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
prefix argument.
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
non-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-key-sequence
or
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
case of 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
becomes 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
quit-flag
non-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
non-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
non-nil
outside this construct. It returns the value of the
last form in body, unless exited by quitting, in which case
it returns nil
.
If inhibit-quit
is nil
on entry to with-local-quit
,
it only executes the body, and setting quit-flag
causes
a normal quit. However, if inhibit-quit
is non-nil
so
that ordinary quitting is delayed, a non-nil
quit-flag
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
already non-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 (signal 'quit
nil)
. This is the same thing that quitting does. (See signal
in Errors.)
To quit without aborting a keyboard macro definition or execution,
you can signal the minibuffer-quit
condition. This has almost
the same effect as the quit
condition except that the error
handling in the command loop handles it without exiting keyboard macro
definition or execution.
You can specify a character other than C-g to use for quitting.
See the function set-input-mode
in Input Modes.