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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" class="active"><strong aria-hidden="true">2.9.2.</strong> Accepting Strings</a></li><li class="chapter-item expanded "><a href="../../idioms/ffi/passing-strings.html"><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="accepting-strings"><a class="header" href="#accepting-strings">Accepting Strings</a></h1>
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<h2 id="description"><a class="header" href="#description">Description</a></h2>
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<p>When accepting strings via FFI through pointers, there are two principles that
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should be followed:</p>
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<ol>
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<li>Keep foreign strings "borrowed", rather than copying them directly.</li>
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<li>Minimize the amount of complexity and <code>unsafe</code> code involved in converting
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from a C-style string to native Rust strings.</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>The strings used in C have different behaviours to those used in Rust, namely:</p>
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<ul>
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<li>C strings are null-terminated while Rust strings store their length</li>
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<li>C strings can contain any arbitrary non-zero byte while Rust strings must be
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UTF-8</li>
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<li>C strings are accessed and manipulated using <code>unsafe</code> pointer operations
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while interactions with Rust strings go through safe methods</li>
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</ul>
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<p>The Rust standard library comes with C equivalents of Rust's <code>String</code> and <code>&str</code>
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called <code>CString</code> and <code>&CStr</code>, that allow us to avoid a lot of the complexity
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and <code>unsafe</code> code involved in converting between C strings and Rust strings.</p>
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<p>The <code>&CStr</code> type also allows us to work with borrowed data, meaning passing
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strings between Rust and C is a zero-cost operation.</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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/// Log a message at the specified level.
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///
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/// # Safety
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///
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/// It is the caller's guarantee to ensure `msg`:
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///
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/// - is not a null pointer
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/// - points to valid, initialized data
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/// - points to memory ending in a null byte
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/// - won't be mutated for the duration of this function call
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#[no_mangle]
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pub unsafe extern "C" fn mylib_log(
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msg: *const libc::c_char,
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level: libc::c_int
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) {
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let level: crate::LogLevel = match level { /* ... */ };
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// SAFETY: The caller has already guaranteed this is okay (see the
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// `# Safety` section of the doc-comment).
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let msg_str: &str = match std::ffi::CStr::from_ptr(msg).to_str() {
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Ok(s) => s,
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Err(e) => {
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crate::log_error("FFI string conversion failed");
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return;
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}
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};
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crate::log(msg_str, level);
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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 is written 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 pointer with an "untracked" lifetime becomes a "tracked" shared
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reference</li>
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</ol>
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<p>Consider an alternative, where the string is actually copied:</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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pub extern "C" fn mylib_log(msg: *const libc::c_char, level: libc::c_int) {
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// DO NOT USE THIS CODE.
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// IT IS UGLY, VERBOSE, AND CONTAINS A SUBTLE BUG.
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let level: crate::LogLevel = match level { /* ... */ };
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let msg_len = unsafe { /* SAFETY: strlen is what it is, I guess? */
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libc::strlen(msg)
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};
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let mut msg_data = Vec::with_capacity(msg_len + 1);
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let msg_cstr: std::ffi::CString = unsafe {
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// SAFETY: copying from a foreign pointer expected to live
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// for the entire stack frame into owned memory
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std::ptr::copy_nonoverlapping(msg, msg_data.as_mut(), msg_len);
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msg_data.set_len(msg_len + 1);
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std::ffi::CString::from_vec_with_nul(msg_data).unwrap()
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}
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let msg_str: String = unsafe {
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match msg_cstr.into_string() {
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Ok(s) => s,
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Err(e) => {
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crate::log_error("FFI string conversion failed");
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return;
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}
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}
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};
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crate::log(&msg_str, level);
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}
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}
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</code></pre>
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<p>This code in inferior to the original in two respects:</p>
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<ol>
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<li>There is much more <code>unsafe</code> code, and more importantly, more invariants it
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must uphold.</li>
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<li>Due to the extensive arithmetic required, there is a bug in this version
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that cases Rust <code>undefined behaviour</code>.</li>
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</ol>
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<p>The bug here is a simple mistake in pointer arithmetic: the string was copied,
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all <code>msg_len</code> bytes of it. However, the <code>NUL</code> terminator at the end was not.</p>
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<p>The Vector then had its size <em>set</em> to the length of the <em>zero padded string</em> --
|
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rather than <em>resized</em> to it, which could have added a zero at the end.
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As a result, the last byte in the Vector is uninitialized memory.
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When the <code>CString</code> is created at the bottom of the block, its read of the
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Vector will cause <code>undefined behaviour</code>!</p>
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<p>Like many such issues, this would be difficult issue to track down.
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Sometimes it would panic because the string was not <code>UTF-8</code>, sometimes it would
|
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put a weird character at the end of the string, sometimes it would just
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completely crash.</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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