after.c

static ERL_NIF_TERM nif_fdb_future_get_keyvalue_array(ErlNifEnv* env, int argc, const ERL_NIF_TERM argv[])
{
    enif_future_t *f;
    const FDBKeyValue * out_kv=NULL;
    int out_count = 0;
    fdb_bool_t out_more = 0;

    if (argc!=1) return enif_make_badarg(env);
    if (get_future(env, argv[0], &f) == 0 )
        return enif_make_badarg(env);
    
    fdb_error_t err = fdb_future_get_keyvalue_array(f->handle, &out_kv, &out_count, &out_more);

    if (err != 0)
    {
        return mk_result(env,err,atom_undefined);
    }
    else
    {
        ERL_NIF_TERM result = enif_make_list(env, 0);

        while (out_count>0)
        {
           out_count = out_count - 1;
           const FDBKeyValue *kv = &(out_kv[out_count]);
           ERL_NIF_TERM k = make_binary(env, kv->key, kv->key_length);
           ERL_NIF_TERM v = make_binary(env, kv->value, kv->value_length);
           ERL_NIF_TERM elem = enif_make_tuple2(env, k, v);
           result = enif_make_list_cell(env, elem, result);
        }
        return mk_result(env,err,enif_make_tuple2(env,result,enif_make_int(env,out_more)));
    }
}

before.c

static ERL_NIF_TERM nif_fdb_future_get_keyvalue_array(ErlNifEnv* env, int argc, const ERL_NIF_TERM argv[])
{
    if (argc!=4) return enif_make_badarg(env);

    //  FDBFuture* f;
    //  FDBKeyValue const** out_kv;
    //  int* out_count;
    //  fdb_bool_t* out_more;

    return error_not_implemented;
}

intermediate.c

static ERL_NIF_TERM nif_fdb_future_get_keyvalue_array(ErlNifEnv* env, int argc, const ERL_NIF_TERM argv[])
{
    enif_future_t *f;
    const FDBKeyValue ** out_kv=NULL;
    int* out_count = NULL;
    fdb_bool_t* out_more = NULL;

    if (argc!=1) return enif_make_badarg(env);
    if (get_future(env, argv[0], &f) == 0 )
        return enif_make_badarg(env);
    
    fdb_error_t err = fdb_future_get_keyvalue_array(f->handle, out_kv, out_count, out_more);

    if (err != 0)
    {
        return mk_result(env,err,atom_undefined);
    }
    else
    {
        ERL_NIF_TERM result = enif_make_list(env, 0);

        while (out_count>0)
        {
           out_count = out_count - 1;
           const FDBKeyValue *kv = out_kv[*out_count];
           ERL_NIF_TERM k = make_binary(env, kv->key, kv->key_length);
           ERL_NIF_TERM v = make_binary(env, kv->value, kv->value_length);
           ERL_NIF_TERM elem = enif_make_tuple2(env, k, v);
           result = enif_make_list_cell(env, elem, result);
        }
        return mk_result(env,err,result);
    }
}