Searching out the task_struct() corresponding to the PID

I need to make a function which can search out the task struct() corresponding to the PID, and store all the fields in a file.How do i get the task_struct() corresponding to the PID?

What i know-

I know I am supposed to use find_task_by_vpid but i don't know how it functions. Or what it returns.And how to i store those in a file(how to write a struct to file)