K 10
svn:author
V 2
ed
K 8
svn:date
V 27
2012-11-11T12:21:51.814570Z
K 7
svn:log
V 1878
MFC r240422, r240426 and r240450:

  Implement LIST_PREV().

  Regular LISTs have been implemented in such a way that the prev-pointer
  does not point to the previous element, but to the next-pointer stored
  in the previous element. This is done to simplify LIST_REMOVE(). This
  macro can be implemented without knowing the address of the list head.

  Unfortunately this makes it harder to implement LIST_PREV(), which is
  why this macro was never here. Still, it is possible to implement this
  macro. If the prev-pointer points to the list head, we return NULL.
  Otherwise we simply subtract the offset of the prev-pointer within the
  structure.

  It's not as efficient as traversing forward of course, but in practice
  it shouldn't be that bad. In almost all use cases, people will want to
  compare the value returned by LIST_PREV() against NULL, so an optimizing
  compiler will not emit code that does more branching than TAILQs.

  While there, add __containerof(). Compared to __member2struct(), this
  macro has the following advantages:

  - It ensures that the type of the pointer is compatible with the member
    field of the structure (or a void pointer).
  - It works properly in combination with volatile and const, though
    unfortunately it drops these qualifiers from the returned value.

  mdf@ proposed to add the container_of() macro, just like Linux has.
  Eventually I decided against this, as <sys/param.h> is included all over
  the place. It seems container_of() on Linux is specific to the kernel,
  not userspace. I'd rather not pollute userspace with this.

  I also thought about adding __container_of(), but this would have two
  advantages. Xorg seems to already have a __container_of(), which is not
  compatible with this version. Also, the underscore in the middle
  conflicts with our existing macros (__offsetof, __rangeof, etc).

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