[Disclaimer: what is below reflects my understanding from reading the
R source, others will correct where deemed necessary]
On 1/2/10 12:00 PM, r-devel-requ...@r-project.org wrote:
Hello,
We are currently making lots of changes to Rcpp (see the open Rcpp
mailing list if interested [1] in the details).
We are now using [2] R_PreserveObject and R_ReleaseObject to manage
garbage collection instead of the PROTECT/UNPROTECT dance. This seems to
work well, but I was wondering if there was documentation about it.
The most precise technical documentation is in memory.c
PROTECT is an alias for Rf_protect, itself an alias for
SEXP protect(SEXP s);
and uses a stack (R_PPStack) to store protected objects.
In particular, if we preserve the same SEXP twice (or more), should we
implement some sort of reference counting ?
This depends on the requirements for your system.
For example, in rpy2 I added a reference counting layer(*) because I
wanted to allow several Python objects to share the same underlying R
object, but that's not currently(*) counting how many times an object
should be freed.
(*: imperfect, but currently doing a very decent job - details upon
request).
That kind of feature could be provided by R's C-level API, since this
could be seen of general use as well as give an opportunity to improve
the performances of the R_PreservedObject/R_ReleaseObject duo whenever
a lot of objects are protected and/or external code is
protecting/releasing objects through a FIFO proxy.
Reading the source (below, from memory.c) I think not, but some
confirmation would help.
I understand the code in memory.c like an object preserved twice needs
to be freed twice: R_PreciousList is just a (linked) list, and
"R_PreserveObject(object)" adds the object to the beginning of the
list while "R_ReleaseObject(object)" removes the first "object" found
from the list.
void R_PreserveObject(SEXP object)
{
R_PreciousList = CONS(object, R_PreciousList);
}
static SEXP RecursiveRelease(SEXP object, SEXP list)
{
if (!isNull(list)) {
if (object == CAR(list))
return CDR(list);
else
CDR(list) = RecursiveRelease(object, CDR(list));
}
return list;
}
void R_ReleaseObject(SEXP object)
{
R_PreciousList = RecursiveRelease(object, R_PreciousList);
}
I'd also be interested if there is some ideas on the relative efficiency
of the preserve/release mechanism compared to PROTECT/UNPROTECT.
PROTECT/UNPROTECT is trading granularity for speed. It is a stack with
only tow operations possible:
- push 1 object into the stack
- pull (unprotect) N last objects from the stack
