C语言offsetof宏的说明

offsetof  ：
    Retrieves the offset of a member from the beginning of its parent structure.

size_t offsetof(structName, memberName);

Parameters：
    structName ： Name of the parent data structure.
    memberName ：Name of the member in the parent data structure for which to determine the offset.

Return Value ： offsetof returns the offset in bytes of the specified member from 
          the beginning of its parent data structure. It is undefined for bit fields.
Remarks ：
    The offsetof macro returns the offset in bytes of memberName from the beginning of the structure specified by structName. You can specify types with the struct keyword.

Note  ：
    offsetof is not a function and cannot be described using a C prototype.

 

 #define offsetof(s, m)   (size_t)&(((s *)0)->m)

s是一个结构名，它有一个名为m的成员（s和m 是宏offsetof的形参，它实际是返回结构s的成员m的偏移地址.

(s *)0 是骗编译器说有一个指向类（或结构）s的指针，其地址值0 

&((s *)0)->m   是要取得类s中成员变量m的地址. 因基址为0，这时m的地址当然就是m在s中的偏移

最后转换size_t 型，即unsigned int。