How truncate file length in Prolog?

Question

How do I truncate the file length in Prolog?

I only find a set_stream_position/2 predicate in the ISO standard. But I don't find a set_stream_length/2 predicate in the major Prolog systems.

Similarly there is a stream property position/1, but I don't see nowhere a length/1 stream property. The latter would help in using set_stream_length/2.

What would be the workaround?

Bye

Answer 1

I think I got it !

see this page...

edit after @false comment, here a sketch of encapsulating code:

set_file_size(Path, Size) :-
    setup_call_cleanup(
        open(Path, update, S),
        (   stream_property(S, reposition(true)),
            % stream_property(S, position(Q)),
            % set_stream_position(S, Q),
            seek(S, Size, bof, Size),
            set_end_of_stream(S)
        ),
        close(S)).

This works, but relies on seek/4 builtin. I can't fully determine the status of such call WRT ISO compliance. It's listed under ISO IO, but untagged as compliant...

Those 2 commented lines served to me to inspect the opaque term position/1. There is a stream_position_data to inquiry the values.