libwebsockets
Lightweight C library for HTML5 websockets
lws_dll Doubly-linked list

Introduction

Lws supports two kinds of doubly-linked list, lws_dll and lws_dll2.

Unless memory is at a big premium, or it has to work on lws < v3.2, it's best to simply use lws_dll2.

lws_dll overview

How to use

The basics are the same for lws_dll and lws_dll2.

The list objects point only to themselves, and you use the lws_container_of macro to get a pointer to your struct that contains the list object. Doing it this way

  • the list object does not have to be the first thing in your struct
  • your struct can contain multiple list objects and appear on lists belonging to multiple owners simultaenously,

lws_dll Minimal example

struct mystruct {
....
lws_dll list;
...
};
lws_dll owner;

Adding a mystruct to the owner list (...add_tail() works the same way but adds to the other end of the list)

struct mystruct *p;
...
lws_dll_add_head(&p->list, &owner);

Removing the list object from its owner

lws_dll2_remove(&p->list, &owner);

If you have a struct lws_dll *d pointing to list in struct mystruct, you can convert it to a struct mystruct *p ike this

struct mystruct *p = lws_container_of(d, struct lws_dll, list);

lws_dll2 Minimal example

struct mystruct {
....
lws_dll2 list;
...
};
lws_dll2_owner owner;

Adding a mystruct to the owner list (...add_tail() works the same way but adds to the other end of the list)

struct mystruct *p;
...
lws_dll2_add_head(&p->list, &owner);

Removing the list object from its owner (notice compared to lws_dll, it doesn't need to be told the owner)

lws_dll2_remove(&p->list);

If you have a struct lws_dll2 *d pointing to list in struct mystruct, you can convert it to a struct mystruct *p ike this

struct mystruct *p = lws_container_of(d, struct lws_dll2, list);

Summary Comparing lws_dll and lws_dll2

  • both offer a doubly-linked list object, and (since v3.2) track both the head and tail in an "list owner" object
  • both are initalized by memsetting to 0
  • for lws_dll, it reuses an lws_dll as the "owner", for lws_dll2, there's a specific lws_dll2_owner structure for that
  • lws_dll2_owner also keeps count of the number of list elements
  • lws_dll2 knows which owner's list it is participating on. So it can remove itself and update the owner without the caller needing to know its owner. In the case there are several potential owners list objects may be on, this is very convenient.
  • lws_dll is simpler and has a smaller footprint (two pointers per entry vs three). But you have to know the exact list owner to perform operations on it.

apis

function lws_dll lws_dll2
add entry at head void lws_dll_add_head(struct lws_dll *d, struct lws_dll *phead) void lws_dll2_add_head(struct lws_dll2 *d, struct lws_dll2_owner *owner)
add entry at tail void lws_dll_add_tail(struct lws_dll *d, struct lws_dll *phead); void lws_dll2_add_tail(struct lws_dll2 *d, struct lws_dll2_owner *owner)
remove entry from its owning list void lws_dll_remove_track_tail(struct lws_dll *d, struct lws_dll *phead) void lws_dll2_add_tail(struct lws_dll2 *d, struct lws_dll2_owner *owner)
get owner (not supported) struct lws_dll2_owner * lws_dll2_owner(const struct lws_dll2 *d)
check if item is detached from any list `lws_dll_is_detached(struct lws_dll *d, struct lws_dll *phead) int lws_dll2_is_detached(const struct lws_dll2 *d)`
iterate through items on list `int lws_dll_foreach_safe(struct lws_dll *phead, void *user, int (*cb)(struct lws_dll *d, void *user)) int lws_dll2_foreach_safe(struct lws_dll2_owner *owner, void *user, int (*cb)(struct lws_dll2 *d, void *user))`