Forking a new process may be a bit heavyweight for my needs. Its more intensive on Windows than on *NIX.

I looked at the progress_simple example, and ended up using check() - which is wait(0).

static int AEC_progress = 0;

// Progress callback updates label
void Update_CB(void* userdata) {
    Fl_Progress* progress = (Fl_Progress*)userdata;
    int val;
    val = AEC_progress;
    progress->value((float)val);              // update progress bar with 0.0 ~ 100 value
    char percent[10] = { "" };
    sprintf_s(percent, "%d%%", val);
    progress->label(percent);
    Fl::repeat_timeout(2.0, Update_CB, (void*)progress);
}

int main()

{

    float mult = 100.f / (total_frames-1);
    Fl_Window win(220, 90);          // create parent window
    win.begin();                                // add progress bar to it..
    Fl_Progress progress (10, 50, 200, 30);
    progress.minimum(0);                      // set progress range to be 0.0 ~ 100
    progress.maximum(100);
    progress.color(0x88888800);               // background color
    progress.selection_color(0x4444ff00);     // progress bar color
    progress.labelcolor(FL_WHITE);            // percent text color
    progress.label("0%%");
    win.end();
    Fl::add_timeout(2.0, Update_CB, (void*)&progress);
    win.show();

// Loop

AEC_progress = (int)(i * mult);
        Fl::check();

// End Loop

}

This works well with the bar indicator, but the text shows some junk. Also, this window goes to the background.