On: ISO C became unusable for operating systems develo...

[josephcsible]

No, that's the difference between unspecified and implementation defined behavior.

[astrobe_ → josephcsible]

This unspecifed behavior is perhaps a bit lesser-known than UB, here is a (hopefully non-null ;-) pointer:

https://stackoverflow.com/questions/18420753/unspecified-und...

[ErikCorry → josephcsible]

If you want to be pedantic about it then I guess Clang and GCC don't implement the standards since they treat implementation defined as unspecified.