[Rd] garbage collection, "preserved" variables, and different outcome depending on "--verbose" or not
Laurent Gautier
lgautier at gmail.com
Mon Jul 21 10:40:51 CEST 2008
I just tried with R-devel (2.8.0 Under development (unstable)
(2008-07-20 r46088),
the problem does not appear with that version.
Thanks,
L.
2008/7/20 Laurent Gautier <lgautier at gmail.com>:
> 2008/7/20 Duncan Murdoch <murdoch at stats.uwo.ca>:
>> On 20/07/2008 10:02 AM, Laurent Gautier wrote:
>>>
>>> I tripped on that while crafting the example.
>>>
>>> The problem still exists when moving the "releases" in the middle,
>>> and removing the last "release".
>
> I also see that the C code contains old/irrelevant comments at the top
> of the function
> lostobject. Sorry about that, the bug I am chasing is elusive...
>
>> I can't spot any problems in the new version of your code, but I can't
>> reproduce the problem, either. So it appears to be system specific -- I was
>> using the identical revision as you, but on Windows, not
>> "x86_64-unknown-linux-gnu". What exact Linux is that?
>
> Ubuntu (hardy heron).
> gcc is 4.2.3
> The R source was built with "make clean; ./configure --enable-R-shlib"
> before running "make; make install"
>
>> Can others using that system (or similar ones) reproduce it?
>>
>> Duncan Murdoch
>>
>>>
>>>
>>> #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
>>>>
>>>
>>> ______________________________________________
>>> R-devel at r-project.org mailing list
>>> https://stat.ethz.ch/mailman/listinfo/r-devel
>>
>>
>
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