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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="true">3.1.6.</strong> Visitor</a></li></ol></li><li class="chapter-item expanded "><a href="../../patterns/creational/intro.html"><strong aria-hidden="true">3.2.</strong> Creational</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../../patterns/creational/builder.html"><strong aria-hidden="true">3.2.1.</strong> Builder</a></li><li class="chapter-item expanded "><a href="../../patterns/creational/fold.html"><strong aria-hidden="true">3.2.2.</strong> Fold</a></li></ol></li><li class="chapter-item expanded "><a href="../../patterns/structural/intro.html"><strong aria-hidden="true">3.3.</strong> Structural</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../../patterns/structural/compose-structs.html"><strong aria-hidden="true">3.3.1.</strong> Compose Structs</a></li><li class="chapter-item expanded "><a href="../../patterns/structural/small-crates.html"><strong aria-hidden="true">3.3.2.</strong> Prefer Small Crates</a></li><li class="chapter-item expanded "><a href="../../patterns/structural/unsafe-mods.html"><strong aria-hidden="true">3.3.3.</strong> Contain unsafety in small modules</a></li></ol></li><li class="chapter-item expanded "><a href="../../patterns/ffi/intro.html"><strong aria-hidden="true">3.4.</strong> Foreign function interface (FFI)</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../../patterns/ffi/export.html"><strong aria-hidden="true">3.4.1.</strong> Object-Based APIs</a></li><li class="chapter-item expanded "><a href="../../patterns/ffi/wrappers.html"><strong aria-hidden="true">3.4.2.</strong> Type Consolidation into Wrappers</a></li></ol></li></ol></li><li class="chapter-item expanded "><a href="../../anti_patterns/index.html"><strong aria-hidden="true">4.</strong> Anti-patterns</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../../anti_patterns/borrow_clone.html"><strong aria-hidden="true">4.1.</strong> Clone to satisfy the borrow checker</a></li><li class="chapter-item expanded "><a href="../../anti_patterns/deny-warnings.html"><strong aria-hidden="true">4.2.</strong> #[deny(warnings)]</a></li><li class="chapter-item expanded "><a href="../../anti_patterns/deref.html"><strong aria-hidden="true">4.3.</strong> Deref Polymorphism</a></li></ol></li><li class="chapter-item expanded "><a href="../../functional/index.html"><strong aria-hidden="true">5.</strong> Functional Programming</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../../functional/paradigms.html"><strong aria-hidden="true">5.1.</strong> Programming paradigms</a></li></ol></li><li class="chapter-item expanded "><a href="../../additional_resources/index.html"><strong aria-hidden="true">6.</strong> Additional Resources</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../../additional_resources/design-principles.html"><strong aria-hidden="true">6.1.</strong> Design principles</a></li></ol></li></ol>
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<h1 id="passing-strings"><a class="header" href="#passing-strings">Passing Strings</a></h1>
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<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
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followed:</p>
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<ol>
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<li>Make the lifetime of owned strings as long as possible.</li>
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<li>Minimize <code>unsafe</code> code during the conversion.</li>
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<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
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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>
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types. However, there are different approaches one can take with strings that
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are being sent to a foreign function call from a Rust function.</p>
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<p>The best practice is simple: use <code>CString</code> in such a way as to minimize
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<code>unsafe</code> code. However, a secondary caveat is that
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<em>the object must live long enough</em>, meaning the lifetime should be maximized.
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In addition, the documentation explains that "round-tripping" a <code>CString</code> after
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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 {
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// other module content
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extern "C" {
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fn seterr(message: *const libc::c_char);
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fn geterr(buffer: *mut libc::c_char, size: libc::c_int) -> libc::c_int;
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}
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fn report_error_to_ffi<S: Into<String>>(
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err: S
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) -> Result<(), std::ffi::NulError>{
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let c_err = std::ffi::CString::new(err.into())?;
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unsafe {
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// 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());
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}
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Ok(())
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// The lifetime of c_err continues until here
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}
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fn get_error_from_ffi() -> Result<String, std::ffi::IntoStringError> {
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let mut buffer = vec![0u8; 1024];
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unsafe {
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// SAFETY: calling an FFI whose documentation implies
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// that the input need only live as long as the call
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let written: usize = geterr(buffer.as_mut_ptr(), 1023).into();
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buffer.truncate(written + 1);
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}
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std::ffi::CString::new(buffer).unwrap().into_string()
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}
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}
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</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>
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<ol>
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<li>The <code>unsafe</code> block is as small as possible.</li>
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<li>The <code>CString</code> lives long enough.</li>
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<li>Errors with typecasts are always propagated when possible.</li>
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</ol>
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<p>A common mistake (so common it's in the documentation) is to not use the
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variable in the first block:</p>
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<pre><code class="language-rust ignore">pub mod unsafe_module {
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// other module content
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fn report_error<S: Into<String>>(err: S) -> Result<(), std::ffi::NulError> {
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unsafe {
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// SAFETY: whoops, this contains a dangling pointer!
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seterr(std::ffi::CString::new(err.into())?.as_ptr());
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}
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Ok(())
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}
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}
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</code></pre>
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<p>This code will result in a dangling pointer, because the lifetime of the
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<code>CString</code> is not extended by the pointer creation, unlike if a reference were
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created.</p>
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<p>Another issue frequently raised is that the initialization of a 1k vector of
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zeroes is "slow". However, recent versions of Rust actually optimize that
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particular macro to a call to <code>zmalloc</code>, meaning it is as fast as the operating
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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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