Sorry to ask about a bit drifted topic, but will there be an alternative API to DATAPTR?
> DATAPTR is not in the API and can't be at least in this form I believe it's vital for ALTREP to return the pointer to the expanded version of a SEXP just like the implementation in base R does [1]. At least, VECSXP has no other measure to expose the pointer if I understand correctly. Best, Yutani [1]: https://github.com/r-devel/r-svn/blob/a3508b75d28164b0e5bcb2c87f816ce5169729a4/src/main/altclasses.c#L186 2024年6月9日(日) 10:43 luke-tierney--- via R-devel <r-devel@r-project.org>: > On Sat, 8 Jun 2024, Ben Bolker wrote: > > > The ASAN errors occur *even if the zero-length object is not actually > > accessed*/is used in a perfectly correct manner, i.e. it's perfectly > legal in > > base R to define `m <- numeric(0)` or `m <- matrix(nrow = 0, ncol = 0)`, > > whereas doing the equivalent in Rcpp will (now) lead to an ASAN error. > > > > i.e., these are *not* previously cryptic out-of-bounds accesses that > are > > now being revealed, but instead sensible and previously legal > definitions of > > zero-length objects that are now causing problems. > > > > I'm pretty sure I'm right about this, but it's absolutely possible > that > > I'm just confused at this point; I don't have a super-simple example to > show > > you at the moment. The closest is this example by Mikael Jagan: > > https://github.com/lme4/lme4/issues/794#issuecomment-2155093049 > > > > which shows that if x is a pointer to a zero-length vector (in plain > C++ > > for R, no Rcpp is involved), DATAPTR(x) and REAL(x) evaluate to > different > > values. > > > > Mikael further points out that "Rcpp seems to cast a (void *) returned > by > > DATAPTR to (double *) when constructing a Vector<REALSXP> from a SEXP, > rather > > than using the (double *) returned by REAL." So perhaps R-core doesn't > want > > to guarantee that these operations give identical answers, in which case > Rcpp > > will have to change the way it does things ... > > It looks like REAL and friends should also get this check, but it's > not high priority at this point, at least to me. DATAPTR has been > using this check for a while in a barrier build, so you might want to > test there as well. I expect we will activate more integrity checks > from the barrier build on the API client side as things are tidied up. > > However: DATAPTR is not in the API and can't be at least in this form: > It allows access to a writable pointer to STRSXP and VECSXP data and > that is too dangerous for memory manager integrity. I'm not sure > exactly how this will be resolve, but be prepared for changes. > > Best, > > luke > > > > > cheers > > Ben > > > > > > > > On 2024-06-08 6:39 p.m., Kevin Ushey wrote: > >> IMHO, this should be changed in both Rcpp and downstream packages: > >> > >> 1. Rcpp could check for out-of-bounds accesses in cases like these, and > >> emit an R warning / error when such an access is detected; > >> > >> 2. The downstream packages unintentionally making these out-of-bounds > >> accesses should be fixed to avoid doing that. > >> > >> That is, I think this is ultimately a bug in the affected packages, but > >> Rcpp could do better in detecting and handling this for client packages > >> (avoiding a segfault). > >> > >> Best, > >> Kevin > >> > >> > >> On Sat, Jun 8, 2024, 3:06 PM Ben Bolker <bbol...@gmail.com > >> <mailto:bbol...@gmail.com>> wrote: > >> > >> > >> A change to R-devel (SVN r86629 or > >> > https://github.com/r-devel/r-svn/commit/92c1d5de23c93576f55062e26d446feface07250 > >> < > https://github.com/r-devel/r-svn/commit/92c1d5de23c93576f55062e26d446feface07250 > > > >> has changed the handling of pointers to zero-length objects, > leading to > >> ASAN issues with a number of Rcpp-based packages (the commit message > >> reads, in part, "Also define STRICT_TYPECHECK when compiling > >> inlined.c.") > >> > >> I'm interested in discussion from the community. > >> > >> Details/diagnosis for the issues in the lme4 package are here: > >> https://github.com/lme4/lme4/issues/794 > >> <https://github.com/lme4/lme4/issues/794>, > >> with a bit more discussion > >> about how zero-length objects should be handled. > >> > >> The short(ish) version is that r86629 enables the > >> CATCH_ZERO_LENGTH_ACCESS definition. This turns on the CHKZLN macro > >> < > https://github.com/r-devel/r-svn/blob/4ef83b9dc3c6874e774195d329cbb6c11a71c414/src/main/memory.c#L4090-L4104 > >> < > https://github.com/r-devel/r-svn/blob/4ef83b9dc3c6874e774195d329cbb6c11a71c414/src/main/memory.c#L4090-L4104 > >>, > >> which returns a trivial pointer (rather than the data pointer that > >> would > >> be returned in the normal control flow) if an object has length 0: > >> > >> /* Attempts to read or write elements of a zero length vector will > >> result in a segfault, rather than read and write random memory. > >> Returning NULL would be more natural, but Matrix seems to > assume > >> that even zero-length vectors have non-NULL data pointers, so > >> return (void *) 1 instead. Zero-length CHARSXP objects still > have > >> a > >> trailing zero byte so they are not handled. */ > >> > >> In the Rcpp context this leads to an inconsistency, where > `REAL(x)` > >> is a 'real' external pointer and `DATAPTR(x)` is 0x1, which in turn > >> leads to ASAN warnings like > >> > >> runtime error: reference binding to misaligned address > 0x000000000001 > >> for type 'const double', which requires 8 byte alignment > >> 0x000000000001: note: pointer points here > >> > >> I'm in over my head and hoping for insight into whether this > >> problem > >> should be resolved by changing R, Rcpp, or downstream Rcpp packages > ... > >> > >> cheers > >> Ben Bolker > >> > >> ______________________________________________ > >> R-devel@r-project.org <mailto:R-devel@r-project.org> mailing list > >> https://stat.ethz.ch/mailman/listinfo/r-devel > >> <https://stat.ethz.ch/mailman/listinfo/r-devel> > >> > > > > > > -- > Luke Tierney > Ralph E. Wareham Professor of Mathematical Sciences > University of Iowa Phone: 319-335-3386 > Department of Statistics and Fax: 319-335-3017 > Actuarial Science > 241 Schaeffer Hall email: luke-tier...@uiowa.edu > Iowa City, IA 52242 WWW: http://www.stat.uiowa.edu/ > ______________________________________________ > R-devel@r-project.org mailing list > https://stat.ethz.ch/mailman/listinfo/r-devel > [[alternative HTML version deleted]] ______________________________________________ R-devel@r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-devel
