You cannot select more than 25 topics
Topics must start with a letter or number, can include dashes ('-') and can be up to 35 characters long.
326 lines
22 KiB
HTML
326 lines
22 KiB
HTML
<!DOCTYPE HTML>
|
|
<html lang="en" class="sidebar-visible no-js rust">
|
|
<head>
|
|
<!-- Book generated using mdBook -->
|
|
<meta charset="UTF-8">
|
|
<title>Accepting Strings - Rust Design Patterns</title>
|
|
|
|
|
|
<!-- Custom HTML head -->
|
|
|
|
<meta name="description" content="A catalogue of Rust design patterns, anti-patterns and idioms">
|
|
<meta name="viewport" content="width=device-width, initial-scale=1">
|
|
<meta name="theme-color" content="#ffffff" />
|
|
|
|
<link rel="icon" href="../../favicon.svg">
|
|
<link rel="shortcut icon" href="../../favicon.png">
|
|
<link rel="stylesheet" href="../../css/variables.css">
|
|
<link rel="stylesheet" href="../../css/general.css">
|
|
<link rel="stylesheet" href="../../css/chrome.css">
|
|
<link rel="stylesheet" href="../../css/print.css" media="print">
|
|
|
|
<!-- Fonts -->
|
|
<link rel="stylesheet" href="../../FontAwesome/css/font-awesome.css">
|
|
<link rel="stylesheet" href="../../fonts/fonts.css">
|
|
|
|
<!-- Highlight.js Stylesheets -->
|
|
<link rel="stylesheet" href="../../highlight.css">
|
|
<link rel="stylesheet" href="../../tomorrow-night.css">
|
|
<link rel="stylesheet" href="../../ayu-highlight.css">
|
|
|
|
<!-- Custom theme stylesheets -->
|
|
|
|
</head>
|
|
<body>
|
|
<!-- Provide site root to javascript -->
|
|
<script>
|
|
var path_to_root = "../../";
|
|
var default_theme = window.matchMedia("(prefers-color-scheme: dark)").matches ? "navy" : "rust";
|
|
</script>
|
|
|
|
<!-- Work around some values being stored in localStorage wrapped in quotes -->
|
|
<script>
|
|
try {
|
|
var theme = localStorage.getItem('mdbook-theme');
|
|
var sidebar = localStorage.getItem('mdbook-sidebar');
|
|
|
|
if (theme.startsWith('"') && theme.endsWith('"')) {
|
|
localStorage.setItem('mdbook-theme', theme.slice(1, theme.length - 1));
|
|
}
|
|
|
|
if (sidebar.startsWith('"') && sidebar.endsWith('"')) {
|
|
localStorage.setItem('mdbook-sidebar', sidebar.slice(1, sidebar.length - 1));
|
|
}
|
|
} catch (e) { }
|
|
</script>
|
|
|
|
<!-- Set the theme before any content is loaded, prevents flash -->
|
|
<script>
|
|
var theme;
|
|
try { theme = localStorage.getItem('mdbook-theme'); } catch(e) { }
|
|
if (theme === null || theme === undefined) { theme = default_theme; }
|
|
var html = document.querySelector('html');
|
|
html.classList.remove('no-js')
|
|
html.classList.remove('rust')
|
|
html.classList.add(theme);
|
|
html.classList.add('js');
|
|
</script>
|
|
|
|
<!-- Hide / unhide sidebar before it is displayed -->
|
|
<script>
|
|
var html = document.querySelector('html');
|
|
var sidebar = 'hidden';
|
|
if (document.body.clientWidth >= 1080) {
|
|
try { sidebar = localStorage.getItem('mdbook-sidebar'); } catch(e) { }
|
|
sidebar = sidebar || 'visible';
|
|
}
|
|
html.classList.remove('sidebar-visible');
|
|
html.classList.add("sidebar-" + sidebar);
|
|
</script>
|
|
|
|
<nav id="sidebar" class="sidebar" aria-label="Table of contents">
|
|
<div class="sidebar-scrollbox">
|
|
