为什么这样0（（键入*）0） - ＆GT;成员用C？

问题描述

container_of（ ） 在Linux内核中宏定义为：

 的#define container_of（PTR，类型，成员）（{\\
        常量的typeof（（（键入*）0） - ＆GT;成员）* __mptr =（PTR）; \\
        （类型*）（（字符*）__ mptr  -  offsetof（类型，成员））;}）
 

为什么这样使用（（类型*）0） - ＆GT;成员，而不是（类型*） - GT;成员？

解决方案

Simply because (type*)->member would be invalid syntax, thus typeof would be impossible. So it uses a NULL pointer, which it doesn't dereference anyway - it's used just so typeof can refer to the member.

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How this works:

• The typeof trick is used to declare a pointer of the type of the member. This pointer gets is initialized with the pointer passed by the caller

• The offset of that member in the struct is subtracted from the address of the pointer: this yields the address of the containing object

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Subtler issue: why not get rid of typeof and just do ptr - offsetof. We're casting it to char * anyway, right ? In that case you could pass anything as ptr and the compiler won't say a thing. So the whole typeof things is there for (rudimentary) type checking.

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