[R] S_alloc or Calloc for return value

[Dirk Eddelbuettel]

On 20 July 2009 at 16:06, Dan Kelley wrote:
| I am trying to write a C function to create a vector of integers that can be
| used by the R calling function.  I do not know the size of the vector in the
| R calling function.  (Well, actually, I have an upper limit on the size, but
| that is so large that R cannot allocate it.  What I'm doing in the function
| is to do a sieving procedure, and the result will be small enough to fit
| into my machine's memory.)
| 
| My reading of sections 6.1.1 and 6.1.2 of the guide on writing R extensions
| has left me a bit confused.   Under R_alloc, I read "R will reclaim the
| memory at the end of the call" which seems to suggest the storage won't be
| made available afterwards, so that's not what I should use.  As for the
| alterntaive, under Calloc, I read "This memory lasts until freed by the
| user", and it's clear that this is to be done within the C function in
| question.
| 
| Summary question: how can I allocate memory withing a C function, making it
| available to an R function that calls the C function?

You want R_alloc(). Here, "end of the call" is the call of the R function
that calls your C function. This is what you want---the data will be
available for the caller of your C code.

FWIW R_alloc() is also the only allocation function used in Rcpp (which
provides an interface between R and C++). 

Hth, Dirk

-- 
Three out of two people have difficulties with fractions.