setup_thread_stack

Hi, I'm totally new to kernel code, just trying to understand the basics of the core subsystems. During the do_fork call, there is a call to dup_task_struct, which in turn calls setup_thread_stack. What's this call supposed to do? Inside I can see a call to the "task_thread_info" macro, but I don't quite understand it: #define task_thread_info(task) ((struct thread_info *)(task)->stack) can a task_struct be cast to a thread_info? perhaps my C knowledge should be improved..I know :( Sorry for the trivial question..just trying to understand :) Regards, Francesco