[Rd] Basic Question regarding PROTECT

Saptarshi Guha saptarshi.guha at gmail.com
Mon Aug 24 15:45:59 CEST 2009

Oh! Yes, this is the GC at work, "root objects" and everything
referenced from them.
I think I'm understanding the need for PROTECT better.

Thank you

On Mon, Aug 24, 2009 at 9:39 AM, Duncan Murdoch<murdoch at stats.uwo.ca> wrote:
> On 8/24/2009 9:33 AM, Saptarshi Guha wrote:
>> Thank you. So the reason I wouldnt need to protect y had I returned to
>> R, is because
>> had i had done something like
>> h<-.Call("boo",a)
>> where "boo" contains y=foo()
>> the assignment "<-" to h would have a PROTECT somewhere, i.e R's
>> assignment is doing the protection for me.
> Once the object is assigned to h, then it is stored in the global
> environment, so it is safe from garbage collection without needing to be
> PROTECTed.  It's just in the short time after creation before storage where
> it's at risk.
> One general rule in R is that things are in use if they are part of
> something that is in use (and the global environment is always in use).
>  PROTECT is another way to mark something as being in use.  When something
> is in use the garbage collector will leave it alone.
> Duncan Murdoch
>> Had I not returned to R, I would have to do it myself.
>> Yes PROTECT is quite cheap, I had thought it to be costly but 1MM
>> PROTECT/UNPROTECT calls, takes <1 second  (on a macbook 2.16ghz)
>> Thanks for the explanation.
>> Regards
>> Saptarshi
>> On Mon, Aug 24, 2009 at 9:24 AM, Duncan Murdoch<murdoch at stats.uwo.ca>
>> wrote:
>>> On 8/24/2009 9:10 AM, Sapsi wrote:
>>>> Hello
>>>> Thank you for the response. So if my call is
>>>> y=foo()
>>>> z=malloc ( by memory allocations , do you mean via R_alloc and
>>>>  allocVector and malloc or just the former two)
>>> Any allocation which is managed by R's memory manager, so that includes
>>> the
>>> former two, and many other kinds of calls which do allocations, i.e.
>>> essentially any call to the R API unless it's documented not to do
>>> allocations. In most cases calling PROTECT is quite cheap, so it is worth
>>> doing if you're not sure.  (There are exceptions:  because the PROTECT
>>> stack
>>> is finite, you can overflow it if you PROTECT too much. That could happen
>>> in
>>> a loop or a deep recursion.)
>>>> Other statements
>>>> Then I need  to protect y. And in my case I don't return to R since I
>>>>  have embedded it.
>>>> Why is this the case I.e if I perform mem allocs , I need to protect y
>>> Because R's memory manager does automatic garbage collection.  If you
>>> don't
>>> protect y, then the memory manager will not know that it is still in use.
>>>  The next time it needs some memory it may decide to free y and re-use
>>> that
>>> space.
>>> Duncan Murdoch
>>>> On Aug 24, 2009, at 8:18 AM, Duncan Murdoch <murdoch at stats.uwo.ca>
>>>> wrote:r
>>>> C
>>>>> On 8/23/2009 11:52 PM, Saptarshi Guha wrote:
>>>>>> Hello,
>>>>>> Suppose I have the function
>>>>>> SEXP foo(){
>>>>>> SEXP s;
>>>>>> PROTECT(s=allocVector(...))
>>>>>> ....
>>>>>> UNPROTECT(1);
>>>>>> return(s)
>>>>>> }
>>>>>> y=foo() // foo is a recusrive call
>>>>>> Q: Am i correct in understanding that one does not need to write
>>>>>> PROTECT(y=foo()) ?(and a corresponding unprotect  later on)
>>>>>> since it is the object that is protected , SEXP is an alias for
>>>>>> SEXPREC* and allocVector probably does some memory allocation which
>>>>>> does not get freed
>>>>>> when foo returns.
>>>>> Whether y needs protecting depends on what happens between the y =
>>>>>  foo()
>>>>> call and the time you return to R.  If nothing happens, i.e.  you just
>>>>> return y to R, then you're safe.  If you do any memory  allocations
>>>>> after
>>>>> that call before returning to R then y will need  to be protected.
>>>>> Duncan Murdoch

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