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< ol class = "chapter" > < li class = "chapter-item expanded " > < a href = "../../intro.html" > < strong aria-hidden = "true" > 1.< / strong > Introduction< / a > < / li > < li class = "chapter-item expanded " > < a href = "../../idioms/index.html" > < strong aria-hidden = "true" > 2.< / strong > Idioms< / a > < / li > < li > < ol class = "section" > < li class = "chapter-item expanded " > < a href = "../../idioms/coercion-arguments.html" > < strong aria-hidden = "true" > 2.1.< / strong > Use borrowed types for arguments< / a > < / li > < li class = "chapter-item expanded " > < a href = "../../idioms/concat-format.html" > < strong aria-hidden = "true" > 2.2.< / strong > Concatenating Strings with format!< / a > < / li > < li class = "chapter-item expanded " > < a href = "../../idioms/ctor.html" > < strong aria-hidden = "true" > 2.3.< / strong > Constructor< / a > < / li > < li class = "chapter-item expanded " > < a href = "../../idioms/default.html" > < strong aria-hidden = "true" > 2.4.< / strong > The Default Trait< / a > < / li > < li class = "chapter-item expanded " > < a href = "../../idioms/deref.html" > < strong aria-hidden = "true" > 2.5.< / strong > Collections Are Smart Pointers< / a > < / li > < li class = "chapter-item expanded " > < a href = "../../idioms/dtor-finally.html" > < strong aria-hidden = "true" > 2.6.< / strong > Finalisation in Destructors< / a > < / li > < li class = "chapter-item expanded " > < a href = "../../idioms/mem-replace.html" > < strong aria-hidden = "true" > 2.7.< / strong > mem::{take(), replace()}< / a > < / li > < li class = "chapter-item expanded " > < a href = "../../idioms/on-stack-dyn-dispatch.html" > < strong aria-hidden = "true" > 2.8.< / strong > On-Stack Dynamic Dispatch< / a > < / li > < li class = "chapter-item expanded " > < a href = "../../idioms/ffi/intro.html" > < strong aria-hidden = "true" > 2.9.< / strong > Foreign function interface (FFI)< / a > < / li > < li > < ol class = "section" > < li class = "chapter-item expanded " > < a href = "../../idioms/ffi/errors.html" > < strong aria-hidden = "true" > 2.9.1.< / strong > Idiomatic Errors< / a > < / li > < li class = "chapter-item expanded " > < a href = "../../idioms/ffi/accepting-strings.html" > < strong aria-hidden = "true" > 2.9.2.< / strong > Accepting Strings< / a > < / li > < li class = "chapter-item expanded " > < a href = "../../idioms/ffi/passing-strings.html" class = "active" > < strong aria-hidden = "true" > 2.9.3.< / strong > Passing Strings< / a > < / li > < / ol > < / li > < li class = "chapter-item expanded " > < a href = "../../idioms/option-iter.html" > < strong aria-hidden = "true" > 2.10.< / strong > Iterating over an Option< / a > < / li > < li class = "chapter-item expanded " > < a href = "../../idioms/pass-var-to-closure.html" > < strong aria-hidden = "true" > 2.11.< / strong > Pass Variables to Closure< / a > < / li > < li class = "chapter-item expanded " > < a href = "../../idioms/priv-extend.html" > < strong aria-hidden = "true" > 2.12.< / strong > Privacy For Extensibility< / a > < / li > < li class = "chapter-item expanded " > < a href = "../../idioms/rustdoc-init.html" > < strong aria-hidden = "true" > 2.13.< / strong > Easy doc initialization< / a > < / li > < li class = "chapter-item expanded " > < a href = "../../idioms/temporary-mutability.html" > < strong aria-hidden = "true" > 2.14.< / strong > Temporary mutability< / a > < / li > < / ol > < / li > < li class = "chapter-item expanded " > < a href = "../../patterns/index.html" > < strong aria-hidden = "true" > 3.< / strong > Design Patterns< / a > < / li > < li > < ol class = "section" > < li class = "chapter-item expanded " > < a href = "../../patterns/behavioural/intro.html" > < strong aria-hidden = "true" > 3.1.< / strong > Behavioural< / a > < / li > < li > < ol class = "section" > < li class = "chapter-item expanded " > < a href = "../../patterns/behavioural/command.html" > < strong aria-hidden = "true" > 3.1.1.< / strong > Command< / a > < / li > < li class = "chapter-item expanded " > < a href = "../../patterns/behavioural/interpreter.html" > < strong aria-hidden = "true" > 3.1.2.< / strong > Interpreter< / a > < / li > < li class = "chapter-item expanded " > < a href = "../../patterns/behavioural/newtype.html" > < strong aria-hidden = "true" > 3.1.3.< / strong > Newtype< / a > < / li > < li class = "chapter-item expanded " > < a href = "../../patterns/behavioural/RAII.html" > < strong aria-hidden = "true" > 3.1.4.< / strong > RAII Guards< / a > < / li > < li class = "chapter-item expanded " > < a href = "../../patterns/behavioural/strategy.html" > < strong aria-hidden = "true" > 3.1.5.< / strong > Strategy< / a > < / li > < li class = "chapter-item expanded " > < a href = "../../patterns/behavioural/visitor.html" > < strong aria-hidden =
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< h1 id = "passing-strings" > < a class = "header" href = "#passing-strings" > Passing Strings< / a > < / h1 >
