How to refer to the unamed structure in page struct

[Dave Hylands]

Hi Jimmy,

On Fri, Nov 4, 2011 at 8:27 AM, Jimmy Pan <dspjm1 at gmail.com> 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