[Rd] garbage collection, "preserved" variables, and different outcome depending on "--verbose" or not

[Laurent Gautier]

I tripped on that while crafting the example.

The problem still exists when moving the "releases" in the middle,
and removing the last "release".


#include <R.h>
#include <Rdefines.h>


SEXP createObject(void)
{
  SEXP x_R;
  int len_x = 1000000;
  PROTECT(x_R = allocVector(REALSXP, len_x));
  Rprintf("Created 'x' at %p\n", x_R);
  Rprintf("  (mode is %i, length is %i)\n", TYPEOF(x_R), LENGTH(x_R));
  Rprintf("  (first element is %d)\n", REAL(x_R)[0]);
  R_PreserveObject(x_R);
  UNPROTECT(1);
  return x_R;
}

void printObject(SEXP sexp)
{
  Rprintf("object at %p\n", sexp);
  Rprintf("  (mode is %i, length is %i, named is %i)\n",
	  TYPEOF(sexp), LENGTH(sexp), NAMED(sexp));
}

SEXP lostobject(SEXP n_R)
{
  /*
   * This function will:
   * 1- create a numerical vector "x" and "preserve it"
   * 2- make call "list(x)"
   * 3- return "x" to R
   */


  SEXP x_R;
  int i;

  int n = INTEGER(n_R)[0];

  /* Create a numerical vector "x_R" */

  for (i=0; i<n; i++) {
    x_R = createObject();
    printObject(x_R);
    R_ReleaseObject(x_R);
    R_gc();
  }

  x_R = createObject();
  printObject(x_R);
  R_gc();

  Rprintf("Returning 'x' at %p\n", x_R);
  Rprintf("  (first element is %d)\n", REAL(x_R)[0]);
  return x_R;
}


2008/7/20 Duncan Murdoch <murdoch at stats.uwo.ca>:
> On 20/07/2008 9:01 AM, Laurent Gautier wrote:
>>
>> Dear list,
>>
>> While trying to identify the root of a problem I am having with
>> garbage collected variables,
>> I have come across the following oddity: depending on whether --verbose is
>> set
>> or not, I obtain different results.
>
> You are working with variables without protecting them, so you just get
> lucky whenever the code works.
>
> More below...
>
>>
>> I have made a small standalone example to demonstrate it.
>> The example is very artificial, but I had a hard time reproducing
>> reliably the problem.
>>
>> So when I do: (the content of test.R is at the end of this email)
>>
>> R --no-save < test.R
>>
>> [The two last lines of the output are:]
>>>
>>> x[1:3]
>>
>> [1] 0 0 0
>>
>> while with
>>
>> R --verbose --no-save < test.R
>>
>> [The two last lines of the output are:]
>>>
>>> x[1:3]
>>
>> [1] 3.733188e-317 3.137345e-317 3.137345e-317
>>
>>
>> The C code is compiled with:
>> R CMD SHLIB test_lostobject.c
>>
>>
>>> sessionInfo()
>>
>> R version 2.7.1 Patched (2008-07-19 r46081)
>> x86_64-unknown-linux-gnu
>>
>> locale:
>>
>> LC_CTYPE=en_US.UTF-8;LC_NUMERIC=C;LC_TIME=en_US.UTF-8;LC_COLLATE=en_US.UTF-8;LC_MONETARY=C;LC_MESSAGES=en_US.UTF-8;LC_PAPER=en_US.UTF-8;LC_NAME=C;LC_ADDRESS=C;LC_TELEPHONE=C;LC_MEASUREMENT=en_US.UTF-8;LC_IDENTIFICATION=C
>>
>> attached base packages:
>> [1] stats     graphics  grDevices utils     datasets  methods   base
>>
>>
>>
>> ### -- file test.R
>>
>> dyn.load("test_lostobject.so")
>>
>> x = .Call("lostobject", as.integer(5))
>>
>> x[1:3]
>>
>>
>> ### ---
>>
>> ###--- file lostobject.c
>>
>> #include <R.h>
>> #include <Rdefines.h>
>>
>>
>>
>> SEXP createObject(void)
>> {
>>  SEXP x_R;
>>  int len_x = 1000000;
>>  PROTECT(x_R = allocVector(REALSXP, len_x));
>>  Rprintf("Created 'x' at %p\n", x_R);
>>  Rprintf("  (mode is %i, length is %i)\n", TYPEOF(x_R), LENGTH(x_R));
>>  Rprintf("  (first element is %d)\n", REAL(x_R)[0]);
>>  R_PreserveObject(x_R);
>>  UNPROTECT(1);
>>  return x_R;
>> }
>>
>> void printObject(SEXP sexp)
>> {
>>  Rprintf("object at %p\n", sexp);
>>  Rprintf("  (mode is %i, length is %i, named is %i)\n",
>>          TYPEOF(sexp), LENGTH(sexp), NAMED(sexp));
>> }
>>
>> SEXP lostobject(SEXP n_R)
>> {
>>  /*
>>   * This function will:
>>   * 1- create a numerical vector "x" and "preserve it"
>>   * 2- make call "list(x)"
>>   * 3- return "x" to R
>>   */
>>
>>
>>  SEXP x_R;
>>  int i;
>>
>>  int n = INTEGER(n_R)[0];
>>
>>  /* Create a numerical vector "x_R" */
>>
>>  for (i=0; i<n; i++) {
>>    x_R = createObject();
>>    R_ReleaseObject(x_R);
>
> At this point, the variable is unprotected, i.e. you have declared that  its
> memory is free for the taking.  You should not try to do anything with it.
>  printObject calls several functions, and one of those may have overwritten
> the memory.  It's not surprising that different flags (--verbose or not)
> result in different behaviour.
>
>>    printObject(x_R);
>>    R_gc();
>>  }
>>
>>  x_R = createObject();
>>  printObject(x_R);
>>  R_gc();
>>  R_ReleaseObject(x_R);
>
> Same thing here.  x_R is unprotected now, so you shouldn't use it.
>
> Duncan Murdoch
>
>>  Rprintf("Returning 'x' at %p\n", x_R);
>>  Rprintf("  (first element is %d)\n", REAL(x_R)[0]);
>>  return x_R;
>> }
>>
>> ______________________________________________
>> R-devel at r-project.org mailing list
>> https://stat.ethz.ch/mailman/listinfo/r-devel
>
>