大綱

• Macro
• Bitwise operation

Macro(continue)

繼上篇簡單提了些macro和gnu extension，我們再來看個我覺得很重要的例子 ＝> container_of， offsetof， typeoff
(參考jserv老師Linux 核心原始程式碼巨集: container_of)

• offsetof : 結構體成員相對結構體的偏移量
• container_of : 藉由結構體成員變量獲得結構體變量的地址
• typeof : 獲取輸入值的類型

//include/linux/stddef.h
#define offsetof(TYPE, MEMBER) ((size_t) &((TYPE *)0)->MEMBER)

typedef struct Fruit{
        char name[13];//default alignment is 4 bytes
        int date;
        int price;
} fruit_t;

// stdout : (size_t) &((fruit_t *)0)->price = 0x14
// 0x14 = 0x10(name[13]) + 0x4(date)

//include/linux/kernel.h
#define container_of(ptr, type, member) ({                      \
const typeof( ((type *)0)->member ) *__mptr = (ptr);    \
(type *)( (char *)__mptr - offsetof(type,member) );})

//ptr = apple->price
//type = fruit_t
//member = price

假設地址分佈如下

|----|0xa100| apple
|----|0xa110 |name
|----|0xa114 |date
|----|0xa118 |price
則__mptr = 0xa114，
而offsetof(fruit_t, price) = 0x14，
所以container_of(apple->price, fruit_t, price) = 0xa100 = apple !

Bitwise Operation

基本指令： and(&), or(|), shift left(<<), shift right(>>), xor(^), not(~)

明天再補一些常用技巧，接著再講一些compiler, linker就要快點進入cpu和os了！