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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 (: When the NOT (LISTEN
stream))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)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)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)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 streamsThe [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 2.49.93+ | Last modified: 2018-02-19 |
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