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’.
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 (see Loading). This is performed by a
function named emacs-session-restore, which is called during
startup. See Summary: Sequence of Actions at Startup.
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
(format "%S" '(with-current-buffer "*scratch*"
(insert "I am restored"))))
nil)