How do I convert an R character vector to a C character pointer?

Question

I'm trying to pass a character vector from R to C and reference it through a C character pointer. However, I don't know which type conversion macro to use. Below is a small test illustrating my problem.

File test.c:

#include <Rinternals.h>

SEXP test(SEXP chars)
{
   char *s;

   s = CHAR(chars);
   return R_NilValue;
}

File test.R:

dyn.load("test.so")

chars <- c("A", "B")

.Call("test", chars)

Output from R:

> source("test.R")
Error in eval(expr, envir, enclos) : 
  CHAR() can only be applied to a 'CHARSXP', not a 'character'

Any clues?

Answer 1

The string in chars can be retrieved by getting each character through the pointer CHAR(STRING_ELT(chars, i)), where 0 <= i < length(chars), and storing it in s[i].

#include <stdlib.h>
#include <Rinternals.h>

SEXP test(SEXP chars)
{
   int n, i;
   char *s;

   n = length(chars);
   s = malloc(n + 1);
   if (s != NULL) {
      for (i = 0; i < n; i++) {
         s[i] = *CHAR(STRING_ELT(chars, i));
      }
      s[n] = '\0';
   } else {
      /*handle malloc failure*/
   }
   return R_NilValue;
}

Answer 2

Character vectors are STRSXP. Each individual element is CHARSXP, so you need something like (untested):

const char *s;
s = CHAR(STRING_ELT(chars, 0));

See the Handling Character Data section of Writing R Extensions. Dirk will be along shortly to tell you how this all will be easier if you just use C++ and Rcpp. :)