Is the "struct hack" technically undefined behavior?

As the C FAQ says:

It's not clear if it's legal or portable, but it is rather popular.

and:

... an official interpretation has deemed that it is not strictly conforming with the C Standard, although it does seem to work under all known implementations. (Compilers which check array bounds carefully might issue warnings.)

The rationale behind the 'strictly conforming' bit is in the spec, section J.2 Undefined behavior, which includes in the list of undefined behavior:

• An array subscript is out of range, even if an object is apparently accessible with the given subscript (as in the lvalue expression a[1][7] given the declaration int a[4][5]) (6.5.6).

Paragraph 8 of Section 6.5.6 Additive operators has another mention that access beyond defined array bounds is undefined:

If both the pointer operand and the result point to elements of the same array object, or one past the last element of the array object, the evaluation shall not produce an overflow; otherwise, the behavior is undefined.

I believe that technically it's undefined behavior. The standard (arguably) doesn't address it directly, so it falls under the "or by the omission of any explicit definition of behavior." clause (§4/2 of C99, §3.16/2 of C89) that says it's undefined behavior.

The "arguably" above depends on the definition of the array subscripting operator. Specifically, it says: "A postfix expression followed by an expression in square brackets [] is a subscripted designation of an array object." (C89, §6.3.2.1/2).

You can argue that the "of an array object" is being violated here (since you're subscripting outside the defined range of the array object), in which case the behavior is (a tiny bit more) explicitly undefined, instead of just undefined courtesy of nothing quite defining it.

In theory, I can imagine a compiler that does array bounds checking and (for example) would abort the program when/if you attempted to use an out of range subscript. In fact, I don't know of such a thing existing, and given the popularity of this style of code, even if a compiler tried to enforce subscripts under some circumstances, it's hard to imagine that anybody would put up with its doing so in this situation.