Re: [R] Calling R from C - part way there but need a push!

From: Jeffrey Horner <jeff.horner_at_vanderbilt.edu>
Date: Wed, 30 Apr 2008 09:25:26 -0500

Maximillian Murphy wrote on 04/29/2008 11:19 AM:
>
> Dear All,
>
>
> I've read the manual on "Writing R Extensions" and in particular the
> part on calling R from C. (Most of the manual is about calling C from
> R, not the other way around.)
>
> The good news is that I can now call _some_ R from C, specifically the R
> functions which have C header files. However it isn't clear to me how
> to call R functions that are written in R. I imagine that there is a
> standard C function of the form
>
> Call_r_from_C("package name", "Rfunction name as a string", arg1, arg2,
> ...);
>
> where arg1 etc are standardised ways of representing R data structures
> in C. However I haven't been able to find such a function prototype in
> the R include/*.h files. See footnote (1). Can you point me in the
> right direction please? Is there a set of examples somewhere that I can
> peruse?
>
>
> Does R keep any state when being called from C? Should I think of it as
> being a co-process, so I can make consecutive calls to it and expect it
> to remember previous calls? I'm thinking of a sequence of calls such as
> "load library", "load R code from a file of my own", "set value of x",
> "make a call and read the values back into my C program"?
>
>
> Help is much appreciated.

I would recommend studying the examples found in the tests/Embedding directory from the R sources. For instance, the following is from RNamedCall.c:

  /*
   Call the function foo() with 3 arguments, 2 of which    are named.
    foo(pch="+", id = 123, c(T,F))

   Note that PrintValue() of the expression seg-faults.    We have to set the print name correctly. */

void
bar1()
{

     SEXP fun, pch;
     SEXP e;

     PROTECT(e = allocVector(LANGSXP, 4));
     fun = findFun(install("foo"), R_GlobalEnv);
     if(fun == R_NilValue) {
	fprintf(stderr, "No definition for function foo. Source foo.R and save 
the session.\n");
	UNPROTECT(1);
	exit(1);
     }
     SETCAR(e, fun);

     SETCADR(e, mkString("+"));
     SET_TAG(CDR(e), install("pch"));

     SETCADDR(e, ScalarInteger(123));
     SET_TAG(CDR(CDR(e)), install("id"));

     pch = allocVector(LGLSXP, 2);
     LOGICAL(pch)[0] = TRUE;
     LOGICAL(pch)[1] = FALSE;
     SETCADDDR(e, pch);

     PrintValue(e);
     eval(e, R_GlobalEnv);

     SETCAR(e, install("foo"));
     PrintValue(e);
     R_tryEval(e, R_GlobalEnv, NULL);

     UNPROTECT(1);

}

Best,

Jeff

>
>
> Regards, Max
>
>
> (1) The most likely include file is Rinterface.h and within that the
> most likely candidates seem to be:
> extern int (*ptr_R_ReadConsole)(const char *, unsigned char *, int, int);
> extern void (*ptr_R_WriteConsole)(const char *, int);
> extern void (*ptr_R_WriteConsoleEx)(const char *, int, int);
>
> but it turns out that they assume that R is running the terminal and
> these are requests to R to display or read from R's console. R isn't
> acting as the back end being given work and returning answers.
>
> (2) Googling "calling r from c" yields precisely five hits, most of
> which just point back at the R extensions document with vague "It's in
> there somewhere"'s. I've looked!
>
> (3) Leads suggested by help.search("C")
> Foreign(base):
> Foreign Function Interface Functions to make calls to compiled
> code that has been loaded into R.
>
> .Internal(base) Call an Internal Function
> '.Internal' performs a call to an internal code which is built in
> to the R interpreter.
>
> Only true R wizards should even consider using this function, and
> only R developers can add to the list of internal functions.
>
> (Definitely not me!)
>
> .Primitive(base) Call a "Primitive" Internal Function
>
> The advantage of '.Primitive' over '.Internal' functions is the
> potential efficiency of argument passing.
>
> (4) R code that I have called successfully is that with prototypes
> defined in e.g. include/R_ext/Applic.h
>
> ______________________________________________
> R-help_at_r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide
> http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.

-- 
http://biostat.mc.vanderbilt.edu/JeffreyHorner

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Received on Wed 30 Apr 2008 - 15:01:13 GMT

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