infinite
, wait_for_input/3
waits indefinitely. If Timeout is 0 or 0.0 this predicate
returns without waiting.90Prior 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)), ...,