/*  Part of SWI-Prolog

    Author:  Jan Wielemaker
    This example code is in the public domain
*/

:- use_module(library(unix)).

/* - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
Simple demo illustrating the combination of   fork/1  and pipe/2 to make
Prolog fork a child to do  some  work   and  get  back  when done. Using
wait_for_input/3 you can make the  main   Prolog  task wait for multiple
childs to return results.
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - */

fork_demo(Result) :-
    pipe(Read, Write),
    fork(Pid),
    (   Pid == child
    ->  close(Read),
        format(Write, '~q.~n', [hello(world)]),
        flush_output(Write),        % stream is fully buffered!
        halt
    ;   close(Write),
        read(Read, Result),
        close(Read)
    ).