I've had some success with a message-based system.

In the forked process I invoke Fl::handle_() with a custom message code. In the main code I establish a handler using Fl::add_handler(). Fl::run() operates as normal.  Something like:

int progress_update(int event)

{

    progress->value((float)event_data); // using FL's event_data global for data transfer

    char percent[10] = {""};

    sprintf_s(percent, "%d%%", (int)event_data);

    progress->copy_label(percent);

}

int main()

{

    Fl::add_handler(progress_update);

// create window as shown earlier

    Fl:run()

}

and in the worker thread:

    event_data = t / 500.0;

    Fl::handle_(PROGRESS_UPDATE,0);