[eru]

> You can't make any useful software in "Portable C" - or any portable language for that matter.

Have you heard of Java or even Python or JavaScript?

> Side effects matter, and they are always non-portable/implementation defined/dependent on the hardware.

Granted. But how does the need for implementation defined excuse undefined behaviour?

> What printf() actaully does is implementation defined [...]

> Well, not every computer has that, so now what?

The standard can be written conditionally: 'if the computer has display, printf shall show something.'