Only the first one is fully standards-compliant C; the others all invoke some non-standard (but widely-accepted) features. (I believe empty unions - and their interpretation as being 0-sized - are non-standard in C, but I am not 100% sure.)

My point was not that those are standards-compliant, but that the use of the GCC extensions is not the primary cause of those examples being unreadable. (which seemed to be the point the person I replied to was making - and I disagree with GCC extensions being bad).