The Magical container_of() Macro and typeof()

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本文深入解析Linux内核中的container_of宏，从其原理到具体实现细节，详细阐述了如何通过该宏获取容器结构的成员指针。通过实际代码示例，逐步拆解宏的构造，解释其内部机制，最终揭示了宏背后的巧妙设计。

When you begin with the kernel, and you start to look around and read thecode, you will eventually come across this magical preprocessor construct.

What does it do? Well, precisely what its name indicates. It takes threearguments – a pointer, type of the container, and the name of the memberthe pointer refers to. The macro will then expand to a new address pointing tothe container which accommodates the respective member. It is indeed aparticularly clever macro, but how the hell can this possibly work? Let meillustrate…

The first diagram illustrates the principle of the container_of(ptr, type,member) macro for who might find the above description too clumsy.

Illustration of how the containter_of macro works.

Bellow is the actual implementation of the macro from Linux Kernel:

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

At first glance, this might look like a whole lot of magic, but it isn’t quiteso. Let’s take it step by step.

Statements in Expressions

The first thing to gain your attention might be the structure of the wholeexpression. The statement should return a pointer, right? But there is justsome kind of weird ({}) block with two statements in it. This in fact is aGNU extension to C language called braced-group within expression. Thecompiler will evaluate the whole block and use the value of the last statementcontained in the block. Take for instance the following code. It will print 5.

int x = ({1; 2;}) + 3;
printf("%d\n", x);

typeof()

This is a non-standard GNU C extension. It takes one argument and returns itstype. Its exact semantics is throughly described in gccdocumentation.

int x = 5;
typeof(x) y = 6;
printf("%d %d\n", x, y);

Zero Pointer Dereference

But what about the zero pointer dereference? Well, it’s a little pointer magicto get the type of the member. It won’t crash, because the expression itselfwill never be evaluated. All the compiler cares for is its type. The samesituation occurs in case we ask back for the address. The compiler againdoesn’t care for the value, it will simply add the offset of the member to theaddress of the structure, in this particular case 0, and return the newaddress.

struct s {
        char m1;
        char m2;
};

/* This will print 1 */
printf("%d\n", &((struct s*)0)->m2);

Also note that the following two definitions are equivalent:

typeof(((struct s *)0)->m2) c;
char c;

offsetof(st, m)

This macro will return a byte offset of a member to the beginning of thestructure. It is even part of the standard library (available in stddef.h ).Not in the kernel space though, as the standard C library is not present there.It is a little bit of the same 0 pointer dereference magic as we saw earlierand to avoid that modern compilers usually offer a built-in function, thatimplements that. Here is the messy version (from the kernel):

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

It returns an address of a member called MEMBER of a structure of type TYPEthat is stored in memory from address 0 (which happens to be the offset we’relooking for).

Putting It All Together

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

When you look more closely at the original definition from the beginning ofthis post, you will start wondering if the first line is really good foranything. You will be right. The first line is not intrinsically important forthe result of the macro, but it is there for type checking purposes. And whatthe second line really does? It subtracts the offset of the structure’smember from its address yielding the address of the container structure.That’s it!

After you strip all the magical operators, constructs and tricks, it is thatsimple :-).