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Comment by phkahler | Hacker News Reader

Something I'd like to know for mixing Rust and C. I know it's possible to access a struct from both C and Rust code and have seen examples. But those all use accessor functions on the Rust side rather than accessing the members directly. Is it possible to define a structure in one of the languages and then via some wrapper or definitions be able to access it idiomatically in the other language? Can you point to some blog or documentation explaining how?

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Rust bindgen[1] will automatically generate native Rust stucts (and unions) from C headers where possible. Note that c_int, c_char, etc. are just aliases for the corresponding native Rust types.

However, not all C constructs have idomatic Rust equivalents. For example, bitfields don't exist in Rust, and unlike Rust enums, C enums can have any value of the underlying type. And for ABI reasons, it's very commom in C APIs to use a pointer to an opaque type paired with what are effectively accessor function and methods, so mapping them to accessors and methods on a "Handle" type in Rust often is the most idomatic Rust representation of the C interface.

[1]: https://github.com/rust-lang/rust-bindgen

Here's one of my recorded talks going through an example of using a `#[repr(C)]` struct (in this case one that's auto-generated by Bindgen): https://youtu.be/LLAUzghhNHg?t=2168

I don't know what examples you've been seeing. The interop structs are just regular Rust structs with the `#[repr(C)]` attribute applied to them, to ensure that the Rust compiler lays the struct out exactly as the C compiler for that target ABI would. Rust code can access their fields just fine. There's no strict need for accessor functions.

And vice versa. Rust code and C code can both operate on each other’s structs natively.

`#[repr(C)]` instructs the compiler to lay the struct out exactly according to C’s rules: order, alignment, padding, size, etc. Without this, the compiler is allowed a lot more freedom when laying out a struct.

Like, repr(C)?

https://doc.rust-lang.org/nomicon/other-reprs.html

I wrote a blog article about exactly this, using a C++ class from C++, C and Rust: https://gaultier.github.io/blog/rust_c++_interop_trick.html

It's idiomatic to access struct fields directly in both languages. What more do you want?

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