infinite, wait_for_input/3
waits indefinitely.63For
compatibility reasons, a Timeout value of 0 (integer) also
waits indefinitely. To call select() without giving up the CPU, pass the
float 0.0. If compatibility with versions older than 5.1.3 is desired,
pass the float value 1.0e-7.
This predicate can be used to implement timeout while reading and to
handle input from multiple sources. The following example will wait for
input from the user and an explicitly opened second terminal. On return,
Inputs may hold user_input or P4 or
both.
?- open('/dev/ttyp4', read, P4),
wait_for_input([user_input, P4], Inputs, 0).
This predicate relies on the select()
call on most operating systems. On Unix this call is implemented for any
stream referring to a file handle, which implies all OS-based streams:
sockets, terminals, pipes, etc. On non-Unix systems select() is
generally only implemented for socket-based streams. See also library(socket)
from the clib package.
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)),
...,