elisp: Session Management
38.18 Session Management
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Emacs supports the X Session Management Protocol, which is used to
suspend and restart applications. In the X Window System, a program
called the “session manager” is responsible for keeping track of the
applications that are running. When the X server shuts down, the
session manager asks applications to save their state, and delays the
actual shutdown until they respond. An application can also cancel the
shutdown.
When the session manager restarts a suspended session, it directs
these applications to individually reload their saved state. It does
this by specifying a special command-line argument that says what saved
session to restore. For Emacs, this argument is ‘--smid SESSION’.
-- Variable: emacs-save-session-functions
Emacs supports saving state via a hook called
‘emacs-save-session-functions’. Emacs runs this hook when the
session manager tells it that the window system is shutting down.
The functions are called with no arguments, and with the current
buffer set to a temporary buffer. Each function can use ‘insert’
to add Lisp code to this buffer. At the end, Emacs saves the
buffer in a file, called the “session file”.
Subsequently, when the session manager restarts Emacs, it loads the
session file automatically (
Loading). This is performed by
a function named ‘emacs-session-restore’, which is called during
startup. Startup Summary.
If a function in ‘emacs-save-session-functions’ returns non-‘nil’,
Emacs tells the session manager to cancel the shutdown.
Here is an example that just inserts some text into ‘*scratch*’ when
Emacs is restarted by the session manager.
(add-hook 'emacs-save-session-functions 'save-yourself-test)
(defun save-yourself-test ()
(insert "(save-current-buffer
(switch-to-buffer \"*scratch*\")
(insert \"I am restored\"))")
nil)
Info Catalog
elisp: Batch Mode
elisp: System Interface
elisp: Desktop Notifications