Basics of Linux kernel - container_of principle and practical application

container_of is often seen in the Linux kernel, and it is also widely used in actual driver writing.

container_of principle

##Function: through a member of the structure The variable address finds the first address of the structure.

is defined as follows:

/**
 * 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) );})

• ptr: pointer to the structure member variable
• type: Structure type
• member: Name of structure member variable

In other words , called:

The address ptr of the member member of the known structure type is known, and the starting address of the structure type is solved.

The calculation formula is: starting address of type = ptr -size (size is the size of member)

Use a picture to illustrate the relationship between ptr, type, and member:

• Brief description of the principle: The beauty of

container_of is that uses 0 as a member variable The base address of member is .

An intermediate variable __mptr is defined, "__" represents internal use, "m" represents middle .

#define offsetof(TYPE, MEMBER) ((size_t) &((TYPE *)0)->MEMBER)

typeof( ((type *)0)->member )是获取member的类型，__mptr = (ptr)判断ptr与member是否为同一类型，offsetof计算成员member的大小size。

驱动中的实际例子

例如内核的pwm驱动，通过成员变量chip，找到结构体bcm2835_pwm：

struct bcm2835_pwm {
 struct pwm_chip chip;
 struct device *dev;
 void __iomem *base;
 struct clk *clk;
};

static inline struct bcm2835_pwm *to_bcm2835_pwm(struct pwm_chip *chip_ptr)
{
 return container_of(chip_ptr, struct bcm2835_pwm, chip);
}

使用container_of通常都会定义一个函数，并且命名为to_xxx或者to_find_xxx，代表要找xxx这个结构体，传参则传入成员变量指针，另外函数也会声明为inline。

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