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
program"?
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.
Duncan Murdoch
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,
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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