On Sat, Nov 5, 2011 at 12:00 AM, Dave Hylands <dhylands@xxxxxxxxx> wrote:
> Hi Jimmy,
>
> On Fri, Nov 4, 2011 at 8:27 AM, Jimmy Pan <dspjm1@xxxxxxxxx> wrote:
>> truct page {
>> unsigned long flags; /* Atomic flags, some possibly
>> * updated asynchronously */
>> atomic_t _count; /* Usage count, see below. */
>> union {
>> atomic_t _mapcount; /* Count of ptes mapped in mms,
>> * to show when page is mapped
>> * & limit reverse map searches.
>> */
>> struct { /* SLUB */
>> u16 inuse;
>> u16 objects;
>> };
>> };
>> union {
>> struct {
>> unsigned long private; /* Mapping-private opaque data:
>> * usually used for buffer_heads
>> * if PagePrivate set; used for
>> * swp_entry_t if PageSwapCache;
>> * indicates order in the buddy
>> * system if PG_buddy is set.
>> */
>> struct address_space *mapping; /* If low bit clear, points to
>> * inode address_space, or NULL.
>> * If page mapped as anonymous
>> * memory, low bit is set, and
>> * it points to anon_vma object:
>> * see PAGE_MAPPING_ANON below.
>> */
>> };
>> #if USE_SPLIT_PTLOCKS
>> spinlock_t ptl;
>> #endif
>> struct kmem_cache *slab; /* SLUB: Pointer to slab */
>> struct page *first_page; /* Compound tail pages */
>> };
>
> You just reference the variable as if the unnamed union weren't there
> (i.e. just like unnamed unions in C++).
>
> Here's an example from the kernel source tree:
> http://lxr.linux.no/linux+v3.1/include/linux/mm.h#L419
>
> --
> Dave Hylands
> Shuswap, BC, Canada
> http://www.davehylands.com
>
Well, I see, but what is the use of the union and structure?
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