1. offsetof 獲得結構體某成員相對於結構體的偏移長度

   /**
    *   計算的結構體中某成員的偏移量
    */
   #define offsetof(TYPE, MEMBER) ((size_t) &((TYPE *)0)->MEMBER)

   這個實現很巧妙，相當於設結構體的整體地址設成0，然後它的成員的地址就相當於成員的偏移

   測試代碼：

   #include <stdio.h>
   #include "list.h"
   
   typedef struct Test_struct
   {
           char ca;
           int ib;
           double dc;
   }Test_struct_t;
   
   int main()
   {
   
           printf("offset ca is %d\n",offsetof(Test_struct_t,ca));
           printf("offset ib is %d\n",offsetof(Test_struct_t,ib));
           printf("offset dc is %d\n",offsetof(Test_struct_t,dc));
   
           return 0;
   }
    
   

   執行結果：
   offset ca is 0
   offset ib is 4
   offset dc is 8

2.container_of 是通過結構體的某成員的地址得到整個結構體的地址，在使用上進而可以訪問到結構體的其余成員

/**
 * 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.
 *  通過結構體中的某個成員的指針，反推出來結構體的指針，可以進而可以訪問其他成員
 */
#define container_of(ptr, type, member) ({              const typeof( ((type *)0)->member ) *__mptr = (ptr);        (type *)( (char *)__mptr - offsetof(type,member) );})
 

實現上實際就是當前這個成員的地址 減去 這個成員相對整個結構體的的偏移量，就是結構體的地址

測試代碼

typedef struct person
{
    char name[20];
    char phone_num[12];
    int index;
}Person_t;

void showInfo(Person_t *ptr)
{
    printf("name :%s\n",ptr->name);
    printf("phone_num :%s\n",ptr->phone_num);
    printf("index :%d\n",ptr->index);
}
// 通過姓名打印所以信息
void getInfoByName(char name[])
{
    if(!name)
        return ;
    Person_t *ptr = container_of(name,Person_t,name);
    if(ptr)
    {
        showInfo(ptr);
    }
}

int main()
{

    Person_t person;
    memset(&person,0x0,sizeof(Person_t));

    person.index = 110;
    strncpy(person.name,"zhangsan",strlen("zhangsan"));
    strncpy(person.phone_num,"15617274954",strlen("15617274954"));

   showInfo(&person);
   printf("\============\n");
   getInfoByName(&person.name);

    return 0;
}
 

執行結果
name :zhangsan
phone_num :15617274954
index :110
\============
name :zhangsan
phone_num :15617274954
index :110

container_of 和 offsetof宏