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

Yes, it is undefined behavior.

C Language Defect Report #051 gives a definitive answer to this question:

The idiom, while common, is not strictly conforming

http://www.open-std.org/jtc1/sc22/wg14/www/docs/dr_051.html

In the C99 Rationale document the C Committee adds:

The validity of this construct has always been questionable. In the response to one Defect Report, the Committee decided that it was undefined behavior because the array p->items contains only one item, irrespective of whether the space exists.