Re: How to refer to the unamed structure in page struct
Hi Jimmy,
On Fri, Nov 4, 2011 at 9:28 AM, Jimmy Pan wrote:
> On Sat, Nov 5, 2011 at 12:00 AM, Dave Hylands wrote:
>> Hi Jimmy,
>>
>> On Fri, Nov 4, 2011 at 8:27 AM, Jimmy Pan 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
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Re: How to refer to the unamed structure in page struct
On Sat, Nov 5, 2011 at 12:00 AM, Dave Hylands wrote:
> Hi Jimmy,
>
> On Fri, Nov 4, 2011 at 8:27 AM, Jimmy Pan 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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Re: How to refer to the unamed structure in page struct
Hi Jimmy,
On Fri, Nov 4, 2011 at 8:27 AM, Jimmy Pan 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
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How to refer to the unamed structure in page struct
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 */
};
-
As you can see in the code， the union in the page structure doesn't
have a name, so does
the structure in the union, how to refer to those two?
Thanks in advance,
Jimmy
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