<ol class="chapter"><li class="chapter-item expanded "><a href="../../intro.html"><strong aria-hidden="true">1.</strong> Introduction</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../../translations.html"><strong aria-hidden="true">1.1.</strong> Translations</a></li></ol></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><li class="chapter-item expanded "><a href="../../idioms/return-consumed-arg-on-error.html"><strong aria-hidden="true">2.15.</strong> Return consumed arg on error</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><li class="chapter-item expanded "><a href="../../functional/generics-type-classes.html"><strong aria-hidden="true">5.2.</strong> Generics as Type Classes</a></li><li class="chapter-item expanded "><a href="../../functional/lenses.html"><strong aria-hidden="true">5.3.</strong> Lenses and Prisms</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>
|
|
</div>
|
|
<div id="sidebar-resize-handle" class="sidebar-resize-handle"></div>
|
|
</nav>
|
|
|
|
<div id="page-wrapper" class="page-wrapper">
|
|
|
|
<div class="page">
|
|
<div id="menu-bar-hover-placeholder"></div>
|
|
<div id="menu-bar" class="menu-bar sticky bordered">
|
|
<div class="left-buttons">
|
|
<button id="sidebar-toggle" class="icon-button" type="button" title="Toggle Table of Contents" aria-label="Toggle Table of Contents" aria-controls="sidebar">
|
|
<i class="fa fa-bars"></i>
|
|
</button>
|
|
<button id="theme-toggle" class="icon-button" type="button" title="Change theme" aria-label="Change theme" aria-haspopup="true" aria-expanded="false" aria-controls="theme-list">
|
|
<i class="fa fa-paint-brush"></i>
|
|
</button>
|
|
<ul id="theme-list" class="theme-popup" aria-label="Themes" role="menu">
|
|
<li role="none"><button role="menuitem" class="theme" id="light">Light</button></li>
|
|
<li role="none"><button role="menuitem" class="theme" id="rust">Rust</button></li>
|
|
<li role="none"><button role="menuitem" class="theme" id="coal">Coal</button></li>
|
|
<li role="none"><button role="menuitem" class="theme" id="navy">Navy</button></li>
|
|
<li role="none"><button role="menuitem" class="theme" id="ayu">Ayu</button></li>
|
|
</ul>
|
|
<button id="search-toggle" class="icon-button" type="button" title="Search. (Shortkey: s)" aria-label="Toggle Searchbar" aria-expanded="false" aria-keyshortcuts="S" aria-controls="searchbar">
|
|
<i class="fa fa-search"></i>
|
|
</button>
|
|
</div>
|
|
|
|
<h1 class="menu-title">Rust Design Patterns</h1>
|
|
|
|
<div class="right-buttons">
|
|
<a href="../../print.html" title="Print this book" aria-label="Print this book">
|
|
<i id="print-button" class="fa fa-print"></i>
|
|
</a>
|
|
<a href="https://github.com/rust-unofficial/patterns" title="Git repository" aria-label="Git repository">
|
|
<i id="git-repository-button" class="fa fa-github"></i>
|
|
</a>
|
|
<a href="https://github.com/rust-unofficial/patterns/edit/main/./idioms/ffi/accepting-strings.md" title="Suggest an edit" aria-label="Suggest an edit">
|
|
<i id="git-edit-button" class="fa fa-edit"></i>
|
|
</a>
|
|
|
|
</div>
|
|
</div>
|
|
|
|
<div id="search-wrapper" class="hidden">
|
|
<form id="searchbar-outer" class="searchbar-outer">
|
|
<input type="search" id="searchbar" name="searchbar" placeholder="Search this book ..." aria-controls="searchresults-outer" aria-describedby="searchresults-header">
|
|
</form>
|
|
<div id="searchresults-outer" class="searchresults-outer hidden">
|
|
<div id="searchresults-header" class="searchresults-header"></div>
|
|
<ul id="searchresults">
|
|
</ul>
|
|
</div>
|
|
</div>
|
|
|
|
