Hi Stefan,

> Yes, I dislike that, too.  The question is why gcc has supported
> the "struct hack" for more than 20 years but needs an annotation
> just for MPX.
The "struct hack" represents a problem for MPX because it intentionally exceeds the bounds of the array declared inside in order to support a variable sized structure. Without the Py_VARIABLE_SIZE annotation, MPX will generate a bounds violation when accessing outside the declared size of the array.

> Is the annotation also needed for the C99 version (ob_array[])?
I tested and for the C99 version the annotation is not needed. Apparently ob_array inherits the bounds of the structure in this case.

Regards,
Florin