> I was under the impression most of how c interacts with memory is part of the undefined part of the spec

For a long time, the memory model, formally speaking, was underspecified. Both C and C++ agreed on and added a memory model in C11 and C++11.

> fencing

You can add a fence via this API: https://en.cppreference.com/w/c/atomic/atomic_thread_fence

> This varies per arch

Right, so what assembly this API will emit depends on the underlying architecture details.

Notably, the Linux kernel does not use the standard C memory model, it uses its own.

To be clear, that’s the definition of the memory model under concurrent execution. I’m pretty sure the single threaded version was well defined

My understanding is that “memory model” is definitionaly about what happens when you add parallelism and/or concurrency.

Regardless, this isn’t a slight against C. Most languages don’t have an explicit memory model at all.

I introduced the word memory model so that’s on me for that error - definitionally memory model does refer to concurrency.

However Op used “memory behavior”. The C standard definitely has defined and defined and undefined memory behaviors even pre C11. For example, dereferencing null is an undefined single threaded memory behavior that bit the kernel when the compilers started optimizing from assumptions about never reaching UB code. But for example there’s lots of defined behaviors like what happens when you dereference an aliased pointer.

And the Linux kernel definitely defined a memory model for itself before C and made all architectures conform to that.

Yeah that's fair. As I said, I'm not trying to imply something was bad here before C11. I thought "underspecified" instead of "unspecified" would communicate "some of it was defined but some of it wasn't but it's been fully so for a while now" but maybe that missed the mark.