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1. Òý×Ó
int a;  
int size = 8;
   
Èç¹ûÈÃaΪ(size =8)µÄÕûÊý±¶±íʾ³É¶þ½øÖÆÓ¦ÊÇʲôÑù×ÓÄØ£¿ÄǾÍÊÇÈÃÕâ¸öÊý±íʾ³É¶þ½øÖÆÊ±µÄ×îºóÈýλΪ0.
¶øÒª´ïµ½ÕâһĿ±ê£¬Ö»ÒªÏÂÃæÕâ¸öÊýÓëËü½øÐÐÓëÔËËã¾Í¿ÉÒÔÁË:
  11111111 11111111 11111111 11111000  
¶øÉÏÃæÕâ¸öÊýʵ¼ÊϾÍÊÇ ~ (size - 1)£¬¿ÉÒÔ½«¸ÃÊý³ÆÎªsizeµÄ¶ÔÆëÑÚÂësize_mask.  
   
¿ÉÕâÑù×öÇó³öµÄÊDZÈaСµÄÄǸö×î´óµÄ8µÄ±¶Êý. Èç¹ûÒªÇó³ö±Èa´óµÄÊDz»ÊÇÐèÒª¼ÓÉÏ8¾Í¿ÉÒÔÁË?
¿ÉÊÇÈç¹ûa±¾Éí¾ÍÊÇ8µÄ±¶Êý, ÕâÑù¼Ó8²»¾Í´íÁËÂð, ËùÒÔÔÚa»ù´¡ÉϼÓÉÏ size - 1, È»ºóÓësizeµÄ¶ÔÆëÑÚÂë½øÐÐÓëÔËËã.
ÕâÑù, ÎÒÃÇ¿ÉÒÔ¶¨ÒåÏÂÃæµÄºê, ÓÃÓÚ¼ÆËãÒ»¸öÊýaÒÔsizeΪ±¶ÊýµÄǰºóÁ½¸öÖµ£º
#define alignment_down(a, size) (a & (~(size-1)) )
#define alignment_up(a, size) ((a+size-1) & (~ (size-1)))
 
ÀýÈç:
a=0, size=8,  Ôòalignment_down(a,size)=0, alignment_up(a,size)=0.
a=6, size=8,  Ôòalignment_down(a,size)=0, alignment_up(a,size)=8.
a=8, size=8,  Ôòalignment_down(a,size)=8, alignment_up(a,size)=8.
a=14, size=8, Ôòalignment_down(a,size)=8, alignment_up(a,size)=16.
 
×¢Ò⣺sizeÓ¦µ±Îª2µÄn´Î·½, ¼´2, 4, 8, 16, 32, 64, 128, 256, 1024, 2048, 4096.....
 
2. ÔÚlinuxÖеÄÓ¦ÓÃ
ÉÏÃæµÄ¼ÆËã·½·¨ÔÚlinuxµÈ´úÂëÖÐÒ²³£³£¿ÉÒÔ¿´µ½,ÏÂÃæ¸ø³ö¼¸¸öÀý×Ó.
 
Àý£±£ºµ±·ÖÅ䵨ַaddrʱ, Òª½«¸ÃµØÖ·ÒÔsizeΪ±¶Êý¶ÔÆë, ¶øÇÒÒªµÃµ½ÊDZÈaddr´óµÄÖµ, ÔòʹÓÃ_ALIGNºê£º
/* align addr on a size boundary - adjust address up if needed -- Cort */
#define _ALIGN(addr,size)   (((addr)+(size)-1)&(~((size)-1)))
Àý2£ºÓëÒ³Ãæ¶ÔÆëÏà¹ØµÄºê
#define PAGE_SIZE               4096
#define PAGE_MASK               (~(PAGE_SIZE-1))
/* to align the pointer to the (next) page boundary */
#define PAGE_ALIGN(addr)        (((addr)+PAGE_SIZE-1) & PAGE_MASK)
 
Àý3£ºÓëskb·ÖÅäʱ¶ÔÆëÏà¹ØµÄºê
#define SKB_DATA_ALIGN(X)  


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