Collections in tor

Smartlists: Neither lists, nor especially smart.

For historical reasons, we call our dynamic-allocated array type “smartlist_t”. It can grow or shrink as elements are added and removed.

All smartlists hold an array of void *. Whenever you expose a smartlist in an API you must document which types its pointers actually hold.

Smartlists are created empty with smartlist_new() and freed with smartlist_free(). See the containers.h module documentation for more information; there are many convenience functions for commonly needed operations.

Digest maps, string maps, and more.

Tor makes frequent use of maps from 160-bit digests, 256-bit digests, or nul-terminated strings to void *. These types are digestmap_t, digest256map_t, and strmap_t respectively. See the containers.h module documentation for more information.

Intrusive lists and hashtables

For performance-sensitive cases, we sometimes want to use “intrusive” collections: ones where the bookkeeping pointers are stuck inside the structures that belong to the collection. If you’ve used the BSD-style sys/queue.h macros, you’ll be familiar with these.

Unfortunately, the sys/queue.h macros vary significantly between the platforms that have them, so we provide our own variants in src/ext/tor_queue.h .

We also provide an intrusive hashtable implementation in src/ext/ht.h . When you’re using it, you’ll need to define your own hash functions. If attacker-induced collisions are a worry here, use the cryptographic siphash24g function to extract hashes.