[Rd] Subsetting a vector in C

[Sean Davis]

I have a pretty simple question.  I have an array of REALSXP allocated as:

rsp = allocVector(REALSXP,n)

and I simply want to pass subsets of rsp of size q (q<<n), for example, to an 
R function in C.  Is there a way to subset rsp without creating a new REALSXP 
vector and copying the values for subsetting into it and then passing it to 
the function?  I'm looking for the R equivalent of rsp[1:q], but in C under 
the R API.  

A pointer to source code in the R distribution would be quite helpful, as I 
haven't found one on my own (at least not one that I recognized as such).

Thanks,
Sean