从基本理解到深入探究 Linux kernel container_of 宏

基本理解container_of 宏

container_of，顾名思义就是某某某的容器，也就是说某某某成员变量所在的结构体是谁。

struct cntner {
	int i;	char c;	float f;
};
struct cntner init_struct ={.i=2019,'a',0.0120};
char *pc = &init_struct.c;
// 参数1：指向该结构体成员变量的指针；参数2：该结构体变量类型；参数3：该结构体成员变量名
struct cntner *pcntner = container_of(pc, struct cntner, c);
// 然后就可以使用pcntner结构体指针寻找其他成员的值了
printf("init_struct.i = %d\n", pcntner->i); 
printf("init_struct.c = %c\n", pcntner->c); 
printf("init_struct.f = %f\n", pcntner->f);

有人会问为什么不用init_struct 访问成员变量呢？如果你看了相关系统源码你就知道，很多时候我们访问不了这个结构体变量，只知道其成员，所以用了此招 container_of。

static int fb_notifier_callback(struct notifier_block *self, unsigned long event, void *data)
{
    struct fb_event *evdata = data;
    int *blank;
    struct xxx_ts_data *xxx_data = container_of(self, struct xxx_ts_data, fb_notif);
    ...
}

一般会使用这个宏接口就行了，若不想放弃请看下面深入探究container_of 宏。

深入探究container_of 宏

计算出结构体cntner 的地址其实就是通过变量的地址减去自己的偏移量，就这么简单。

cntner_addr = addr_c - offsetof_c

其中变量c的偏移量offsetof_c = &((struct cntner *)0)->c，0被强制转化为结构体是不是很高级，其实就是起始地址为0的结构体，然后指向变量c取地址就是c的偏移量。

注：本文代码测试在64位的ubuntu系统中，关于上面字节对齐疑问请参见：https://blog.csdn.net/u014134180/article/details/78335328#6_488

我们看看下面几个container_of 宏的实现：

container_of 宏 版本一

这个宏定义出自 msm-4.9\drivers\gpu\drm\nouveau\include\nvif\list.h

/**
 * Returns a pointer to the container of this list element.
 *
 * Example:
 * struct foo* f;
 * f = container_of(&foo->entry, struct foo, entry);
 * assert(f == foo);
 *
 * @param ptr Pointer to the struct list_head.
 * @param type Data type of the list element.
 * @param member Member name of the struct list_head field in the list element.
 * @return A pointer to the data struct containing the list head.
 */
#ifndef container_of
#define container_of(ptr, type, member) \
    (type *)((char *)(ptr) - (char *) &((type *)0)->member)
#endif

下面我们编写一个测试代码

#include 
#define container_of(ptr, type, member) ( (type*) ((char*)(ptr) - ((char*)&((type*)0)->member)) ) 

struct cntner {
	int i;	char c;	float f;
};

int main(void)
{
	struct cntner init_struct ={.i=2019,'c',2019.0122};
	char *pc = &init_struct.c;	
	
	struct cntner *pcntner = container_of(pc, struct cntner, c);
	printf("                   &init_struct  = %p\n", &init_struct);
	printf("                       = pcntner = %p\n", pcntner);
	printf("                           (pc)  = %p\n", (pc) );
	printf("((char*)&((struct cntner*)0)->c) = %p\n", ((char*)&((struct cntner*)0)->c) );
	//warning: format ‘%p’ expects argument of type ‘void *’, but argument 2 has type ‘long int’ 
	//printf(" = %p\n", ((char*)(pc) - ((char*)&((struct cntner*)0)->c)) );
	printf("(struct cntner*) ((char*)(pc) - ((char*)&((struct cntner*)0)->c)) = %p\n", \
			(struct cntner*)((char*)(pc) - ((char*)&((struct cntner*)0)->c)) );
	// output other people
	printf("init_struct.i = %d\n", pcntner->i); 
	printf("init_struct.c = %c\n", pcntner->c); 
	printf("init_struct.f = %f\n", pcntner->f); 
	
