4
(global-set-key [?\S- ] 'set-mark-command) 

(define-key global-map (kbd "C-c SPC") 'ace-jump-mode)

What's the differences and when to use what?

4

The manual node on Commands for Key Bindings, which I got from the first hit of a google search for "emacs global-set-key", says they're the same:

Command: global-set-key key binding

This function sets the binding of key in the current global map to binding.

         (global-set-key key binding)
         ==
         (define-key (current-global-map) key binding)

If you look at the function definition for global-set-key (C-h f global-set-key, then follow the link to the source code), the function only does the following (ignoring the error checking code):

(define-key (current-global-map) key command)

And the docstring for current-global-map (again, C-h f current-global-map) says:

Return the current global keymap.

  • The docs aren't always right, @Dan :) – abo-abo Dec 28 '14 at 13:20
  • @abo-abo: +1 for the interesting edge case in your answer. My answer was meant more for pedagogical purposes: how one would go about answering this question on one's own. – Dan Dec 28 '14 at 13:53
4

It's more or less the same. The first thing expands to:

(progn (or (vectorp [?\S-]) (stringp [?\S-])
           (signal 'wrong-type-argument (list 'arrayp [?\S-])))
       (define-key (current-global-map) [?\S-] 'set-mark-command))

And since:

(eq (current-global-map) global-map)
; -> t

it's mostly equivalent to the second thing. I do prefer the global-set-key though, since they're nicer semantic-wise.

update: they aren't the same

(setq foobar (copy-keymap global-map))
(define-key foobar (kbd "C-.") 'forward-char)
(use-global-map foobar)
(eq (current-global-map) foobar)
;; -> t
(eq (current-global-map) global-map)
;; -> nil 

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