How could I mock void type function?

I have a void insert function and I want to test if it inserts correctly. I'm using CMocka framework for testing.

I've tried to do dpl_insert(ret, "mock_value") instead of the will_return() but it seems that no value is appended to the list.

t_list **dpl_create();
int ft_list_size(t_list *self);

void __wrap_dpl_insert(t_list **self, void *data)
{
   check_expected(self);
   check_expected(data);
}

static void test_dpl_insert_empty_list(void **state)
{
    (void) state;
    t_list **ret = dpl_create(); //Initialization of an empty struct.
    int ret_size;

    expect_value(__wrap_dpl_insert, self, ret);
    expect_value(__wrap_dpl_insert, data, "mock_value");

    will_return(__wrap_dpl_insert, XXX); //PROBLEM RESIDES HERE.

    ret_size = ft_list_size(*ret);
    fprintf(stderr, "Size of the list=>%d\n", ret_size);
}

With dpl_insert(ret, "mock_value") the fprintf() prints 0, like if no element was added.

My objective is to obtain 1 from fprintf().