[R] Error in dyn.load

[Philip Silva]

Hi,

I've created a shared library from including this code (with g++ 
-I/usr/local/lib/R/include -I/usr/local/include -c Rtest.cpp; g++ 
-shared -L/usr/local/lib -o Rtest.so Rtest.cpp):
void check_data (SEXP data) {
	int l=length (data);
}

But when I try to load it in R I get this error message:
unable to load shared library ...
...
  undefined symbol: _Z9Rf_lengthP7SEXPREC

How can I solve this problem?

Philip