Quartz 4

container_of

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container_of(ptr, type, member)宏根据指向结构体type中成员member的指针ptr获取该结构体的首地址，也就是获取成员member所在的结构体container。

/**
 * container_of - cast a member of a structure out to the containing structure
 * @ptr:	the pointer to the member.
 * @type:	the type of the container struct this is embedded in.
 * @member:	the name of the member within the struct.
 *
 * WARNING: any const qualifier of @ptr is lost.
 */
#define container_of(ptr, type, member) ({				\
	void *__mptr = (void *)(ptr);					\
	static_assert(__same_type(*(ptr), ((type *)0)->member) ||	\
		      __same_type(*(ptr), void),			\
		      "pointer type mismatch in container_of()");	\
	((type *)(__mptr - offsetof(type, member))); })

container_of宏实现的基本思想是从指向member的指针ptr中减去member在该结构体中的偏移量。Line 9通过typeof运算符member的实际类型，并将指向member的指针ptr转成具有该实际类型的__mptr指针。Line 10首先通过offsetof(type, member)宏获取member在结构体类型type中的偏移量。由于该偏移量表示的是字节数，因此在做指针减法运算之前，需要先将__mptr经过强制类型转换，转成char *类型。(char *)__mptr - offsetof(type,member)得到了结构体的首地址，但是是char *类型，最后还需要经过(type *)强制类型转换。

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