Hi Jimmy, On Fri, Nov 4, 2011 at 9:28 AM, Jimmy Pan <[email protected]> wrote: > On Sat, Nov 5, 2011 at 12:00 AM, Dave Hylands <[email protected]> wrote: >> Hi Jimmy, >> >> On Fri, Nov 4, 2011 at 8:27 AM, Jimmy Pan <[email protected]> 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
> Well, I see, but what is the use of the union and structure? Unions and structures are basic elements of the C language. http://en.wikipedia.org/wiki/Union_(computer_science) http://publications.gbdirect.co.uk/c_book/chapter6/unions.html -- Dave Hylands Shuswap, BC, Canada http://www.davehylands.com _______________________________________________ Kernelnewbies mailing list [email protected] http://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies
