If you look at the definition of csum in the sk_buff , you may find its 4-byte memory space is shared with another two variables: csum_start and csum_ offset . Explain the usages of these two...

If you look at the definition of csum in the sk_buff , you may find its 4-byte memory space is shared with another two variables: csum_start and csum_ offset . Explain the usages of these two variables and why both variables share with csum the same memory space.