20.8.2 Reading One Event

The lowest level functions for command input are read-event, read-char, and read-char-exclusive.

Function: read-event &optional prompt inherit-input-method seconds

This function reads and returns the next event of command input, waiting if necessary until an event is available.

The returned event may come directly from the user, or from a keyboard macro. It is not decoded by the keyboard’s input coding system (see Terminal I/O Encoding).

If the optional argument prompt is non-nil, it should be a string to display in the echo area as a prompt. Otherwise, read-event does not display any message to indicate it is waiting for input; instead, it prompts by echoing: it displays descriptions of the events that led to or were read by the current command. See The Echo Area.

If inherit-input-method is non-nil, then the current input method (if any) is employed to make it possible to enter a non-ASCII character. Otherwise, input method handling is disabled for reading this event.

If cursor-in-echo-area is non-nil, then read-event moves the cursor temporarily to the echo area, to the end of any message displayed there. Otherwise read-event does not move the cursor.

If seconds is non-nil, it should be a number specifying the maximum time to wait for input, in seconds. If no input arrives within that time, read-event stops waiting and returns nil. A floating point seconds means to wait for a fractional number of seconds. Some systems support only a whole number of seconds; on these systems, seconds is rounded down. If seconds is nil, read-event waits as long as necessary for input to arrive.

If seconds is nil, Emacs is considered idle while waiting for user input to arrive. Idle timers—those created with run-with-idle-timer (see Idle Timers)—can run during this period. However, if seconds is non-nil, the state of idleness remains unchanged. If Emacs is non-idle when read-event is called, it remains non-idle throughout the operation of read-event; if Emacs is idle (which can happen if the call happens inside an idle timer), it remains idle.

If read-event gets an event that is defined as a help character, then in some cases read-event processes the event directly without returning. See Help Functions. Certain other events, called special events, are also processed directly within read-event (see Special Events).

Here is what happens if you call read-event and then press the right-arrow function key:

(read-event)
     ⇒ right

Function: read-char &optional prompt inherit-input-method seconds

This function reads and returns a character of command input. If the user generates an event which is not a character (i.e., a mouse click or function key event), read-char signals an error. The arguments work as in read-event.

In the first example, the user types the character 1 (ASCII code 49). The second example shows a keyboard macro definition that calls read-char from the minibuffer using eval-expression. read-char reads the keyboard macro’s very next character, which is 1. Then eval-expression displays its return value in the echo area.

(read-char)
     ⇒ 49

;; We assume here you use M-: to evaluate this.
(symbol-function 'foo)
     ⇒ "^[:(read-char)^M1"

(execute-kbd-macro 'foo)
     -| 49
     ⇒ nil

Function: read-char-exclusive &optional prompt inherit-input-method seconds

This function reads and returns a character of command input. If the user generates an event which is not a character, read-char-exclusive ignores it and reads another event, until it gets a character. The arguments work as in read-event.

None of the above functions suppress quitting.

Variable: num-nonmacro-input-events

This variable holds the total number of input events received so far from the terminal—not counting those generated by keyboard macros.

We emphasize that, unlike read-key-sequence, the functions read-event, read-char, and read-char-exclusive do not perform the translations described in Translation Keymaps. If you wish to read a single key taking these translations into account, use the function read-key:

Function: read-key &optional prompt

This function reads a single key. It is “intermediate” between read-key-sequence and read-event. Unlike the former, it reads a single key, not a key sequence. Unlike the latter, it does not return a raw event, but decodes and translates the user input according to input-decode-map, local-function-key-map, and key-translation-map (see Translation Keymaps).

The argument prompt is either a string to be displayed in the echo area as a prompt, or nil, meaning not to display a prompt.

Function: read-char-choice prompt chars &optional inhibit-quit

This function uses read-key to read and return a single character. It ignores any input that is not a member of chars, a list of accepted characters. Optionally, it will also ignore keyboard-quit events while it is waiting for valid input. If you bind help-form (see Help Functions) to a non-nil value while calling read-char-choice, then pressing help-char causes it to evaluate help-form and display the result. It then continues to wait for a valid input character, or keyboard-quit.