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Update Interoperability section #351

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Update Interoperability section #351

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Update Interoperability section
abt8601 May 6, 2023
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Fix formatting
abt8601 May 6, 2023
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51 changes: 26 additions & 25 deletions src/interoperability/index.md
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Expand Up @@ -2,40 +2,42 @@

Interoperability between Rust and C code is always dependent
on transforming data between the two languages.
For this purposes there are two dedicated modules
For this purpose, there is a dedicated module
in the `stdlib` called
[`std::ffi`](https://doc.rust-lang.org/std/ffi/index.html) and
[`std::os::raw`](https://doc.rust-lang.org/std/os/raw/index.html).
[`std::ffi`](https://doc.rust-lang.org/std/ffi/index.html).

`std::os::raw` deals with low-level primitive types that can
be converted implicitly by the compiler
because the memory layout between Rust and C
is similar enough or the same.
`std::ffi` provides type definitions for C primitive types,
such as `char`, `int`, and `long`.
It also provides some utility for converting more complex
types such as strings, mapping both `&str` and `String`
to C types that are easier and safer to handle.

`std::ffi` provides some utility for converting more complex
types such as Strings, mapping both `&str` and `String`
to C-types that are easier and safer to handle.

Neither of these modules are available in `core`, but you can find a `#![no_std]`
compatible version of `std::ffi::{CStr,CString}` in the [`cstr_core`] crate, and
most of the `std::os::raw` types in the [`cty`] crate.
As of Rust 1.30,
functionalities of `std::ffi` are available
in either `core::ffi` or `alloc::ffi`
depending on whether or not memory allocation is involved.
The [`cty`] crate and the [`cstr_core`] crate
also offer similar functionalities.

[`cstr_core`]: https://crates.io/crates/cstr_core
[`cty`]: https://crates.io/crates/cty

| Rust type | Intermediate | C type |
|------------|--------------|--------------|
| String | CString | *char |
| &str | CStr | *const char |
| () | c_void | void |
| u32 or u64 | c_uint | unsigned int |
| etc | ... | ... |
| Rust type | Intermediate | C type |
|----------------|--------------|----------------|
| `String` | `CString` | `char *` |
| `&str` | `CStr` | `const char *` |
| `()` | `c_void` | `void` |
| `u32` or `u64` | `c_uint` | `unsigned int` |
| etc | ... | ... |

As mentioned above, primitive types can be converted
by the compiler implicitly.
A value of a C primitive type can be used
as one of the corresponding Rust type and vice versa,
since the former is simply a type alias of the latter.
For example, the following code compiles on platforms
where `unsigned int` is 32-bit long.

```rust,ignore
unsafe fn foo(num: u32) {
fn foo(num: u32) {
let c_num: c_uint = num;
let r_num: u32 = c_num;
}
Expand All @@ -51,7 +53,6 @@ We are collecting examples and use cases for this on our issue tracker in

[issue #61]: https://github.com/rust-embedded/book/issues/61


## Interoperability with RTOSs

Integrating Rust with an RTOS such as FreeRTOS or ChibiOS is still a work in
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