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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 a

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?

Arnavion 1y ago
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.
- stouset 1y ago
  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.
pmalynin 1y ago
Like, repr(C)? https://doc.rust-lang.org/nomicon/other-reprs.html https://doc.rust-lang.org/nomicon/other-reprs.html
GrantMoyer 1y ago
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 https://github.com/rust-lang/rust-bindgen
oconnor663 1y ago
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 https://youtu.be/LLAUzghhNHg?t=2168
broken_broken_ 1y ago
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 https://gaultier.github.io/blog/rust_c++_interop_trick.html
IshKebab 1y ago
It's idiomatic to access struct fields directly in both languages. What more do you want?

Arnavion 1y ago

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.

stouset 1y ago
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.

pmalynin 1y ago

Like, repr(C)? https://doc.rust-lang.org/nomicon/other-reprs.html https://doc.rust-lang.org/nomicon/other-reprs.html

GrantMoyer 1y ago

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 https://github.com/rust-lang/rust-bindgen

oconnor663 1y ago

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 https://youtu.be/LLAUzghhNHg?t=2168

broken_broken_ 1y ago

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 https://gaultier.github.io/blog/rust_c++_interop_trick.html

IshKebab 1y ago

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