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points by LordDragonfang 2 days ago | 0 comments
> the compiler can invoke nasal demons if it makes the code go faster

I had never heard that phrase, and was delighted at the answer when I googled it its origin, thank you:

http://www.catb.org/jargon/html/N/nasal-demons.html

> “When the compiler encounters [a given undefined construct] it is legal for it to make demons fly out of your nose” (the implication is that the compiler may choose any arbitrarily bizarre way to interpret the code without violating the ANSI C standard)

[–] cowlevel 2 days ago
However this is limited by the compiler's actual abilities and code. The C standard doesn't forbid nasal demons, but the compiler isn't programmed to invoke nasal demons and isn't running on hardware with that capability, so it won't happen.

I've heard nasal demons invoked as a literal possibility. No they are not a literal possibility.

[–] Xirdus 2 days ago
Maybe your compiler isn't, but GCC is designed to summon every nasal demon it can. It especially loves to elide null pointer checks. There was a PR that almost made it to Linux mainline that did some weird stuff in kernel space where memory mapped to address 0 is valid and contains important stuff. It got a pointer, read data from it, then checked if pointer is null before giving root permissions for some operation. Reading the code, everything seems alright. Even stepping through it with debugger shows that it works. But when compiled with -O2 or higher, GCC removed the null check and now everybody got root permissions unconditionally - because reading from null pointer is UB regardless of circumstances, and GCC treats UB as dead code even if it is reachable.
[–] cowlevel 2 days ago
> GCC is designed to summon every nasal demon it can.

Which is zero, to be clear. There's not even a standard USB device class for a nasal demon releaser

A null check elision is not a demon up my nose - it's a null check elision.

[–] Xirdus 1 day ago
Sure, and char[] is no string because you can't tie anything with it. And don't get me started on the insects that allegedly live inside digital source files - that's an absurd concept, no self respecting computer specialist should be talking like that.
[–] neomantra 2 days ago
Having made devices for other human holes, you can just use the standardized HID class. It can be adapted for nasal demons incoming and outgoing.
[–] miki123211 2 days ago
Wait, so if you're running on embedded, you're not allowed to store anything at address 0, requiring you to waste that little bit of RAM?

That's... a bizarre choice, considering how religious C is about not making any assumptions about what a machine can and cannot do, even going to the trouble of working with non-IEEE floats and int types that are not a multiple of 8 bits.

[–] Xirdus 1 day ago
Unfortunately yes. ISO/IEC 9899:1999 footnote 83: "Among the invalid values for dereferencing a pointer by the unary * operator are a null pointer".