	return 0;
}

运行结果如下，读者用其他变量可以测试一下container_of

wucb0122@wucb0122-ubuntu14:~/Codes/test$ gcc  container_of.c
wucb0122@wucb0122-ubuntu14:~/Codes/test$ ./a.out 
                   &init_struct  = 0x7ffe1a9a7d80
                       = pcntner = 0x7ffe1a9a7d80
                           (pc)  = 0x7ffe1a9a7d84
((char*)&((struct cntner*)0)->c) = 0x4
(struct cntner*) ((char*)(pc) - ((char*)&((struct cntner*)0)->c)) = 0x7ffe1a9a7d80
init_struct.i = 2019
init_struct.c = c
init_struct.f = 2019.012207
wucb0122@wucb0122-ubuntu14:~/Codes/test$ 

可以用预处理命令查看宏替换结果（预处理命令请参看文章 C语言编译流程）

wucb0122@wucb0122-ubuntu14:~/Codes/test$ gcc -E -o container_of.i container_of.c
wucb0122@wucb0122-ubuntu14:~/Codes/test$ cat container_of.i 
... ...
int main(void)
{
 struct cntner init_struct ={.i=2019,'c',2019.0122};
 char *pc = &init_struct.c;

 struct cntner *pcntner = ( (struct cntner*) ((char*)(pc) - ((char*)&((struct cntner*)0)->c)) );
... ...

上面用都把地址用(char*)转换，是以防类型不一致导致编译错误。

container_of 宏 版本二

这个宏定义出自 msm-4.9\include\linux\kernel.h，版本二的 container_of（ptr, type, member）宏的实现包括两部分：

• 做类型检查。检查ptr是否是指向结构成员member的指针，如果我们用typeof求出来的类型和ptr不一致，那么编译器会报错，因为ptr和member都是手工输入的参数，宏要保证它们是结构体成员和其相关联指针的一致性。
• 计算size大小。结构体的起始地址 = (type *)((char *)mptr - offset) (注：强转为该结构体指针)

/**
 - 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.
 -  */
#define container_of(ptr, type, member) ({			\
	const typeof( ((type *)0)->member ) *__mptr = (ptr);	\
	(type *)( (char *)__mptr - offsetof(type,member) );})
	
#define offsetof(TYPE, MEMBER) ((size_t) &((TYPE *)0)->MEMBER)

替换结果为：

int main(void)
{
 struct cntner init_struct ={.i=2019,'c',2019.0122};
 char *pc = &init_struct.c;

 struct cntner *pcntner = ({ const typeof(((struct cntner *)0)->c) *__mptr=(pc); (struct cntner *) ((char *)__mptr - ((size_t) &((struct cntner *)0)->c)); });
 

container_of 宏 版本三

发现在kernel 4.13 的container_of 宏，发现有2个新变化，显得更加高大上了.

• 用void *取代了char *来做减法；
• 用__same_type宏来做类型检测，显得更加的直观明了，错了还可以有提示。

https://elixir.bootlin.com/linux/v4.13/source/include/linux/kernel.h#L858

/**
 * 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.
 *
 */
#define container_of(ptr, type, member) ({				\
	void *__mptr = (void *)(ptr);					\
	BUILD_BUG_ON_MSG(!__same_type(*(ptr), ((type *)0)->member) &&	\
			 !__same_type(*(ptr), void),			\
			 "pointer type mismatch in container_of()");	\
	((type *)(__mptr - offsetof(type, member))); })

// include/linux/compiler.h
/* Are two types/vars the same type (ignoring qualifiers)? */
#ifndef __same_type
# define __same_type(a, b) __builtin_types_compatible_p(typeof(a), typeof(b))                  
#endi

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《从基本理解到深入探究Linux kernel container_of 宏》：https://blog.csdn.net/u014134180/article/details/86547550