how to call multiple functions with parameters and ... in C using R's C API

R_C_API2.c

/* evaluate 2 functions with ... (but must take same named args) */
SEXP C_example(SEXP call, SEXP rho){

   SEXP args = CDR(call);

   /* make up some 'arguments' for our two functions we want to call */
   SEXP a1 = PROTECT(allocVector(REALSXP,1));
   SEXP b1 = PROTECT(allocVector(REALSXP,1));
   REAL(a1)[0] = 5.;
   REAL(b1)[0] = 10.;

   /* get the symbols to the functions */
   SEXP afun = install("afun");
   args = CDR(args);

   SEXP bfun = install("bfun");
   args = CDR(args);
  
   /* make the function calls; note that both functions must take same args */
   SEXP a1call = PROTECT(LCONS(afun, LCONS(a1, LCONS(R_DotsSymbol, R_NilValue))));
   SEXP b1call = PROTECT(LCONS(bfun, LCONS(b1, LCONS(R_DotsSymbol, R_NilValue))));

   /* evaluate */
   SEXP aRes = R_forceAndCall(a1call, 1, rho);
   SEXP bRes = R_forceAndCall(b1call, 1, rho);

   SEXP out = PROTECT(allocVector(VECSXP,2));
   SET_VECTOR_ELT(out,0,aRes);
   SET_VECTOR_ELT(out,1,bRes);

   UNPROTECT(5);

   return out;
 }

/* R wrapper using .Call */
#' test
#' @useDynLib mypackage C_rdiff
#' @export
test <- function(afun,bfun,...){
  call <- match.call(expand.dots = FALSE)
  .Call(C_example,call,environment())
}

afun <- function(x,y){return(x+10+y)}
bfun <- function(x,y){return(x+5)}
test(afun,bfun,y=2)