Emacs usually shows a menu bar at the top of each frame.
See Menu Bars in The GNU Emacs Manual. Menu bar items are
subcommands of the fake function key
menu-bar, as defined
in the active keymaps.
To add an item to the menu bar, invent a fake function key of your
own (let’s call it key), and make a binding for the key sequence
[menu-bar key]. Most often, the binding is a menu keymap,
so that pressing a button on the menu bar item leads to another menu.
When more than one active keymap defines the same function key for the menu bar, the item appears just once. If the user clicks on that menu bar item, it brings up a single, combined menu containing all the subcommands of that item—the global subcommands, the local subcommands, and the minor mode subcommands.
The variable overriding-local-map is normally ignored when
determining the menu bar contents. That is, the menu bar is computed
from the keymaps that would be active if overriding-local-map
nil. See Active Keymaps.
Here’s an example of setting up a menu bar item:
;; Make a menu keymap (with a prompt string) ;; and make it the menu bar item’s definition. (define-key global-map [menu-bar words] (cons "Words" (make-sparse-keymap "Words")))
;; Define specific subcommands in this menu. (define-key global-map [menu-bar words forward] '("Forward word" . forward-word))
(define-key global-map [menu-bar words backward] '("Backward word" . backward-word))
A local keymap can cancel a menu bar item made by the global keymap by
rebinding the same fake function key with
undefined as the
binding. For example, this is how Dired suppresses the ‘Edit’ menu
(define-key dired-mode-map [menu-bar edit] 'undefined)
edit is the fake function key used by the global map for
the ‘Edit’ menu bar item. The main reason to suppress a global
menu bar item is to regain space for mode-specific items.
Normally the menu bar shows global items followed by items defined by the local maps.
This variable holds a list of fake function keys for items to display at
the end of the menu bar rather than in normal sequence. The default
(help-menu); thus, the ‘Help’ menu item normally appears
at the end of the menu bar, following local menu items.
This normal hook is run by redisplay to update the menu bar contents, before redisplaying the menu bar. You can use it to update menus whose contents should vary. Since this hook is run frequently, we advise you to ensure that the functions it calls do not take much time in the usual case.
Next to every menu bar item, Emacs displays a key binding that runs
the same command (if such a key binding exists). This serves as a
convenient hint for users who do not know the key binding. If a
command has multiple bindings, Emacs normally displays the first one
it finds. You can specify one particular key binding by assigning an
:advertised-binding symbol property to the command. See Keys in Documentation.