R devel - Jan 2008 - Pb with defineVar() example in the "Writing R Extensions" manual

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 :-)

Cheers,
H.

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);}

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

-- 
   O__  ---- Peter Dalgaard             ?ster Farimagsgade 5, Entr.B
  c/ /'_ --- Dept. of Biostatistics     PO Box 2099, 1014 Cph. K
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~~~~~~~~~~ - (p.dalgaard at biostat.ku.dk)                  FAX: (+45) 35327907