https://gcc.gnu.org/bugzilla/show_bug.cgi?id=128412
--- Comment #1 from Joseph S. Myers <jsm28 at gcc dot gnu.org> --- That's not an attribute following the identifier, it's the "array-declarator attribute-specifier-sequence[opt]" case of direct-declarator. As stated in 6.7.7.3 Array declarators, "The optional attribute specifier sequence appertains to the array.". If you want the attribute on the declared entity, put it after the identifier and before the [100].
