Why do people define their own offsetof?

Question

Having just seen What does the following macro do? I gotta ask my own question: why do so many applications' headers define offsetof themselves? Is there some reason why <stddef.h> is not to be relied upon?

Answer 1

我不认为他们不信任标准offsetof -至少从我所看到的来看，通常他们只是不知道它。

Answer 2

Is there some reason why is not to be relied upon?

I know one of the reasons. GCC produces a warning when standard offsetof() is used on fields of C++ classes. That leads some people to roll out their own version which doesn't trigger the warning.

Answer 3

又或者它是来自C编译器的不符合ANSI且没有offsetof的旧代码？