Next: , Previous: read-char, Up: Streams Dictionary



21.2.18 read-char-no-hang [Function]

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 read or 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[<--~]<<|
     =>  (#\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.