converting R objects to C types in .Call

SEXP testfn(SEXP chstr)
{
  char * charptr = CHAR(chstr);
  printf("%s", charptr);
}

I am sure I am making an obvious mistake, but can someone help me to 
sort it out? Thanks in advance. Please cc me, I'm not subscribed.

Firstly, R is expecting an SEXP as a return value! And secondly, your 
SEXP chstr is still a vector - so you need to get an element from it. If 
there's only one element, then that'll be the zeroth element. Here's my 
code:

#include <R.h>
#include <Rinternals.h>
#include <stdio.h>

SEXP testfn(SEXP chstr)
{
   char * charptr = CHAR(VECTOR_ELT(chstr,0));
   printf("%s", charptr);
   return(chstr);
}

  - I'm just returning the same object back in the return() statement, 
and using VECTOR_ELT(chstr,0) to get to the 0'th element.

  SO in R:

  > foo = .Call("testfn","fnord")

should print 'fnord' and return foo as "fnord".

  > foo = .Call("testfn",c("bar","baz"))

will print 'bar' and return foo as c("bar","baz")

Baz