Linux Kernel Macro container_of

Let's start with a simple version.

1 /* given a pointer @ptr to the field @member embedded into type (usually2*/3
   
     #define
    4     ((Type *) ((
    char *) (PTR)-(
    Char *) (& ((type *)
    0)- >member)))
   

Function: Mainly used for the structure, given a pointer ptr to a struct type instance of the member member, returns the first address of this struct instance, that is, the struct instance of the pointer, verbose a heap, not as an example:

1typedefstructabc{2     CharA;3     Charb;4     intC;5 }abc;6 7 intMainintargcConst Char*argv[])8 {9 ABC ins;Ten  Oneprintf"%p\n", Container_of (&ins.c, ABC, c)); Aprintf"%p\n", &ins); -     return 0; -}

The printed value should be equal to the first address of the struct ins. We will expand the macro:

Container_of (&ins.c, ABC, c); // after expansion ((ABC *) ((char *) (&INS.C)-(Char *) (& ((ABC *)0)->c)));

The 2 sides of the minus sign are 2 sections, the front: (char *) (&INS.C), which is the pointer to the member C of INS, followed by: (char *) (& (((ABC *) 0)->c) is the offset of member C relative to the structure's ABC pointer, For example, in this example:

The pointer to member C (&INS.C) minus the offset of member C (offset) then gets the struct INS first address. If I only know the pointer value of a struct member, then you can get the struct pointer by container_of macro, then you can access any member of it.

Upgrade version:

1 #define Offsetof (TYP,MEMB) ((long) (long_ptr_t) ((char *) & ((((Typ *) 0)->memb))2#define container_of (PTR, type, member) ({            3     consttypeof((type *)0) Member) *__mptr = (PTR);     4     (Type *) ((char *) __mptr-offsetof (Type,member));})

What is the improvement of the upgraded version? I'm going to summarize what I know:

1.const modification to ensure that the incoming PTR pointer is not modified. 2. Use an intermediate variable, define a variable pointer __mptr for a member member type, and assign the PTR to it. Why would you do that? Just in case, if you make a mistake. The pointer is not pointing to the member member, the compiler will have at least one warning.

3.OFFSET macro uses (long) (long_ptr_t) 2 data types for strong turn I didn't read too much, (long long) 64 bits? Is it possible for a member to have an offset of more than 32 bits? It's __mptr. This address value may exceed 32 bits (addressing more than 4G). Puzzled.

Reference:

http://broken.build/2012/11/10/magical-container_of-macro/

Linux 3.13.0 kernel.

Linux Kernel Macro container_of