Couple of questions on Linux C

hello

Here are the questions:

1. Given a file pointer, how do i get the corresponding   file descriptor in Linux(kno how to do it in dos)

2.How do i lock a file using only file pointer ?

3.When going through the header files and sources, i came across many functions and variables beginning with underscore(_) and double underscore(__) What do these represent and what is their significance ?

4. Any good links on advanced pointer use.

Thanks in advance