Passing lists from R to C, extracting elements, and sending lists back again

I have data that is stored in R lists.  I would like to pass the list to 
C code using the .Call function, perform mathematical operations on the 
elements of the lists (which might be other lists), and return results 
in a new list.

As a simple example, suppose I have a list of n matrices (dimension 
k-by-k) that I want to invert, and return the result in another list. 
I wrote the C code below that reads the list as a SEXP.  Then, for each 
of the i=1..n matrices, I convert the R vector to a gsl matrix, perform 
the linear algebra operations and populate the new list with the 
inverted matrix.

The problem is that I can't figure out the best way to get the real 
elements into the list SEXP to return it to R.  When I run the code 
below, the compiler (R CMD SHLIB sexp1.c) gives me:

-bash-3.00$ R CMD SHLIB sexp1.c
gcc -I/usr/lib64/R/include -I/usr/lib64/R/include -I/usr/include -I/usr/local/include   -fpic  -O2 -g -c sexp1.c -o sexp1.o
sexp1.c: In function `invMatList':
sexp1.c:37: warning: passing arg 3 of `SET_VECTOR_ELT' from incompatible pointer type gcc -shared -L/usr/local/lib64 -o sexp1.so sexp1.o -L/usr/lib64 -lgsl -lgslcblas -L/usr/lib  -L/usr/lib64/R/lib -lR

	SEXP r = PROTECT(allocVector(VECSXP,n));
	double rm[k][k];

	for (i=0; i<n; i=i+1) {

		for (a=0; a<k; ++a){ // convert to gsl matrix
			for (b=0; b<k; ++b){
				ind = b*k + a;
				m = REAL(VECTOR_ELT(list,i))[ind];
				gsl_matrix_set(mat, b, a, m);
			}
		} // end conversion loops

		gsl_linalg_LU_decomp(mat, p, &s);
		gsl_linalg_LU_invert(mat, p, inv);


		for (a=0; a<k; ++a){ // convert from gsl matrix
			for (b=0; b<k; ++b){
				rm[b][a] = gsl_matrix_get(inv,b,a);
			}
		}
		SET_VECTOR_ELT(r, i, rm);
	}
	UNPROTECT(1);
	return (r);
}