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

That particular way of doing it is not explicitly defined in any C standard, but C99 does include the "struct hack" as part of the language. In C99, the last member of a struct may be a "flexible array member", declared as char foo[] (with whatever type you desire in place of char).