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Comment by bengl3rt | Hacker News Reader

I only read the paper (and the code in the paper) but not the complete source code, so maybe this would become clearer if I had, but...

Does Rust fundamentally guarantee that if you make a struct, its fields will lay out in memory in the order that you defined them? Can it be used to interact with APIs (really ABIs) who expect a C struct (or pointer to one)?

I think my main frustration with stuff like Go and Swift in this case is that their structs are not binary-compatible with C structs in this way because they rearrange things to be better aligned/packed/whatever.

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Yes, Rust supports annotating your types with a `#[repr(...)]` attribute to control how it gets laid out in memory. There's a "C" repr that gives you representations interoperable with C.

https://doc.rust-lang.org/reference/type-layout.html#the-c-r...

> Does Rust fundamentally guarantee that if you make a struct, its fields will lay out in memory in the order that you defined them? Can it be used to interact with APIs (really ABIs) who expect a C struct (or pointer to one)?

You have to specify this behavior with #[repr(C)]. Otherwise, the compiler will rearrange fields to try to optimize packing and alignment.

Or, more precisely, reserves the right to do so in a future version. rustc does not currently do this. There are some compiler flags to randomize the struct layout just to make sure that you’re not implicitly depending on the order.

https://doc.rust-lang.org/reference/type-layout.html

https://github.com/rust-lang/compiler-team/issues/457

> Or, more precisely, reserves the right to do so in a future version. rustc does not currently do this

Does not currently do what ?

Rust certainly will re-arrange the layout of a default repr(Rust) struct to make it smaller, for example: https://rust.godbolt.org/z/7KsqvnE9o

[Edited to provide a nicer Godbolt example which compares the two layout strategies]

I stand corrected

The source[0] has `repr(packed)` but they should be using `repr(C, packed)`. That'll blow up in a future rust version (possibly 1.80) since packed alone is not sufficient to guarantee ordering[1].

[0] https://github.com/bootreer/vroom/blob/37bd8a22f5e0550b2cbc9... [1] https://doc.rust-lang.org/reference/type-layout.html#the-ali...

Go doesn't rearrange struct fields, which is also why there's a linter/analyzer which shows you whether or not you could be using less memory with better field sorting

https://pkg.go.dev/golang.org/x/tools/go/analysis/passes/fie...

> I think my main frustration with stuff like Go and Swift in this case is that their structs are not binary-compatible with C structs in this way because they rearrange things to be better aligned/packed/whatever.

If you need binary-compatibility with C structs in Swift, you can define them in a bridging header.

in rust you can control the layout and alignment of fields in a struct with `#[repr(...)]`

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