[Rd] PROTECT help

Duncan Murdoch murdoch.duncan at gmail.com
Tue Mar 27 15:49:15 CEST 2012

On 12-03-27 9:22 AM, Terry Therneau wrote:
> I received the following note this AM.  The problem is, I'm not quite
> sure how to fix it.
> Can one use PROTECT(coxlist(eval(PROTECT.... , do I create an
> intermediate variable, or otherwise?

I think both would work.  The usual style in R sources is to use an 
intermediate variable, assigned within the PROTECT call, e.g.

PROTECT(var = f());

but this would act the same as

var = PROTECT(f());

I don't know where the best docs are, but here is my understanding of 

What PROTECT(x) does is to make a copy of the pointer x in a stack of 
protected pointers.  When garbage collection happens, nothing in that 
stack will be released.  It is safe to protect things that don't need 
protection, but it is a little inefficient.  (You shouldn't call 
PROTECT() on a pointer that isn't an R object declared as a SEXP, but it 
will only cause trouble in certain debugging modes.)

PROTECT(x) does return the value of x, so f(PROTECT(x)) should evaluate 
the same as f(x) (but x will be protected from collection).

The main thing to watch when you use PROTECT is that you keep track of 
how many times it is called, because UNPROTECT just pops a number of 
pointers off the protection stack.

> I'm willing to update the code if someone will give me a pointer to the
> right documentation.  This particular chunk was written when there was a
> lot of change going on in the callback mechanism and so there might be a
> safer and/or simpler and/or more standard aproach by now. The routine in
> question has to do with penalized Cox models, the C code needs to get
> the value of the penalty and the penalty is an arbitrary S expression
> passed down from top level.
> Terry T
> ----------------------------
> In survival_2.36-12 (and earlier), in the function cox_callback() at
> cox_Rcallback.c:40:
>       PROTECT(coxlist=eval(lang2(fexpr,data),rho));
> the return value of the call to lang2() is vulnerable if allocations
> within eval() give rise to garbage collection.
> (Discovered during CXXR development.)
> Andrew
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