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Sorry, still not getting it.

If an implementation provides offsetof and it doesn't work, that implementation would go out of fashion in a heart beat.

Where is the practical use of this information?

How is this even an argument?

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The use of the Offsetof macro in a program will cause undefined behavior when compiled with GCC or Clang and they are still "in fashion" as far as I am aware. The practical use of this information is to prevent the occurrence of undefined behavior by knowing not to use offsetof for example with GCC and instead use __builtin_offsetof in the specific case of GCC. The argument appears to be that you are claiming that what sgl_baseof does is not undefined behavior and I am claiming that it is not possible to infer that without specifying the implementation of the offsetof macro.

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