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Update the primitive types explanation in the Rust coding standards

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Part of #25368.
Includes c_double in anticipation of #23061.
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teor 2018-02-27 15:55:13 +11:00
parent 54e25ab124
commit 01a977b492
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22
doc/HACKING/CodingStandardsRust.md
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@ -284,12 +284,26 @@ Here are some additional bits of advice and rules:
}
}
3. Pass only integer types and bytes over the boundary
3. Pass only C-compatible primitive types and bytes over the boundary
The only non-integer type which may cross the FFI boundary is
Rust's C-compatible primitive types are integers and floats.
These types are declared in the [libc crate](https://doc.rust-lang.org/libc/x86_64-unknown-linux-gnu/libc/index.html#types).
Most Rust objects have different [representations](https://doc.rust-lang.org/libc/x86_64-unknown-linux-gnu/libc/index.html#types)
in C and Rust, so they can't be passed using FFI.
Tor currently uses the following Rust primitive types from libc for FFI:
* defined-size integers: `uint32_t`
* native-sized integers: `c_int`
* native-sized floats: `c_double`
* native-sized raw pointers: `* c_void`, `* c_char`, `** c_char`
TODO: C smartlist to Stringlist conversion using FFI
The only non-primitive type which may cross the FFI boundary is
bytes, e.g. `&[u8]`. This SHOULD be done on the Rust side by
passing a pointer (`*mut libc::c_char`) and a length
(`libc::size_t`).
passing a pointer (`*mut libc::c_char`). The length can be passed
explicitly (`libc::size_t`), or the string can be NUL-byte terminated
C string.
One might be tempted to do this via doing
`CString::new("blah").unwrap().into_raw()`. This has several problems: