commit 01a977b492327f39e7490e9ffbc5a27f15734dc9 Author: teor teor2345@gmail.com Date: Tue Feb 27 15:55:13 2018 +1100

Update the primitive types explanation in the Rust coding standards

Part of #25368. Includes c_double in anticipation of #23061. --- doc/HACKING/CodingStandardsRust.md | 22 ++++++++++++++++++---- 1 file changed, 18 insertions(+), 4 deletions(-)

diff --git a/doc/HACKING/CodingStandardsRust.md b/doc/HACKING/CodingStandardsRust.md index d0b17c160..731a7bb85 100644 --- a/doc/HACKING/CodingStandardsRust.md +++ b/doc/HACKING/CodingStandardsRust.md @@ -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#type...). + Most Rust objects have different [representations](https://doc.rust-lang.org/libc/x86_64-unknown-linux-gnu/libc/index.html#type...) + 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: