[Rd] Basic Question regarding PROTECT

[Duncan Murdoch]

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