关于 C-g 在 Emacs Manual中的Quitting and Aborting 有一段说明

C-g works by setting the variable quit-flag to t the instant C-g is typed; Emacs Lisp checks this variable frequently, and quits if it is non-nil. C-g is only actually executed as a command if you type it while Emacs is waiting for input. In that case, the command it runs is keyboard-quit. 

也就是说, C-g 有两种工作模式:

当Emacs在等待用户输入时, C-g 实际上是调用了 keyboard-quit 命令，关于这个命令的说明很简单，它仅仅是抛出一个quit信号而已:

Signal a ‘quit’ condition.
During execution of Lisp code, this character causes a quit directly.
At top-level, as an editor command, this simply beeps.

在其他情况下, C-g 设置 quit-flag 为 t, Emacs不断检测该变量，当检测到 quit-flag 不为 nil 时，则会退出正在执行的elisp代码。 关于 quit-flag 的说明如下:

Non-nil causes ‘eval’ to abort, unless ‘inhibit-quit’ is non-nil.
If the value is t, that means do an ordinary quit.
If the value equals ‘throw-on-input’, that means quit by throwing
to the tag specified in ‘throw-on-input’; it’s for handling ‘while-no-input’.
Typing C-g sets ‘quit-flag’ to t, regardless of ‘inhibit-quit’,
but ‘inhibit-quit’ non-nil prevents anything from taking notice of that.

知道 C-g 会做什么，我们就知道如何对 C-g 进行拦截了。

如果我们拦截等待输入时的 C-g, 那么只需要忽略信号quit即可.我们可以使用 condition-case 捕获信息:

(condition-case sig
    (read-string "请输入字符串，按C-g退出")
  (quit (message "收到信号(%s),退出" sig)))

收到信号((quit)),退出

若是不允许通过 C-g 中断某段elisp代码，则可以通过设置 inhibit-quit 来进行:

(let ((inhibit-quit t))
  (while t)
  (setq quit-flag nil))

执行上面语句后你再按 C-g 会发现无法退出死循环(要想退出这个死循环需要快速按下三次 C-g)