time is unavailable. (On file systems that do not support symbolic links, a regular file is used instead, with contents of the form
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.
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 option
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 is not locked.
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
tsays to grab the lock on the file. Then this user may edit the file and
other-userloses the lock.
A value of
nilsays to ignore the lock and let this user edit the file anyway.
This function may instead signal a
file-lockederror, 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
fileis the name of the file and
other-useris 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.