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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="../CONTRIBUTING.html"><strong aria-hidden="true">2.</strong> Contributing</a></li><li class="chapter-item expanded "><a href="../idioms/index.html"><strong aria-hidden="true">3.</strong> Idioms</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../idioms/coercion-arguments.html"><strong aria-hidden="true">3.1.</strong> Use borrowed types for arguments</a></li><li class="chapter-item expanded "><a href="../idioms/concat-format.html"><strong aria-hidden="true">3.2.</strong> Concatenating Strings with format!</a></li><li class="chapter-item expanded "><a href="../idioms/ctor.html"><strong aria-hidden="true">3.3.</strong> Constructor</a></li><li class="chapter-item expanded "><a href="../idioms/default.html"><strong aria-hidden="true">3.4.</strong> The Default Trait</a></li><li 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class="active"><strong aria-hidden="true">3.9.2.</strong> Accepting Strings</a></li><li class="chapter-item expanded "><a href="../idioms/ffi-passing-strings.html"><strong aria-hidden="true">3.9.3.</strong> Passing Strings</a></li></ol></li><li class="chapter-item expanded "><a href="../idioms/option-iter.html"><strong aria-hidden="true">3.10.</strong> Iterating over an Option</a></li><li class="chapter-item expanded "><a href="../idioms/pass-var-to-closure.html"><strong aria-hidden="true">3.11.</strong> Pass Variables to Closure</a></li><li class="chapter-item expanded "><a href="../idioms/priv-extend.html"><strong aria-hidden="true">3.12.</strong> Privacy For Extensibility</a></li><li class="chapter-item expanded "><a href="../idioms/rustdoc-init.html"><strong aria-hidden="true">3.13.</strong> Easy doc initialization</a></li><li class="chapter-item expanded "><a href="../idioms/temporary-mutability.html"><strong aria-hidden="true">3.14.</strong> Temporary mutability</a></li></ol></li><li class="chapter-item expanded "><a href="../patterns/index.html"><strong aria-hidden="true">4.</strong> Design Patterns</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../patterns/builder.html"><strong aria-hidden="true">4.1.</strong> Builder</a></li><li class="chapter-item expanded "><a href="../patterns/compose-structs.html"><strong aria-hidden="true">4.2.</strong> Compose Structs</a></li><li class="chapter-item expanded "><a href="../patterns/entry.html"><strong aria-hidden="true">4.3.</strong> Entry API</a></li><li class="chapter-item expanded "><a href="../patterns/ffi-intro.html"><strong aria-hidden="true">4.4.</strong> Foreign function interface usage</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../patterns/ffi-export.html"><strong aria-hidden="true">4.4.1.</strong> Object-Based APIs</a></li><li class="chapter-item expanded "><a href="../patterns/ffi-wrappers.html"><strong aria-hidden="true">4.4.2.</strong> Type Consolidation into Wrappers</a></li></ol></li><li class="chapter-item expanded "><a href="../patterns/fold.html"><strong aria-hidden="true">4.5.</strong> Fold</a></li><li class="chapter-item expanded "><a href="../patterns/newtype.html"><strong aria-hidden="true">4.6.</strong> Newtype</a></li><li class="chapter-item expanded "><a href="../patterns/RAII.html"><strong aria-hidden="true">4.7.</strong> RAII Guards</a></li><li class="chapter-item expanded "><a href="../patterns/small-crates.html"><strong aria-hidden="true">4.8.</strong> Prefer Small Crates</a></li><li class="chapter-item expanded "><a href="../patterns/unsafe-mods.html"><strong aria-hidden="true">4.9.</strong> Contain unsafety in small modules</a></li><li class="chapter-item expanded "><a href="../patterns/visitor.html"><strong aria-hidden="true">4.10.</strong> Visitor</a></li></ol></li><li class="chapter-item expanded "><a href="../anti_patterns/index.html"><strong aria-hidden="true">5.</strong> Anti-patterns</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../anti_patterns/deny-warnings.html"><strong aria-hidden="true">5.1.</strong> #[deny(warnings)]</a></li><li class="chapter-item expanded "><a href="../anti_patterns/deref.html"><strong aria-hidden="true">5.2.</strong> Deref Polymorphism</a></li></ol></li><li class="chapter-item expanded "><a href="../functional/index.html"><strong aria-hidden="true">6.</strong> Functional Programming</a></li><li class="chapter-item expanded "><a href="../additional_resources.html"><strong aria-hidden="true">7.</strong> Additional Resources</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../design-principles.html"><strong aria-hidden="true">7.1.</strong> Design principles</a></li></ol></li></ol>
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<main>
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<h1><a class="header" href="#accepting-strings" id="accepting-strings">Accepting Strings</a></h1>
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<h2><a class="header" href="#description" id="description">Description</a></h2>
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<p>When accepting strings via FFI through pointers, there are two principles that 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 <code>unsafe</code> code during the conversion.</li>
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</ol>
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<h2><a class="header" href="#motivation" id="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.
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However, there are different approaches one can take with strings that are being accepted from a foreign caller of a Rust function.</p>
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<p>The best practice is simple: use <code>CStr</code> in such a way as to minimize unsafe code, and create a borrowed slice.
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If an owned String is needed, call <code>to_string()</code> on the string slice.</p>
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<h2><a class="header" href="#code-example" id="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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#[no_mangle]
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pub extern "C" fn mylib_log(msg: *const libc::c_char, level: libc::c_int) {
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let level: crate::LogLevel = match level { /* ... */ };
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let msg_str: &str = unsafe {
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// SAFETY: accessing raw pointers expected to live for the call,
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// and creating a shared reference that does not outlive the current
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// stack frame.
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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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};
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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><a class="header" href="#advantages" id="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 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. 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 must uphold.</li>
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<li>Due to the extensive arithmetic required, there is a bug in this version 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, all <code>msg_len</code> bytes of it.
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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> -- rather than <em>resized</em> to it, which could have added a zero at the end. 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 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 put a weird character at the end of the string, sometimes it would just completely crash.</p>
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<h2><a class="header" href="#disadvantages" id="disadvantages">Disadvantages</a></h2>
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<p>None?</p>
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