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 | Last modified: 2010-07-07 |