[Rd] help with rchk warnings on Rf_eval(Rf_lang2(...))

[Ben Bolker]

 Thanks, that's really useful.  One more question for you, or someone
else here:

const ArrayXd glmLink::linkFun(const ArrayXd& mu) const {
        return as<ArrayXd>(::Rf_eval(::Rf_lang2(as<SEXP>(d_linkFun),

as<SEXP>(Rcpp::NumericVector(mu.data(),

     mu.data() + mu.size()))
						), d_rho);
    }


I guess I need that to read
PROTECT(::Rf_eval(PROTECT(::Rf_lang2(...),...) , but as written it
doesn't seem I have anywhere to squeeze in an UNPROTECT(2).  Do I need
to define a temporary variable so I can UNPROTECT(2) before I return the
value?

Or is there a way I can use Shield() since this an Rcpp-based project
anyway?

  Sorry for all the very basic questions, but I'm flying nearly blind
here ...

  cheers
   Ben Bolker



On 2020-03-23 4:01 p.m., Tomas Kalibera wrote:
> On 3/23/20 8:39 PM, Ben Bolker wrote:
>> Dear r-devel folks,
>>
>>    [if this is more appropriate for r-pkg-devel please let me know and
>> I'll repost it over there ...]
>>
>> I'm writing to ask for help with some R/C++ integration idioms that are
>> used in a package I'm maintaining, that are unfamilar to me, and that
>> are now being flagged as problematic by Tomas Kalibera's 'rchk'
>> machinery (https://github.com/kalibera/rchk); results are here
>> https://raw.githubusercontent.com/kalibera/cran-checks/master/rchk/results/lme4.out
>>
>>
>> The problem is with constructions like
>>
>> ::Rf_eval(::Rf_lang2(fun, arg), d_rho)
>>
>> I *think* this means "construct a two-element pairlist from fun and arg,
>> then evaluate it within expression d_rho"
>>
>> This leads to warnings like
>>
>> "calling allocating function Rf_eval with argument allocated using
>> Rf_lang2"
>>
>> Is this a false positive or ... ? Can anyone help interpret this?
> This is a true error. You need to protect the argument of eval() before
> calling eval, otherwise eval() could destroy it before using it. This is
> a common rule: whenever passing an argument to a function, that argument
> must be protected (directly or indirectly). Rchk tries to be smart and
> doesn't report a warning when it can be sure that in that particular
> case, for that particular function, it is safe. This is easy to fix,
> just protect the result of lang2() before the call and unprotect (some
> time) after.
>> Not sure why this idiom was used in the first place: speed? (e.g., see
>> https://stat.ethz.ch/pipermail/r-devel/2019-June/078020.html ) Should I
>> be rewriting to avoid Rf_eval entirely in favor of using a Function?
>> (i.e., as commented in
>> https://stackoverflow.com/questions/37845012/rcpp-function-slower-than-rf-eval
>>
>> : "Also, calling Rf_eval() directly from a C++ context is dangerous as R
>> errors (ie, C longjmps) will bypass the destructors of C++ objects and
>> leak memory / cause undefined behavior in general. Rcpp::Function tries
>> to make sure that doesn't happen.")
> 
> Yes, eval (as well as lang2) can throw an error, this error has to be
> caught via R API and handled (e.g. by throwing as exception or something
> else, indeed that exception then needs to be caught and possibly
> converted back when leaving again to C stack frames). An R/C API you can
> use here is R_UnwindProtect. This is of course a bit of a pain, and one
> does not have to worry when programming in plain C.
> 
> I suppose Rcpp provides some wrapper around R_UnwindProtect, that would
> be a question for Rcpp experts/maintainers.
> 
> Best
> Tomas
> 
>>
>>   Any tips, corrections, pointers to further documentation, etc. would be
>> most welcome ... Web searching for this stuff hasn't gotten me very far,
>> and it seems to be deeper than most of the introductory material I can
>> find (including the Rcpp vignettes) ...
>>
>>    cheers
>>     Ben Bolker
>>
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>> https://stat.ethz.ch/mailman/listinfo/r-devel
> 
>