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

[Maximillian Murphy]

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.


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