R: pointer to c function

问题

How to pass a pointer to a function from C code to R (using External R) and after call that function from R??

Something like:

typedef void (* FunctionPtr)();
SEXP ans;
PROTECT(ans = /* ?some code? */);
R_tryEval(ans, R_GlobalEnv, NULL);
UNPROTECT(1);

callback_function()

EDIT: @Romain Francois's post was very helpful.
myapp code:

namespace
{
    void callback()
    {
        std::cout << "callback\n" << std::flush;
    }
}
class Worker
{
public:
/*...*/
    void initialize(argc, argv)
    {
       Rf_initEmbeddedR(argc, argv);

        SEXP ans, val;
        typedef void (* FunctionPtr)();

        PROTECT(ans = Rf_lang2(Rf_install("source"), Rf_mkString("script.R")));
        R_tryEval(ans, R_GlobalEnv, NULL);
        UNPROTECT(1);

        PROTECT(val = Rf_ScalarInteger((int)(&callback)));
        /* pass the address of the pointer to a function */
        PROTECT(ans = Rf_lang2(Rf_install("setCallback"), val));
        R_tryEval(ans, R_GlobalEnv, NULL);
        UNPROTECT(2);
    }
    void uninitialize()
    {
        Rf_endEmbeddedR(0);
    }
};

R and Rcpp
script.R

###################
sourceCpp("utils.cpp")
###################
callback <- function()
{
  callCallback()
}

utils.cpp

#include <Rcpp.h>

using namespace Rcpp;

typedef void (* Callback)();
static Callback spCallback = 0;

// [[Rcpp::export]]
void callCallback()
{
  if (spCallback) {
    spCallback();
  } else {
    Rprintf("ERROR: callback is not set");
  }
}
// [[Rcpp::export]]
void setCallback(const int address)
{
  spPlaceOrder = (Callback)address;
}

回答1:

External pointers are what you are looking for. They ley you encapsulate pointers to arbitrary data strucures. The word data is important here and function pointers are a different beast. If you want to use C++ and Rcpp, what I would suggest is to create a small class that encapsulates the function pointer:

typedef void (* FunctionPtr)();
class FunctionPointer {
  FunctionPtr ptr;
public:
    FunctionPointer( FunctionPtr ptr_) : ptr(ptr_){}
} ;

and then create an XPtr<FunctionPointer>:

#include <Rcpp.h>
using namespace Rcpp ;

// your callback function
void callback(){
    Rprintf( "hello from callback\n" ) ;
}

// The function that creates an external pointer to your 
// callback
// [[Rcpp::export]]
XPtr<FunctionPointer> create_ptr(){
    return XPtr<FunctionPointer>( new FunctionPointer(callback) );
}

// The function that invokes the callback
// [[Rcpp::export]]
void invokeCallback( XPtr<FunctionPointer> callback){
    callback->ptr() ;
}

On the R side, you can use lexical scoping for example to wrap the external pointer into an R function:

callback <- local( {
    ptr <- create_ptr()
    function(){
        invokeCallback( ptr )
        invisible(NULL)
    } 
} )
callback()

回答2:

Another way is to use the InternalFunction class in Rcpp:

#include <Rcpp.h>
using namespace Rcpp ;

void callback(){
    Rprintf( "hello from calback\n" ) ;    
}

// [[Rcpp::export]]
InternalFunction get_callback(){
    return InternalFunction(callback) ;    
}

The implementation is similar to what I described in the other answer. Things on the R side are taken care of by Rcpp:

callback <- get_callback()
callback()

回答3:

First load the lib:

dyn.load("yourDLL.dll")

Then use:

Functions to make calls to compiled code that has been loaded into R.

See The R FFI

EDIT:

First compile your C-code. On Windows that is the following, but on other OS's similar:

gcc mylib.c -shared -o mylib.dll

and then in R:

dyn.load("mylib.dll")     
tmp1  <- as.raw(rep(0,4))   # 4 bytes to be sent to the function's argument
tmp2<-.C("yourFunctionName", tmp1)   # tmp2 now has the data returned by the func

Note that in c++ (not c) the function name has to be the mangled name

来源：https://stackoverflow.com/questions/13904786/r-pointer-to-c-function

标签

c++

pointers

rcpp