How to wrap 3rd party DLL for use in R? - c++

Question

I need to use an existing 3rd party API that comes with a *.h and a *.dll file to load data into R. The functions provided by the dll are not callable directly, so I need to wrap them to call them from R. In order to familiarize myself with this, I made little example dll (based on the HOWTO from the MINGW page here, I have put the source code of the files at the end of the post). There is just one function in it that doubles an integer input. I can compile the dll just fine and also use it in a exe file, so it is functional. This is on Windows 10.
I am not sure how to correctly use this in R. I have created a package (named testwithdll2 ), placed the header file and the dll in "src", together with the wrapper function. When I try to compile the package, I get the follwing error messages with the undefined reference:
C:/Rtools/mingw_64/bin/gcc -I"C:/PROGRA~1/R/R-35~1.1/include" -DNDEBUG
-O2 -Wall -std=gnu99 -mtune=generic -c mydouble_c.c -o mydouble_c.o
C:/Rtools/mingw_64/bin/gcc -shared -s -static-libgcc -o testwithdll2.dll
tmp.def mydouble_c.o -LC:/PROGRA~1/R/R-35~1.1/bin/x64 -lR
mydouble_c.o:mydouble_c.c:(.text+0xc): undefined reference to `__imp_timestwo'
collect2.exe: error: ld returned 1 exit status
Any pointers on what might have gone wrong are greatly appreciated.
example_dll.h:
#ifndef EXAMPLE_DLL_H
#define EXAMPLE_DLL_H
#ifdef __cplusplus
extern "C" {
#endif
#ifdef BUILDING_EXAMPLE_DLL
#define EXAMPLE_DLL __declspec(dllexport)
#else
#define EXAMPLE_DLL __declspec(dllimport)
#endif
int EXAMPLE_DLL timestwo(int x);
#ifdef __cplusplus
}
#endif
#endif // EXAMPLE_DLL_H
example_dll.cpp:
#include <stdio.h>
#include "example_dll.h"
int timestwo(int x)
{
return 2 * x;
}
mydouble.c (in the src folder of the r package):
#include "example_dll.h"
void mydouble(int* a){
*a = timestwo(*a);
}
timestwo.R (wrapper function, in the R folder):
#' #useDynLib testwithdll2 mydouble
#' #export
timestwo <- function(n){
.C("mydouble",n )
n
}

Answer 1

I figured out what to do.
It was necessary to use a makevars file with the following lines:
MAKEVARS:
PKG_CPPFLAGS= -I.
PKG_LIBS= -L. -lexample_dll
It was also necessary to add the useDynlib call to the example_dll.dll in the namespace before the call to the testwithdll2.dll. This also meant that the .C call needed the PACKAGE parameter to be specified, so I had to change the r wrapper to:
timestwo.R
#' #useDynLib example_dll
#' #useDynLib testwithdll2
#' #export
timestwo <- function(n){
.C("mydouble",n, PACKAGE = "testwithdll2")[[1]]
}
Now everything works as expected.