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38.20 Notifications on File Changes

Several operating systems support watching of filesystems for changes of files. If configured properly, Emacs links a respective library like inotify, kqueue, gfilenotify, or w32notify statically. These libraries enable watching of filesystems on the local machine.

It is also possible to watch filesystems on remote machines, see Remote Files This does not depend on one of the libraries linked to Emacs.

Since all these libraries emit different events on notified file changes, there is the Emacs library filenotify which provides a unique interface.

— Function: file-notify-add-watch file flags callback

Add a watch for filesystem events pertaining to file. This arranges for filesystem events pertaining to file to be reported to Emacs.

The returned value is a descriptor for the added watch. Its type depends on the underlying library, it cannot be assumed to be an integer as in the example below. It should be used for comparison by equal only.

If the file cannot be watched for some reason, this function signals a file-notify-error error.

Sometimes, mounted filesystems cannot be watched for file changes. This is not detected by this function, a non-nil return value does not guarantee that changes on file will be notified.

flags is a list of conditions to set what will be watched for. It can include the following symbols:

watch for file changes
watch for file attribute changes, like permissions or modification time

If file is a directory, changes for all files in that directory will be notified. This does not work recursively.

When any event happens, Emacs will call the callback function passing it a single argument event, which is of the form

          (descriptor action file [file1])

descriptor is the same object as the one returned by this function. action is the description of the event. It could be any one of the following symbols:

file was created
file was deleted
file's contents has changed; with w32notify library, reports attribute changes as well
file has been renamed to file1
a file attribute was changed
watching file has been stopped

Note that the w32notify library does not report attribute-changed events. When some file's attribute, like permissions or modification time, has changed, this library reports a changed event. Likewise, the kqueue library does not report reliably file attribute changes when watching a directory.

The stopped event reports, that watching the file has been stopped. This could be because file-notify-rm-watch was called (see below), or because the file being watched was deleted, or due to another error reported from the underlying library.

file and file1 are the name of the file(s) whose event is being reported. For example:

          (require 'filenotify)
               ⇒ filenotify
          (defun my-notify-callback (event)
            (message "Event %S" event))
               ⇒ my-notify-callback
            "/tmp" '(change attribute-change) 'my-notify-callback)
               ⇒ 35025468
          (write-region "foo" nil "/tmp/foo")
               ⇒ Event (35025468 created "/tmp/.#foo")
                  Event (35025468 created "/tmp/foo")
                  Event (35025468 changed "/tmp/foo")
                  Event (35025468 deleted "/tmp/.#foo")
          (write-region "bla" nil "/tmp/foo")
               ⇒ Event (35025468 created "/tmp/.#foo")
                  Event (35025468 changed "/tmp/foo")
                  Event (35025468 deleted "/tmp/.#foo")
          (set-file-modes "/tmp/foo" (default-file-modes))
               ⇒ Event (35025468 attribute-changed "/tmp/foo")

Whether the action renamed is returned, depends on the used watch library. Otherwise, the actions deleted and created could be returned in a random order.

          (rename-file "/tmp/foo" "/tmp/bla")
               ⇒ Event (35025468 renamed "/tmp/foo" "/tmp/bla")
          (delete-file "/tmp/bla")
               ⇒ Event (35025468 deleted "/tmp/bla")
— Function: file-notify-rm-watch descriptor

Removes an existing file watch specified by its descriptor. descriptor should be an object returned by file-notify-add-watch.

— Function: file-notify-valid-p descriptor

Checks a watch specified by its descriptor for validity. descriptor should be an object returned by file-notify-add-watch.

A watch can become invalid if the file or directory it watches is deleted, or if the watcher thread exits abnormally for any other reason. Removing the watch by calling file-notify-rm-watch also makes it invalid.

          (make-directory "/tmp/foo")
               ⇒ Event (35025468 created "/tmp/foo")
          (setq desc
                  "/tmp/foo" '(change) 'my-notify-callback))
               ⇒ 11359632
          (file-notify-valid-p desc)
               ⇒ t
          (write-region "bla" nil "/tmp/foo/bla")
               ⇒ Event (11359632 created "/tmp/foo/.#bla")
                  Event (11359632 created "/tmp/foo/bla")
                  Event (11359632 changed "/tmp/foo/bla")
                  Event (11359632 deleted "/tmp/foo/.#bla")
          ;; Deleting a file in the directory doesn't invalidate the watch.
          (delete-file "/tmp/foo/bla")
               ⇒ Event (11359632 deleted "/tmp/foo/bla")
          (write-region "bla" nil "/tmp/foo/bla")
               ⇒ Event (11359632 created "/tmp/foo/.#bla")
                  Event (11359632 created "/tmp/foo/bla")
                  Event (11359632 changed "/tmp/foo/bla")
                  Event (11359632 deleted "/tmp/foo/.#bla")
          ;; Deleting the directory invalidates the watch.
          ;; Events arrive for different watch descriptors.
          (delete-directory "/tmp/foo" 'recursive)
               ⇒ Event (35025468 deleted "/tmp/foo")
                  Event (11359632 deleted "/tmp/foo/bla")
                  Event (11359632 deleted "/tmp/foo")
                  Event (11359632 stopped "/tmp/foo")
          (file-notify-valid-p desc)
               ⇒ nil