[julia-users] Creating a tuple in C and returning it to Julia

[Dominique Orban]

My use case is a C function that creates a sparse matrix in triplet form 
(rows, cols, vals). What's the appropriate way to return this to Julia? 
I've tried the following, thinking that Julia accepts C pointers as return 
values, so why not tuples of C pointers:

jl_value_t *my_c_function(void) {

  jl_tuple_t *tuple = jl_alloc_tuple(3);
  long *rows, *cols;
  double *vals;

  // allocate and populate rows, cols and vals...

  jl_tupleset(tuple, 0, rows);
  jl_tupleset(tuple, 1, cols);
  jl_tupleset(tuple, 2, vals);

  return (jl_value_t*)tuple;
}

(The intention is then to call ptr_to_array() on each component of the 
tuple.) In Julia, I call this C function as follows:

(rows, cols, vals) = ccall((:my_c_function, "mylib"), Ptr{Any}, ())

But that gives me

ERROR: no method start(Ptr{Any})

I'm sure there's more than one mistake in that code, but I'm not finding 
documentation on the Julia API.

Thanks!