Wait for input on one of the streams in ListOfStreams and
return a list of streams on which input is available in ReadyList.
Each element of ListOfStreams is either a stream or an
integer. Integers are consider waitable OS handles. This can be used to
(also) wait for handles that are not associated with Prolog streams such
as UDP sockets. See tcp_setopt/2.
This predicate waits for at most TimeOut seconds. TimeOut
may be specified as a floating point number to specify fractions of a
second. If TimeOut equals infinite, wait_for_input/3
waits indefinitely. If Timeout is 0 or 0.0 this predicate
returns without waiting.108Prior
to 7.3.23, the integer value‘0’was the same as infinite.
This predicate can be used to implement timeout while reading and to
handle input from multiple sources and is typically used to wait for
multiple (network) sockets. On Unix systems it may be used on any stream
that is associated with a system file descriptor. On Windows it can only
be used on sockets. If ListOfStreams contains a stream that
is not associated with a supported device, a domain_error(waitable_stream,
Stream) is raised.
The example below waits for input from the user and an explicitly
opened secondary terminal stream. On return, Inputs may hold
user_input or P4 or both.
?- open('/dev/ttyp4', read, P4),
wait_for_input([user_input, P4], Inputs, 0).
When
available, the implementation is based on the poll() system call.
The poll() puts no additional restriction on the number of open
files the process may have. It does limit the time to 2^31-1
milliseconds (a bit less than 25 days). Specifying a too large timeout
raises a
representation_error(timeout) exception. If poll()
is not supported by the OS, select() is used. The select()
call can only handle file descriptors up to FD_SETSIZE. If
the set contains a descriptor that exceeds this limit a
representation_error(’FD_SETSIZE’) is raised.
Note that wait_for_input/3
returns streams that have data waiting. This does not mean you can, for
example, call read/2
on the stream without blocking as the stream might hold an incomplete
term. The predicate
set_stream/2
using the option timeout(Seconds) can be used to make the
stream generate an exception if no new data arrives within the timeout
period. Suppose two processes communicate by exchanging Prolog terms.
The following code makes the server immune for clients that write an
incomplete term:
...,
tcp_accept(Server, Socket, _Peer),
tcp_open(Socket, In, Out),
set_stream(In, timeout(10)),
catch(read(In, Term), _, (close(Out), close(In), fail)),
...,
