21.5. Non-Blocking Input and Output
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21.5. Non-Blocking Input and Output

In addition to the standard functions LISTEN and READ-CHAR-NO-HANG, CLISP provides the following functionality facilitating non-blocking input and output, both binary and character.

(EXT:READ-CHAR-WILL-HANG-P stream)

EXT:READ-CHAR-WILL-HANG-P queries the stream's input status. It returns NIL if READ-CHAR and PEEK-CHAR with a peek-type of NIL will return immediately. Otherwise it returns T. (In the latter case the standard LISTEN function would return NIL.)

Note the difference with (NOT (LISTEN stream)): When the end-of-stream is reached, LISTEN returns NIL, whereas EXT:READ-CHAR-WILL-HANG-P returns NIL.

Note also that EXT:READ-CHAR-WILL-HANG-P is not a good way to test for end-of-stream: If EXT:READ-CHAR-WILL-HANG-P returns T, this does not mean that the stream will deliver more characters. It only means that it is not known at this moment whether the stream is already at end-of-stream, or will deliver more characters.

(EXT:READ-BYTE-LOOKAHEAD stream)
To be called only if stream's STREAM-ELEMENT-TYPE is (UNSIGNED-BYTE 8) or (SIGNED-BYTE 8). Returns T if READ-BYTE would return immediately with an INTEGER result. Returns :EOF if the end-of-stream is already known to be reached. If READ-BYTE's value is not available immediately, returns NIL instead of waiting.
(EXT:READ-BYTE-WILL-HANG-P stream)
To be called only if stream's STREAM-ELEMENT-TYPE is (UNSIGNED-BYTE 8) or (SIGNED-BYTE 8). Returns NIL if READ-BYTE will return immediately. Otherwise it returns true.
(EXT:READ-BYTE-NO-HANG stream &OPTIONAL eof-error-p eof-value)
To be called only if stream's STREAM-ELEMENT-TYPE is (UNSIGNED-BYTE 8) or (SIGNED-BYTE 8). Returns an INTEGER or does end-of-stream handling, like READ-BYTE, if that would return immediately. If READ-BYTE's value is not available immediately, returns NIL instead of waiting.

LISTEN on binary streams

The [ANSI CL standard] specification for LISTEN mentions “character availability” as the criterion that determines the return value. Since a CHARACTER is never available on a binary STREAM (i.e., a stream with STREAM-ELEMENT-TYPE being a subtype of INTEGER), LISTEN returns NIL for such streams. (You can use SOCKET:SOCKET-STATUS to check binary streams). Any other behavior would be hard to make consistent: consider a bivalent stream, i.e., a STREAM that can be operated upon by both READ-CHAR and READ-BYTE. What should LISTEN return on such a stream if what is actually available on the stream at the moment is only a part of a multi-byte character? Right now one can use first SOCKET:SOCKET-STATUS to check if anything at all is available and then use LISTEN to make sure that a full CHARACTER is actually there.

These notes document CLISP version 2.49Last modified: 2010-07-07
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