R help - Jun 2011 - C-side: Applying a function (given as param) to data (given as param)

Hello,

I'm implementing a package (C-extension),
where one function gets data and a function
that needs to be applied to the data.

I want to apply the function to (parts of)
the data on the C-side.

1) how do I apply a function (given via SEXP) to data
2) how do I select parts of the data?

Comment on 2: I want to implement a moving/rolling/running
 apply on the data, so I want to select a data window.
How can this be done, and how can it be done efficiently?

For example on page 101 of "Writing R extensions" there was a trick
on how to use REAL on the result, before using it in a loop.
Can this be used for efficiency reasons in general?

What do these macros do?
Do they convert data or cast datatypes?

Ciao,
   Oliver