Ending the list_t scourge

Most people who dig through the kernel source eventually run into struct list_head, the structure used for the management of generic, doubly-linked lists in the kernel. The kernel list implementation has some interesting features, including the fact that every entry in the list is a "list head." The lists are circular, and no one node is special.

Recently, a typedef (list_t) was added as an equivalent name for the list_head structure; rumor has it Ingo Molnar added the name to help keep his source lines within 80 columns. One would think that people would not get overly worked up about this addition, but this is the kernel hacker community we are dealing with. The prevailing opinion among kernel hackers has swung strongly against typedef in recent times. Use of typedef is seen as a useless hiding of information that programmers need to see. Defined types also complicate include file dependencies. Structures can be "predeclared" with a line like:

    struct my_struct;

and references to that structure (pointers, in particular) can be used as long as the internals of the structure are not accessed. Defined types can not be predeclared in this way, making it harder to mix mutually-dependent types across files.

So Rusty Russell posted a patch which removes list_t from the kernel. Nobody really complained about that change, but some wondered: why not rename the list_head structure to struct list at the same time. As William Irwin rather graphicly put it: "Throw the whole frog in the blender, please, not just half."

In the end, a big renaming of struct list_head throughout the kernel tree (and external code) wasn't to most peoples' taste. And Linus isn't into blended frogs. So the patch removing list_t went into Linus's BitKeeper tree (and will be in 2.5.34), but struct list_head remains.

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