[Rd] SET_TYPEOF no longer allowed, how should I call R from within C?

[Duncan Murdoch]

On 2024-06-25 5:25 a.m., Matthew Fidler wrote:
> Hi,
> 
> I have adapted code to run R from within C from the writing R extensions
> here
> 
> https://colinfay.me/writing-r-extensions/system-and-foreign-language-interfaces.html

That was written in 2017.  You should use the one that came with R, or 
even better, the one that comes with the development version of R.

Duncan Murdoch

> 
> As a more comprehensive example of constructing an R call in C code and
> evaluating, consider the following fragment of printAttributes in
> src/main/print.c.
> 
>      /* Need to construct a call to
> 
>         print(CAR(a), digits=digits)
> 
>         based on the R_print structure, then eval(call, env).
> 
>         See do_docall for the template for this sort of thing.
> 
>      */
> 
>      SEXP s, t;
> 
>      t = s = PROTECT(allocList(3));
> 
>      SET_TYPEOF(s, LANGSXP);
> 
>      SETCAR(t, install("print")); t = CDR(t);
> 
>      SETCAR(t, CAR(a)); t = CDR(t);
> 
>      SETCAR(t, ScalarInteger(digits));
> 
>      SET_TAG(t, install("digits"));
> 
>      eval(s, env);
> 
>      UNPROTECT(1);
> 
> At this point CAR(a) is the R object to be printed, the current attribute.
> There are three steps: the call is constructed as a pairlist of length 3,
> the list is filled in, and the expression represented by the pairlist is
> evaluated.
> 
> A pairlist is quite distinct from a generic vector list, the only
> user-visible form of list in R. A pairlist is a linked list (with CDR(t)
> computing the next entry), with items (accessed by CAR(t)) and names or
> tags (set by SET_TAG). In this call there are to be three items, a symbol
> (pointing to the function to be called) and two argument values, the first
> unnamed and the second named. Setting the type to LANGSXP makes this a call
> which can be evaluated.
> 
> 
> 
> New checks  tells me that this is no longer allowed since it was not part
> of the public api any longer.
> 
> 
> So, how does one call R from C then?
> 
> Also should the writing R extensions be updated with the new approved
> approach?
> 
> 
> Thanks in advance.
> 
> Matt
> 
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> 
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