commit 8fff331bb095dc6f5e2fe2ecfc9ab08ea9e2fe97 Author: Isis Lovecruft isis@torproject.org Date: Thu Feb 8 20:06:19 2018 +0000

rust: Add macro for passing static borrowed strings from Rust to C.

* ADD a new macro, tor_util::string::cstr!() which takes Rust strings, concatenates them together, appends a NUL byte, and converts it into a std::ffi::CStr for handing to C. --- src/rust/tor_util/strings.rs | 136 +++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 136 insertions(+)

diff --git a/src/rust/tor_util/strings.rs b/src/rust/tor_util/strings.rs index 9321ce4f8..659e792a3 100644 --- a/src/rust/tor_util/strings.rs +++ b/src/rust/tor_util/strings.rs @@ -80,3 +80,139 @@ pub fn empty_static_cstr() -> &'static CStr {

empty } + +/// Create a `CStr` from a literal byte slice, appending a NUL byte to it first. +/// +/// # Warning +/// +/// The literal byte slice which is taken as an argument *MUST NOT* have any NUL +/// bytes (`b"\0"`) in it, anywhere, or else an empty string will be returned +/// (`CStr::from_bytes_with_nul_unchecked(b"\0")`) so as to avoid `panic!()`ing. +/// +/// # Examples +/// +/// ``` +/// #[macro_use] +/// extern crate tor_util; +/// +/// use std::ffi::CStr; +/// +/// # fn do_test() -> Result<&'static CStr, &'static str> { +/// let message: &'static str = "This is a test of the tsunami warning system."; +/// let tuesday: &'static CStr; +/// let original: &str; +/// +/// tuesday = cstr!("This is a test of the tsunami warning system."); +/// original = tuesday.to_str().or(Err("Couldn't unwrap CStr!"))?; +/// +/// assert!(original == message); +/// # +/// # Ok(tuesday) +/// # } +/// # fn main() { +/// # do_test(); // so that we can use the ? operator in the test +/// # } +/// ``` +/// It is also possible to pass several string literals to this macro. They +/// will be concatenated together in the order of the arguments, unmodified, +/// before finally being suffixed with a NUL byte: +/// +/// ``` +/// #[macro_use] +/// extern crate tor_util; +/// # +/// # use std::ffi::CStr; +/// # +/// # fn do_test() -> Result<&'static CStr, &'static str> { +/// +/// let quux: &'static CStr = cstr!("foo", "bar", "baz"); +/// let orig: &'static str = quux.to_str().or(Err("Couldn't unwrap CStr!"))?; +/// +/// assert!(orig == "foobarbaz"); +/// # Ok(quux) +/// # } +/// # fn main() { +/// # do_test(); // so that we can use the ? operator in the test +/// # } +/// ``` +/// This is useful for passing static strings to C from Rust FFI code. To do so +/// so, use the `.as_ptr()` method on the resulting `&'static CStr` to convert +/// it to the Rust equivalent of a C `const char*`: +/// +/// ``` +/// #[macro_use] +/// extern crate tor_util; +/// +/// use std::ffi::CStr; +/// use std::os::raw::c_char; +/// +/// pub extern "C" fn give_static_borrowed_string_to_c() -> *const c_char { +/// let hello: &'static CStr = cstr!("Hello, language my parents wrote."); +/// +/// hello.as_ptr() +/// } +/// # fn main() { +/// # let greetings = give_static_borrowed_string_to_c(); +/// # } +/// ``` +/// Note that the C code this static borrowed string is passed to *MUST NOT* +/// attempt to free the memory for the string. +/// +/// # Note +/// +/// An unfortunate limitation of the rustc compiler (as of 1.25.0-nightly), is +/// that the above code compiles, however if we were to change the assignment of +/// `tuesday` as follows, it will fail to compile, because Rust macros are +/// expanded at parse time, and at parse time there is no symbols table +/// available. +/// +/// ```ignore +/// tuesday = cstr!(message); +/// ``` +/// with the error message `error: expected a literal`. +/// +/// # Returns +/// +/// If the string literals passed as arguments contain no NUL bytes anywhere, +/// then an `&'static CStr` containing the (concatenated) bytes of the string +/// literal(s) passed as arguments, with a NUL byte appended, is returned. +/// Otherwise, an `&'static CStr` containing a single NUL byte is returned (an +/// "empty" string in C). +#[macro_export] +macro_rules! cstr { + ($($bytes:expr),*) => ( + ::std::ffi::CStr::from_bytes_with_nul( + concat!($($bytes),*, "\0").as_bytes() + ).unwrap_or( + unsafe{ + ::std::ffi::CStr::from_bytes_with_nul_unchecked(b"\0") + } + ) + ) +} + +#[cfg(test)] +mod test { + use std::ffi::CStr; + + #[test] + fn cstr_macro() { + let _: &'static CStr = cstr!("boo"); + } + + #[test] + fn cstr_macro_multi_input() { + let quux: &'static CStr = cstr!("foo", "bar", "baz"); + + assert!(quux.to_str().unwrap() == "foobarbaz"); + } + + #[test] + fn cstr_macro_bad_input() { + let waving: &'static CStr = cstr!("waving not drowning o/"); + let drowning: &'static CStr = cstr!("\0 drowning not waving"); + + assert!(waving.to_str().unwrap() == "waving not drowning o/"); + assert!(drowning.to_str().unwrap() == "") + } +}