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
waits indefinitely. If Timeout is 0 or 0.0 this predicate
returns without waiting.105Prior
to 7.3.23, the integer value‘0' was the same as
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
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).
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
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
..., tcp_accept(Server, Socket, _Peer), tcp_open(Socket, In, Out), set_stream(In, timeout(10)), catch(read(In, Term), _, (close(Out), close(In), fail)), ...,