I wonder why this wasn't the original design; I've seen the "two-allocate" method a few times in other code and it's always seemed rather silly to allocate that separate little structure just to point to the things you're linking together anyway, so I'm curious how that way of doing it became somewhat commonplace.
That use case is possible with the particular implementation that is now used by curl:
struct fileinfo {
struct curl_fileinfo info;
struct curl_llist_element list;
};
I think one of your other points is more likely, or simply "it was fast enough and we went for more interesting features than for such optimizations".
struct A {
int x, y;
};
struct B {
union {
struct A 2d;
struct { int x, y; };
}
int z;
};
...
struct B foo;
foo.x = foo.2d.y = foo.z = 0;