typedef struct { node_t* next } node_t;

    typedef struct { ... } payload_t;

    typedef struct {
        node_t node;
        payload_t payload;
    } user_data_t;

> typedef struct { node_t list1_node; node_t list2_node; node_t list3_node; payload_t payload; } user_data_t;

In C, at any rate, that doesn't give you "inclusion into multiple lists". It gives you "inclusion into at most 3 lists. The extrusive example I posted gives "inclusion into multiple lists".

So, yeah, I'm still not seeing your point.

It's not, for the reason they put in parentheses:

> note: no pointers in user_data_t

An allocation of user_data_t also allocates space for payload_t, rather than just allocating space for a pointer to payload_t. Your structure requires an additional allocation to point the payload_t* at something. The fact that they hid the next node_t* in a struct doesn't matter though.

>> note: no pointers in user_data_t

I feel like I am taking crazy pills: where, in my intrusive example, are pointers in the `user_data_t`?

My intrusive code sample had no pointers for the user_data_t. It's exactly the same as GP's intrusive example.