4.18 Status of streams

[det]wait_for_input(+ListOfStreams, -ReadyList, +TimeOut)

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.^{107Prior 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)),
    ...,

byte_count(+Stream, -Count)

Byte position in Stream. For binary streams this is the same as character_count/2. For text files the number may be different due to multi-byte encodings or additional record separators (such as Control-M in Windows).

character_count(+Stream, -Count)

Unify Count with the current character index. For input streams this is the number of characters read since the open; for output streams this is the number of characters written. Counting starts at 0.

line_count(+Stream, -Count)

Unify Count with the number of lines read or written. Counting starts at 1.

line_position(+Stream, -Count)

Unify Count with the position on the current line. Note that this assumes the position is 0 after the open. Tabs are assumed to be defined on each 8-th character, and backspaces are assumed to reduce the count by one, provided it is positive.