21.8.3 Modifying and Translating Input Events

Emacs modifies every event it reads according to extra-keyboard-modifiers, then translates it through keyboard-translate-table (if applicable), before returning it from read-event.

variable extra-keyboard-modifiers

This variable lets Lisp programs “press" the modifier keys on the keyboard. The value is a character. Only the modifiers of the character matter. Each time the user types a keyboard key, it is altered as if those modifier keys were held down. For instance, if you bind extra-keyboard-modifiers to ?\C-\M-a, then all keyboard input characters typed during the scope of the binding will have the control and meta modifiers applied to them. The character ?\C-@, equivalent to the integer 0, does not count as a control character for this purpose, but as a character with no modifiers. Thus, setting extra-keyboard-modifiers to zero cancels any modification.

When using a window system, the program can press any of the modifier keys in this way. Otherwise, only the CTL and META keys can be virtually pressed.

Note that this variable applies only to events that really come from the keyboard, and has no effect on mouse events or any other events.

variable keyboard-translate-table

This terminal-local variable is the translate table for keyboard characters. It lets you reshuffle the keys on the keyboard without changing any command bindings. Its value is normally a char-table, or else nil. (It can also be a string or vector, but this is considered obsolete.)

If keyboard-translate-table is a char-table (see Char-Tables), then each character read from the keyboard is looked up in this char-table. If the value found there is non-nil, then it is used instead of the actual input character.

Note that this translation is the first thing that happens to a character after it is read from the terminal. Record-keeping features such as recent-keys and dribble files record the characters after translation.

Note also that this translation is done before the characters are supplied to input methods (see Input Methods). Use translation-table-for-input (see Translation of Characters), if you want to translate characters after input methods operate.

function keyboard-translate from to

This function modifies keyboard-translate-table to translate character code from into character code to. It creates the keyboard translate table if necessary.

Here’s an example of using the keyboard-translate-table to make C-x, C-c and C-v perform the cut, copy and paste operations:

(keyboard-translate ?\C-x 'control-x)
(keyboard-translate ?\C-c 'control-c)
(keyboard-translate ?\C-v 'control-v)
(global-set-key [control-x] 'kill-region)
(global-set-key [control-c] 'kill-ring-save)
(global-set-key [control-v] 'yank)

On a graphical terminal that supports extended ASCII input, you can still get the standard Emacs meanings of one of those characters by typing it with the shift key. That makes it a different character as far as keyboard translation is concerned, but it has the same usual meaning.

See Translation Keymaps, for mechanisms that translate event sequences at the level of read-key-sequence.