Saptarshi,
R_PreserveObject and R_ReleaseObject are, as far as I know, intended for
the rare situations where you need to maintain a SEXP in C that is not
pointed to by any R level symbols, and across e.g. .Calls.
If you are returning an object to R ("v" in this case) and intend to do
.Call("cfunc2", v) later, there is no benefit at all to having called
R_PreserveObject on it. The only case where your object would lose
protection (v and all other R symbols being rm'd/going out of scope) would
also cause to to lose your only reference to the SEXP, so by R_Preserve'ing
all you will have done is create an unreachable protected pointer.
If you are keeping a static pointer to the SEXP down in C code that R can't
see, then R_PreserveObject would be appropriate, but the situations where
doing that is a good idea are rare (though they do exist).
HTH,
~G
On Thu, Mar 6, 2014 at 2:32 PM, Saptarshi Guha <saptarshi.g...@gmail.com>wrote:
> Hello,
>
> This is a question that probably reveals my lack of understanding.
> In a C function (call it cfunc), i created a SEXP, called S, and then
> called R_PreserveObject on S.
>
> I returned the SEXP to the calling R function (call it rfunc). Note, I
> didn't call
> R_ReleaseObject on S.
>
> v <- .Call("cfunc")
>
> So, are the following statements correct
>
> 1. S is 'doubly' protected from the GC by being associated both with 'v'
> and because it has been added to the precious list (via a call to
> R_PreserveObject without ReleaseObject being called)
>
> 2. I have another C function called cfunc2. In cfunc2, I call
> R_ReleaseObject on S. S , however, is still protected from the GC, because
> it is associated with 'v'
>
> Is (1) and (2) correct?
>
> I have not used R_protect/unprotect, because if I return from cfunc without
> the equivalent number of unprotects, i get 'unbalanced stack' warnings. I'd
> rather not have to worry about that because i intend to balance it later.
>
> Regards
> Saptarshi
>
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>
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>
--
Gabriel Becker
Graduate Student
Statistics Department
University of California, Davis
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