[ External Interface | The ECLiPSe Built-In Predicates | Reference Manual | Alphabetic Index ]

yield(+ToC, -FromC)

Yield control to the C/C++ main program that has invoked ECLiPSe. The arguments are used for passing data in and out.
+ToC
A term.
-FromC
A variable.

Description

When ECLiPSe is used as an embedded component within an application written in C/C++, the ECLiPSe execution is conceptually a thread. Therefore, a yield-resume control flow model is used. On the C/C++ side, the ec_resume()/EC_resume() functions pass control to the ECLiPSe thread, while on the ECLiPSe side, the yield/2 predicate is used to pass control back. Data can be passed both ways: the ToC argument of yield/2 is passed to C/C++ via the second argument of ec_resume()/EC_resume() when it returns. Similarly a ec_resume()/EC_resume() first argument is passed to ECLiPSe as the FromC argument of yield/2.

This mechanism is supposed to be used such that an ECLiPSe server loop is set up, and the host program resumes ECLiPSe repeatedly to have a request served.

Note that, by default, ECLiPSe is set up as a server which calls posted goals. This is such a general mechanism that it is often not necessary to write a special-purpose server loop.

Fail Conditions

None.

Resatisfiable

No.

Examples

    % ECLiPSe server code
    start_server :-
        eclipse_server(dummy).

    eclipse_server(PrevResult) :-
        yield(PrevResult, Request),
        process_request(Request, NewResult),
        eclipse_server(NewResult).


    // C++ client code
    ec_init();
    post_goal("start_server");
    if (EC_resume() == EC_yield)
    {
        for(;;)
        {
	    // create a request
            ...
            if (EC_resume(request, result) != EC_yield);
                break;
            ...
            // use the result
        }
    }

See Also

xget / 3, xset / 3