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
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PLEASE do read the posting guide
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and provide commented, minimal, self-contained, reproducible code.
--
http://biostat.mc.vanderbilt.edu/JeffreyHorner
______________________________________________
R-help@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.
