mpg-ref-at_end_of_stream (SICStus Prolog)

11.3.14 `at_end_of_stream/[0,1]` ISO

Synopsis

at_end_of_stream

at_end_of_stream(+Stream)

Tests whether the end has been reached for the current input stream or for the input stream Stream.

Arguments

Stream

stream_object, must be ground

A valid Prolog input stream, defaults to the current input stream.

Description

Checks whether the end has been reached for the specified input stream. An input stream reaches the end when all items (characters or bytes) except ‘EOF’ (-1) of the stream have been read. It remains at the end after ‘EOF’ has been read.

Exceptions

Stream errors (see ref-iou-sfh-est).

Comments

Note that at_end_of_stream/[0,1] never blocks. If reading ahead would block, then at_end_of_stream/[0,1] will fail, even if the stream is actually at its end. If you want to ensure that the end-of-stream condition is always properly detected, even if that entails blocking until further input is possible, then you can use peek_code/[1,2] or peek_byte/[1,2], e.g. something like the following:

blocking_at_end_of_stream(Alias) :-
        atom(Alias),
        stream_property(S, alias(Alias)),
        !,
        blocking_at_end_of_stream(S).
blocking_at_end_of_stream(S) :-
        at_end_of_stream(S), !.
blocking_at_end_of_stream(S) :-
        stream_property(S, type(text)), !,
        peek_code(S, -1).
blocking_at_end_of_stream(S) :-
        stream_property(S, type(binary)), !,
        peek_byte(S, -1).

This is similar to what SICStus 3 and some other Prolog implementations do.

Please note: The design of at_end_of_stream/[0,1] makes it inherently unreliable. It is present only for ISO standards compliance. It is better to read or peek until one of the end of file indications is returned.

11.3.14 at_end_of_stream/[0,1] ISO