A pair of Rust kernel modules

Posted Sep 15, 2022 22:33 UTC (Thu) by Wol (subscriber, #4433)
In reply to: A pair of Rust kernel modules by mathstuf
Parent article: A pair of Rust kernel modules

> On one hand, a UB-less C would be "safer", but its portability would tank because "it worked on my x86_64" means diddly squat when you compile it for aarch64.

You've missed "implementation defined" and "hardware defined".

If something is "hardware defined" then yes, just because it works on x86_64, you can't expect the SAME code to work on aarch64, but firstly the programmer will KNOW that they need to check behaviour, and secondly they can put the ifdefs and whatever in there, and know that that IS DEFINED behaviour.

The *only* grounds for UB should be because "we can't define it because we can't get the logic to add up". There's no need for the C/C++ standard to define everything itself - it can defer the definition to something else - but all behaviour should be defined *somewhere*, if a definition is possible.

Take for example the size of a byte. In *PRACTICE* it's always 8-bit nowadays. I wouldn't be surprised if it's actually already implementation or hardware defined, but that's a perfect example of something that makes perfect sense as hardware-defined. In places, bytes are 6 bits, and if the programmer doesn't account for it it will cause a major problem if they're targetting an old platform. But the standard CAN, and SHOULD, address it.

Cheers,
Wol