< h2 id = "description" > < a class = "header" href = "#description" > Description< / a > < / h2 >
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< p > When passing strings to FFI functions, there are four principles that should be
followed:< / p >
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< ol >
< li > Make the lifetime of owned strings as long as possible.< / li >
< li > Minimize < code > unsafe< / code > code during the conversion.< / li >
< li > If the C code can modify the string data, use < code > Vec< / code > instead of < code > CString< / code > .< / li >
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< li > Unless the Foreign Function API requires it, the ownership of the string
should not transfer to the callee.< / li >
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< / ol >
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< h2 id = "motivation" > < a class = "header" href = "#motivation" > Motivation< / a > < / h2 >
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< p > Rust has built-in support for C-style strings with its < code > CString< / code > and < code > CStr< / code >
types. However, there are different approaches one can take with strings that
are being sent to a foreign function call from a Rust function.< / p >
< p > The best practice is simple: use < code > CString< / code > in such a way as to minimize
< code > unsafe< / code > code. However, a secondary caveat is that
< em > the object must live long enough< / em > , meaning the lifetime should be maximized.
In addition, the documentation explains that " round-tripping" a < code > CString< / code > after
modification is UB, so additional work is necessary in that case.< / p >
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< h2 id = "code-example" > < a class = "header" href = "#code-example" > Code Example< / a > < / h2 >
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< pre > < code class = "language-rust ignore" > pub mod unsafe_module {
// other module content
extern " C" {
fn seterr(message: *const libc::c_char);
fn geterr(buffer: *mut libc::c_char, size: libc::c_int) -> libc::c_int;
}
fn report_error_to_ffi< S: Into< String> > (
err: S
) -> Result< (), std::ffi::NulError> {
let c_err = std::ffi::CString::new(err.into())?;
unsafe {
// SAFETY: calling an FFI whose documentation says the pointer is
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// const, so no modification should occur
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seterr(c_err.as_ptr());
}
Ok(())
// The lifetime of c_err continues until here
}
fn get_error_from_ffi() -> Result< String, std::ffi::IntoStringError> {
let mut buffer = vec![0u8; 1024];
unsafe {
// SAFETY: calling an FFI whose documentation implies
// that the input need only live as long as the call
let written: usize = geterr(buffer.as_mut_ptr(), 1023).into();
buffer.truncate(written + 1);
}
std::ffi::CString::new(buffer).unwrap().into_string()
}
}
< / code > < / pre >
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< h2 id = "advantages" > < a class = "header" href = "#advantages" > Advantages< / a > < / h2 >
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< p > The example is written in a way to ensure that:< / p >
< ol >
< li > The < code > unsafe< / code > block is as small as possible.< / li >
< li > The < code > CString< / code > lives long enough.< / li >
< li > Errors with typecasts are always propagated when possible.< / li >
< / ol >
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< p > A common mistake (so common it's in the documentation) is to not use the
variable in the first block:< / p >
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< pre > < code class = "language-rust ignore" > pub mod unsafe_module {
// other module content
fn report_error< S: Into< String> > (err: S) -> Result< (), std::ffi::NulError> {
unsafe {
// SAFETY: whoops, this contains a dangling pointer!
seterr(std::ffi::CString::new(err.into())?.as_ptr());
}
Ok(())
}
}
< / code > < / pre >
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< p > This code will result in a dangling pointer, because the lifetime of the
< code > CString< / code > is not extended by the pointer creation, unlike if a reference were
created.< / p >
< p > Another issue frequently raised is that the initialization of a 1k vector of
zeroes is " slow" . However, recent versions of Rust actually optimize that
particular macro to a call to < code > zmalloc< / code > , meaning it is as fast as the operating
system's ability to return zeroed memory (which is quite fast).< / p >
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< h2 id = "disadvantages" > < a class = "header" href = "#disadvantages" > Disadvantages< / a > < / h2 >
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< p > None?< / p >
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