Comment by gizmo686

Comment by gizmo686 2 days ago

In practice C let's you control memory layout just fine. You might need to use __attribute__((packed)), which is technically non standard.

I've written hardware device drivers in pure C where you need need to peek and poke at specific bits on the memory bus. I defined a struct that matched the exact memory layout that the hardware specifies. Then cast an integer to a pointer to that struct type. At which point I could interact with the hardware by directly reading/writing fields if the struct (most of which were not even byte aligned).

It is not quite that simple, as you also have to deal with bypassing the cache, memory barriers, possibly virtual memory, finding the erreta that clarifies the originaly published register address was completely wrong. But I don't think any of that is what people mean when they say "memory layout".

pjmlp a day ago

Now split the struct across registers in C.

You are aware that some of those casting tricks are UB, right?

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gizmo686 a day ago

Casting integers to pointers in C is implementation defined, not UB. In practice compilers define these casts as the natural thing for the architecture you are compiling to. Since mainstream CPUs don't do anything fancy with pointer tagging, that means the implementation defined behave does exactly what you expect it to do (unless you forget that you have paging enabled and cannot simply point to a hardware memory address).
If you want to control register layout, then C is not going to help you, but that is not typically what is meant by "memory layout".
And if you want to control cache usage ... Some architectures do expose some black magic which you would need to go to assembly to access. But for the most part controlling cache involves understanding how the cache works, then controlling the memory layout and accesses to work well with the cache.

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