结构成员变量偏移量的两个应用

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作者联系方式：李先静 <xianjimli at hotmail dot com>

最近在阅读linux kernel的代码，发现很多地方都使用了一个名container_of的宏，根据上下文可以看出，它的功能是得到包含某个结构成员的结构的指针，看了一下宏的定义，原来并没有用什么高深的技巧，只是C语言中常识而已，其实现如下：

(kernel.h)

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

#definecontainer_of(ptr,type,member)({
consttypeof(((type*)0)->member)*__mptr=(ptr);
(type*)((char*)__mptr-offsetof(type,member));})

想起以前学习COM时，看过< COM本质论>，里面提到了类似的方法：

(COM本质论：inttable.h)

#defineBASE_OFFSET(ClassName,BaseName)
(DWORD(static_cast<BaseName*>(reinterpret_cast<ClassName*>(0x10000000)))-0x10000000)


#defineCOMPOSITE_OFFSET(ClassName,BaseName,MemberType,MemberName)
(DWORD(static_cast<BaseName*>(reinterpret_cast<MemberType*>(0x10000000+offsetof(ClassName,MemberName))))-0x10000000)

#defineBEGIN_INTERFACE_TABLE(ClassName)
typedefClassName_InterfaceTableClassName;
staticconstINTERFACE_ENTRY*GetInterfaceTable(void)
...{
staticconstINTERFACE_ENTRYtable[]=...{

#defineINTERFACE_TABLE_ENTRY(piid,pfn,dwData)
...{piid,pfn,dwData},


#defineIMPLEMENTS_INTERFACE(ItfName)
...{&IID_##ItfName,ENTRY_IS_OFFSET,BASE_OFFSET(_InterfaceTableClassName,ItfName)},


#defineIMPLEMENTS_INTERFACE_AS(RequestedItfName,BaseClassName)
...{&IID_##RequestedItfName,ENTRY_IS_OFFSET,BASE_OFFSET(_InterfaceTableClassName,BaseClassName)},


#defineIMPLEMENTS_INTERFACE_WITH_COMPOSITE(RequestedItfName,DataMemberType,DataMemberName)
...{&IID_##RequestedItfName,ENTRY_IS_OFFSET,COMPOSITE_OFFSET(_InterfaceTableClassName,RequestedItfName,DataMemberType,DataMemberName)},


#defineEND_INTERFACE_TABLE()
...{0,0,0}
};
returntable;
}