I believe the modulo operator is implementation defined. As should be signed integer overflow, though it isn't.

One big problem about "undefined" is, when some platform go bananas over some stuff, the behaviour is standardised as undefined for all platforms, allowing the compilers to go bananas. Which they do, because every bit of undefined helps them perform some micro-optimization they couldn't do before.

I long for the day where the standard forces compilers to provide options such as -fwrap or -fno-strict on platforms that can reasonably support it.