Introduction to the container of() function

container of() Function introduction

#ifndef offsetof
#define offsetof(TYPE, MEMBER) ((size_t) &((TYPE *)0)->MEMBER)
#else
#undef offsetof
#define offsetof(TYPE, MEMBER) ((size_t) &((TYPE *)0)->MEMBER)
#endif

#ifndef container_of
#define container_of(ptr, type, member) ({
       \ const typeof(((type *)0)->member) * __mptr = (ptr); \ (type *)((char *)__mptr - offsetof(type, member)); })
#else
#undef container_of
#define container_of(ptr, type, member) ({
       \ const typeof(((type *)0)->member) * __mptr = (ptr); \ (type *)((char *)__mptr - offsetof(type, member)); })
#endif

container_of The role of ： Known structures type Members of member The address of ptr, Solving structure type From .

type From =ptr-size.

1、offsetof Medium 0 Use of the pointer

#include <stdio.h>
#include <stdint.h>

typedef struct
{
    
	char a;
	int b;
	char c;
} test_t;

int main()
{
    

	test_t test;

	printf("&test=%p\n", &test);
	printf("&test.c=%p\n", &test.c);

	printf("&((test_t*)0)->c=%d\n", (int)&((test_t *)0)->c);

	return 0;
}

The output is ：

&test=000000b6e7dffd04
&test.c=000000b6e7dffd0c
&((test_t*)0)->c=8

In the custom structure, the byte alignment is required , So the size of the structure is 4bytes*3=12bytes.

(test_t *)0: ad locum 0 Be coerced into test_t * type , It serves as a pointer to the starting address of the structure .

&((test_t *)0)->c: The function of the teacher is to seek c Number of bytes to start address ,, In fact, it is to find the relative address , The starting address is 0, be &c Namely size Size .（ Printing requires integer , So there's a int Strong go ）.

2、const typeof( ((type *)0)->member ) *__mptr = (ptr) Rigor in

If there is a problem with the parameters entered by the developer ：ptr And member Type mismatch , At compile time there will be warnning, But if you take it out and change careers , That's gone , And this warning is exactly necessary （ To prevent mistakes, you don't know where the mistakes are ）, Its function is to get member That's all .

3、 summary

container_of（ptr, type,member） The implementation of the function consists of two parts ：

   1.   Judge ptr  And  member  Is it consent type 

   2.   Calculation size size , The starting address of the structure  = (type *)((char *)ptr - size)   ( notes ： Strong to the struct pointer )

Now we know that container_of() Its function is to find the first address of a structure variable through the address of a member in the structure variable .

container_of(ptr,type,member), There are ptr,type,member Each represents a pointer 、 type 、 member .
Finally, modify the code as follows ：

int main()
{
    

	test_t test;

	printf("&test=%p\n", &test);
	printf("&test.c=%p\n", &test.c);
	printf("&((test_t*)0)->c=%d\n", (int)&((test_t *)0)->c);

	printf("&test=%p\n", container_of(&test.c, test_t, c));
	printf("&test=%p\n", container_of(&test.c, typeof(test), c));

	return 0;
}

Output is ：

&test=00000011341ffc44
&test.c=00000011341ffc4c
&((test_t*)0)->c=8
&test=00000011341ffc44
&test=00000011341ffc44