Is the “struct hack” technically undefined behavior?

Question

What I am asking about is the well known \"last member of a struct has variable length\" trick. It goes something like this:

struct T {
    int len;
    char         


        

Answer 1

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.