1. Pre-C99, no flexible array was allowed.

2. Gradually, people started using the "size-1 array at the end of a sturct but write beyond" hack as flexible array.

3. As an attempt to do the hack in a more ordered manner, some compilers, including GCC, started officially supporting the non-standard "size-0" array extension.

4. C99 added flexible array with indefinite length (array[]), while prohibiting both the undefined-behavior "array[1]" and the non-standard extentios "array[0]".

So when people say "size-0" array, it could mean either of these three things, and it does get a bit confusing. But fundamentally the idea was the same, all of the three techniques are used to achieve the same practical effect.