[Rd] Transferring ownership of R-managed buffer

[Hervé Pagès]

On 03/29/2017 11:04 AM, Hervé Pagès wrote:
> Hi Tim,
>
> On 03/29/2017 08:24 AM, Tim Keitt wrote:
>> I have a use case where I would like to create an SEXP around an existing
>> buffer that is managed by R, thus avoiding a copy operation.
>
> What to you mean exactly by "an existing buffer managed by R"?
>
>> If I have
>> something like:
>>
>> void *p = (void*) RAW(PROTECT(Rf_allocVector(RAWSXP, n)));
>> ... additional maniupulation ...
>> SEXP x = somefunc(SXPTYPE, n, p);  // ????
>>
>> Is there a "placement" constructor available?
>
> What's a "placement" constructor?
>
>> (I have arranged for the
>> corresponding UNPROTECT.) I've looked at and experimented with
>> R_allocator
>> and allocVector3, but can't quite get it right. I know this is odd,
>> but it
>> makes sense for my use case.
>
> Not sure I follow what you are trying to do. Note that an SEXP is a
> pointer to a C struct called SEXPREC. I think that trying to point an
> SEXPREC struct to data pointed to by an existing SEXPREC struct is very
> likely to lead to a disaster.
>
> Note that if the existing buffer managed by R is an SEXP (e.g. b) and
> your code has access to this SEXP then you don't need to create another
> SEXP around its data. Since SEXPs are pointers doing
>
>   SEXP x = b;
>
> is fine and doesn't generate any copy.
>
> And if your code only has access to a "naked" pointer to the buffer
> managed by R (e.g. to RAW(b) is the buffer is actually in an SEXP)
                                ^^
                                if

sorry

H.

> then why would you need to wrap it inside an SEXP?
>
> H.
>
>>
>> Thanks for any pointers.
>>
>> THK
>>
>> https://urldefense.proofpoint.com/v2/url?u=http-3A__www.keittlab.org_&d=DwICAg&c=eRAMFD45gAfqt84VtBcfhQ&r=BK7q3XeAvimeWdGbWY_wJYbW0WYiZvSXAJJKaaPhzWA&m=Ceu5dLU_mdGwmpIk_iUqE0dPNjqwy8wiRy6hS_lWF9k&s=h04DJujKDfqzbLz4FmP3_fZ5bYS3t7UEjSwpLrW5mL0&e=
>>
>>
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>>
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>>
>>
>

-- 
Hervé Pagès

Program in Computational Biology
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