[R] Calling R from C - part way there but need a push!
murdoch at stats.uwo.ca
Tue Apr 29 19:29:00 CEST 2008
On 29/04/2008 12:19 PM, Maximillian Murphy wrote:
> 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, ...);
Not really. eval() is the main function to call from C. You construct
an R expression, then evaluate it.
> 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?
Section 5.10 (Evaluating R expressions from C) of the Writing R
Extensions manual gives a couple of examples.
> 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
The normal way to do it is to let R be in charge, and only occasionally
call out to C: so then R keeps its state. What you are trying to do
sounds more like writing a new front-end to R, which is much more
involved: see Chapter 8 of that manual.
> Help is much appreciated.
> 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,
> 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 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
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> and provide commented, minimal, self-contained, reproducible code.
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