The context: Anne with her brothers and they all have their sandwiches.
"Where do you mean to have lunch ?" asked Anne. "If we stay here much longer, I shall really have to have something to eat - and it's not nearly time yet. I wish I didn't always feel so hungry in the open air".
1-Why is "shall" being used ?
Does it have a special meaning or is it simply expressing future ?
Can 'will' be used instead without any change in the meaning ?
2- What does it mean "I'll have to have something to eat ?"
I can't figure out why there is the second "have".
How is Anne's sentence different than: "If we stay here much longer, I'll really have something to eat ..."