tree.h implements intrusive data structures using code generation. Even in C++, to support an object being in multiple containers intrusively the templates get quite complex because you need to parameterize on the member field name. Some older implementation simply fallback to external code generation, including use of macros.
C application code using these data structures, particularly of the finely honed interfaces defined by queue.h and tree.h in BSD, tend to be extremely clear and simple. And at least the implementation of intrusive lists, particularly queue.h, is also quite straightforward, even considering the backslash-escaped multiline macros. (tree.h is admittedly hairier, but it's implementing an intrusive, non-recursive, type-safe red-black tree. Multiline macros are the least of your problems.)
None of the C-like languages make this easy at the type level. I'm not familiar with any statically-typed functional languages; but I doubt it's any easier because intrusive, inside-out struct/record object relations tend to be even more foreign. By resorting to rudimentary string interpolation (i.e. C macros), it's counter intuitively quite easy to implement this stuff in C. I suppose it's also easier in s-expression based languages, but s-expressions lend themselves to dynamic code generation.
This is indeed one of the two ways to get "generics" in C.
The other way is to set up a set of #defines before #including a file, so the #defines rename everything in the include. (The include can also be included multiple times in this pattern.) The latter seems to be more rare.
