[R] S_alloc or Calloc for return value

[Dan Kelley]

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?

Thanks.  Dan.
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