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

[Duncan Murdoch]

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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