Hi Dave,
it's great that you posted your code, but an image is not very helpful.
You can attach your source file (.cxx, .cpp, whatever) here w/o issues.
If we had the source code we could easily (test and) modify it to show
you how to proceed.
Another important info would be the OS and compiler/toolchain you're
using. If it's Windows (maybe VS ?), then printf doesn't work as you may
assume.
That said, your code does only print the value right at the beginning of
the program (what does it print, BTW?) but not during runtime when the
slider is moved.
(See also Ian's post which I saw while writing this.)
I understand that the latter is what you want. If you want to access the
value during runtime you need either a callback or another means like a
button or timer to access the value whenever you need. The most direct
way would likely be a callback with an appropriate when() condition that
gets triggered when the slider is moved, something like (untested!):
void slider_cb(Fl_Widget *w, void *v) {
Fl_Value_Slider *sl = (Fl_Value_Slider *)w;
printf("Slider value = %d\n", sl->value());
}
But note, this is not going to output anything if you're using Windows.
Please ask again if you need further advice.