Typing C-g while a Lisp function is running causes Emacs to quit whatever it is doing. This means that control returns to the innermost active command loop.
Typing C-g while the command loop is waiting for keyboard input
does not cause a quit; it acts as an ordinary input character. In the
simplest case, you cannot tell the difference, because C-g
normally runs the command
keyboard-quit, whose effect is to quit.
However, when C-g follows a prefix key, they combine to form an
undefined key. The effect is to cancel the prefix key as well as any
In the minibuffer, C-g has a different definition: it aborts out of the minibuffer. This means, in effect, that it exits the minibuffer and then quits. (Simply quitting would return to the command loop within the minibuffer.) The reason why C-g does not quit directly when the command reader is reading input is so that its meaning can be redefined in the minibuffer in this way. C-g following a prefix key is not redefined in the minibuffer, and it has its normal effect of canceling the prefix key and prefix argument. This too would not be possible if C-g always quit directly.
When C-g does directly quit, it does so by setting the variable
quit-flag to t. Emacs checks this variable at appropriate
times and quits if it is not
nil. Setting quit-flag
nil in any way thus causes a quit.
At the level of C code, quitting cannot happen just anywhere; only at the special places that check quit-flag. The reason for this is that quitting at other places might leave an inconsistency in Emacs’s internal state. Because quitting is delayed until a safe place, quitting cannot make Emacs crash.
Certain functions such as
read-quoted-char prevent quitting entirely even though they wait
for input. Instead of quitting, C-g serves as the requested
input. In the case of
read-key-sequence, this serves to bring
about the special behavior of C-g in the command loop. In the
read-quoted-char, this is so that C-q can be used
to quote a C-g.
You can prevent quitting for a portion of a Lisp function by binding
the variable inhibit-quit to a non-
nil value. Then,
although C-g still sets quit-flag to t as usual, the
usual result of this—a quit—is prevented. Eventually,
inhibit-quit will become
nil again, such as when its
binding is unwound at the end of a
let form. At that time, if
quit-flag is still non-
nil, the requested quit happens
immediately. This behavior is ideal when you wish to make sure that
quitting does not happen within a critical section of the program.
In some functions (such as
read-quoted-char), C-g is
handled in a special way that does not involve quitting. This is done
by reading the input with inhibit-quit bound to t, and
setting quit-flag to
nil before inhibit-quit
nil again. This excerpt from the definition of
read-quoted-char shows how this is done; it also shows that
normal quitting is permitted after the first character of input.
(defun read-quoted-char (&optional prompt) "…documentation…" (let ((message-log-max nil) done (first t) (code 0) char) (while (not done) (let ((inhibit-quit first) …) (and prompt (message "%s-" prompt)) (setq char (read-event)) (if inhibit-quit (setq quit-flag nil))) …set the variable code…) code))
If this variable is non-
nil, then Emacs quits immediately, unless
inhibit-quit is non-
nil. Typing C-g ordinarily sets
nil, regardless of inhibit-quit.
This variable determines whether Emacs should quit when quit-flag
is set to a value other than
nil. If inhibit-quit is
nil, then quit-flag has no special effect.
This macro executes body forms in sequence, but allows quitting, at
least locally, within body even if inhibit-quit was
nil outside this construct. It returns the value of the
last form in body, unless exited by quitting, in which case
If inhibit-quit is
nil on entry to
it only executes the body, and setting quit-flag causes
a normal quit. However, if inhibit-quit is non-
that ordinary quitting is delayed, a non-
triggers a special kind of local quit. This ends the execution of
body and exits the
with-local-quit body with
quit-flag still non-
nil, so that another (ordinary) quit
will happen as soon as that is allowed. If quit-flag is
nil at the beginning of body, the local quit
happens immediately and the body doesn’t execute at all.
This macro is mainly useful in functions that can be called from timers, process filters, process sentinels, pre-command-hook, post-command-hook, and other places where inhibit-quit is normally bound to t.
This function signals the
quit condition with
nil). This is the same thing that quitting does. (See
You can specify a character other than C-g to use for quitting.
See the function
set-input-mode in Input Modes.