[Rd] Pb with defineVar() example in the "Writing R Extensions" manual

[Peter Dalgaard]

Herve Pages wrote:
> Hi Peter,
>
> Peter Dalgaard wrote:
>   
>> Herve Pages wrote:
>>     
>>> Hi,
>>>
>>> I'm wondering if this code from the "Writing R Extensions" manual
>>> is really safe:
>>>
>>>      SEXP mkans(double x)
>>>      {
>>>          SEXP ans;
>>>          PROTECT(ans = allocVector(REALSXP, 1));
>>>          REAL(ans)[0] = x;
>>>          UNPROTECT(1);
>>>          return ans;
>>>      }
>>>
>>>      double feval(double x, SEXP f, SEXP rho)
>>>      {
>>>          defineVar(install("x"), mkans(x), rho);
>>>          return(REAL(eval(f, rho))[0]);
>>>      }
>>>
>>> In C, the order in which function arguments are evaluated before the
>>> function itself is called is undefined. Hence there is no guarantee
>>> that install("x") will be evaluated before mkans(x). What happens if
>>> mkans(x) is evaluated first? Then install("x") will be called and
>>> eventually trigger garbage collection while the SEXP returned by
>>> mkans(x) is still unprotected.
>>>
>>> I'm asking because I'm getting all sorts of problems with
>>>
>>>   defineVar(install(somekey), mkans(x), rho);
>>>
>>> In my code this line is inside a big loop (hundred of thousands of
>>> iterations) so I end up with a lot of symbols in the rho environment.
>>>
>>> The problems I've seen are hard to reproduce: sometimes it's a segfault,
>>> sometimes a "cons memory exhausted" error, or sometimes everything looks
>>> fine except that, later, when I retrieve values from the rho environment
>>> with findVar(), some of them are altered!
>>>
>>> But if I replace the above line by:
>>>
>>>   PROTECT(ans = mkans(x));
>>>   defineVar(install(somekey), ans, rho);
>>>   UNPROTECT(1);
>>>
>>> then everything works fine :-)
>>>
>>>   
>>>       
>> Sounds like you are right. You don't really have the "smoking gun", but
>> it doesn't seem to be worth trying to catch the actual bug in action
>> with hardware watchpoints and whatnot.
>>
>> The opposite fix should work too (does it?):
>>
>> { SEXP sym = install(somekey) ; defineVar(sym, mkans(x), rho);}
>>     
>
> So now you are protected against install(somekey) eventually triggering
> garbage collection but you are still not protected against defineVar() itself
> triggering garbage collection. Maybe defineVar() does not do that, and will
> never do it, but isn't it risky to rely on this kind of assumption?
>
> Thanks!
> H.
>   
That's not the problem you raised (argument evaluation order), but
there's a CONS inside defineVar, and as far as I can see, it doesn't
protect its arguments, so you could well be right.

>   
>> (I don't think you need to  PROTECT elements in the symbol table)
>>
>>     
>
>   


-- 
   O__  ---- Peter Dalgaard             Øster Farimagsgade 5, Entr.B
  c/ /'_ --- Dept. of Biostatistics     PO Box 2099, 1014 Cph. K
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