A technique commonly used in C, for example in the Linux kernel. You embed "links" to other nodes in your structs, and find the struct from the link based on offsetof. Linux kernel uses linked lists and red-black trees this way. Probably other data structures too.

Intrusive containers are cache friendly and typically require no dynamic allocations (in addition to allocating the data itself).

See the link in the post above for more about linked lists in the linux kernel.

Interesting. At first I thought you were claiming this technique made linked lists cache friendly. After looking at it, I have concluded you meant merely that the next and prev pointers are near your data. Am I understanding you correctly?

That's my understanding too. Instead of going to your data structure, and then dereferencing a pointer to your data, after going to your data structure, you're already there.

Yes, there's one pointer chase less when you don't have a pointer from the link node to the data, but you can get it with offsetof. This can save a significant number of cache misses in some algorithms.

It is a container that intrudes on your data structure. For example in a linked list the next pointer is inside your structure instead of your data structure being wrapped in a node type.

The two other responses already described what intrusive containers are. Have a look at https://troydhanson.github.io/uthash/ to see an implementation of them that I personally like and use in my C projects. It's really simple to have a struct that is indexed by multiple keys by basically just adding two UT_hash_handle values to your struct and then using the uthash functions. On top of that, it's a really nice implementation that is completely self-contained in a single C header file.