offsetof at compile time

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The offsetof() macro is a compile-time construct. There is no standard-compliant way to define it, but every compiler must have some way of doing it.

One example would be:

#define offsetof( type, member ) ( (size_t) &( ( (type *) 0 )->member ) )

While not being a compile-time construct technically (see comments by user “litb”), every compiler must have at least one such expression that it is able to resolve at compile time, which is exactly what offsetof() is defined to in <stddef.h>.

There is probably some other error to your code – a missing include of <stddef.h>, or some other thing irritating your compiler.

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