This is in contrast to the other category that exists, which is "implementation-defined".
Just don't do unaligned access, it's a dialect that doesn't exist currently, and should never exist.
This is plain wrong. Undefined behaviour, means the C standard specifies no restriction on the behaviour of the program, which is what the implementation chooses to emit. An implementation can very well choose to emit any program it pleases, including programs that encrypt your harddisk, but also programs that stick to well defined rules.
Writing C (or any language) means adhering to the standard, because that's the definition of the language.
The C standard reminds me of Posix. You have a rough guideline if you ever wanted to port a program, but you actually have to learn the new compiler and its actual behavior before doing so.
Side effects matter, and they are always non-portable/implementation defined/dependent on the hardware.
What printf() actaully does is implementation defined - what does "printing mean", does a console even exist? Maybe a user expects it to show graphical ascii/utf8 glyphs on a LCD display? Well, not every computer has that, so now what?
