read-char-no-hang (original) (raw)

ANSI Common Lisp 21 Streams 21.2 Dictionary of Streams

Syntax:

read-char-no-hang &optional input-stream eof-error-p eof-value recursive-p char

Arguments and Values:

input-stream -- an input stream designator. The default is standard input.

eof-error-p - a generalized boolean. The default is true.

eof-value - an object. The default is nil.

recursive-p - a generalized boolean. The default is false.

char - a character or nil or the eof-value.

Description:

read-char-no-hang returns a character from input-stream if such a character is available. If no character is available, read-char-no-hang returns nil.

If recursive-p is true, this call is expected to be embedded in a higher-level call to reador a similar function used by the Lisp reader.

If an _end of file_2 occurs and eof-error-p is false, eof-value is returned.

Examples:

;; This code assumes an implementation in which a newline is not ;; required to terminate input from the console. (defun test-it () (unread-char (read-char)) (list (read-char-no-hang) (read-char-no-hang) (read-char-no-hang))) TEST-IT ;; Implementation A, where a Newline is not required to terminate ;; interactive input on the console. (test-it) a (#\a NIL NIL) ;; Implementation B, where a Newline is required to terminate ;; interactive input on the console, and where that Newline remains ;; on the input stream. (test-it) a[Return] (#\a #\Newline NIL)

Affected By:

*standard-input*,*terminal-io*.

Exceptional Situations:

If an _end of file_2 occurs when eof-error-p is true, an error of type end-of-file is signaled .

See Also:

listen

Notes:

read-char-no-hang is exactly like read-char, except that if it would be necessary to wait in order to get a character (as from a keyboard), nil is immediately returned without waiting.

Allegro CL Implementation Details:

None.