<!-- Apply ARIA attributes after the sidebar and the sidebar toggle button are added to the DOM -->
|
|
<script>
|
|
document.getElementById('sidebar-toggle').setAttribute('aria-expanded', sidebar === 'visible');
|
|
document.getElementById('sidebar').setAttribute('aria-hidden', sidebar !== 'visible');
|
|
Array.from(document.querySelectorAll('#sidebar a')).forEach(function(link) {
|
|
link.setAttribute('tabIndex', sidebar === 'visible' ? 0 : -1);
|
|
});
|
|
</script>
|
|
|
|
<div id="content" class="content">
|
|
<main>
|
|
<h1 id="accepting-strings"><a class="header" href="#accepting-strings">Accepting Strings</a></h1>
|
|
<h2 id="description"><a class="header" href="#description">Description</a></h2>
|
|
<p>When accepting strings via FFI through pointers, there are two principles that
|
|
should be followed:</p>
|
|
<ol>
|
|
<li>Keep foreign strings "borrowed", rather than copying them directly.</li>
|
|
<li>Minimize the amount of complexity and <code>unsafe</code> code involved in converting
|
|
from a C-style string to native Rust strings.</li>
|
|
</ol>
|
|
<h2 id="motivation"><a class="header" href="#motivation">Motivation</a></h2>
|
|
<p>The strings used in C have different behaviours to those used in Rust, namely:</p>
|
|
<ul>
|
|
<li>C strings are null-terminated while Rust strings store their length</li>
|
|
<li>C strings can contain any arbitrary non-zero byte while Rust strings must be
|
|
UTF-8</li>
|
|
<li>C strings are accessed and manipulated using <code>unsafe</code> pointer operations
|
|
while interactions with Rust strings go through safe methods</li>
|
|
</ul>
|
|
<p>The Rust standard library comes with C equivalents of Rust's <code>String</code> and <code>&str</code>
|
|
called <code>CString</code> and <code>&CStr</code>, that allow us to avoid a lot of the complexity
|
|
and <code>unsafe</code> code involved in converting between C strings and Rust strings.</p>
|
|
<p>The <code>&CStr</code> type also allows us to work with borrowed data, meaning passing
|
|
strings between Rust and C is a zero-cost operation.</p>
|
|
<h2 id="code-example"><a class="header" href="#code-example">Code Example</a></h2>
|
|
<pre><code class="language-rust ignore">pub mod unsafe_module {
|
|
|
|
// other module content
|
|
|
|
/// Log a message at the specified level.
|
|
///
|
|
/// # Safety
|
|
///
|
|
/// It is the caller's guarantee to ensure `msg`:
|
|
///
|
|
/// - is not a null pointer
|
|
/// - points to valid, initialized data
|
|
/// - points to memory ending in a null byte
|
|
/// - won't be mutated for the duration of this function call
|
|
#[no_mangle]
|
|
pub unsafe extern "C" fn mylib_log(
|
|
msg: *const libc::c_char,
|
|
level: libc::c_int
|
|
) {
|
|
let level: crate::LogLevel = match level { /* ... */ };
|
|
|
|
// SAFETY: The caller has already guaranteed this is okay (see the
|
|
// `# Safety` section of the doc-comment).
|
|
let msg_str: &str = match std::ffi::CStr::from_ptr(msg).to_str() {
|
|
Ok(s) => s,
|
|
Err(e) => {
|
|
crate::log_error("FFI string conversion failed");
|
|
return;
|
|
}
|
|
};
|
|
|
|
crate::log(msg_str, level);
|
|
}
|
|
}</code></pre>
|
|
<h2 id="advantages"><a class="header" href="#advantages">Advantages</a></h2>
|
|
<p>The example is is written to ensure that:</p>
|
|
<ol>
|
|
<li>The <code>unsafe</code> block is as small as possible.</li>
|
|
<li>The pointer with an "untracked" lifetime becomes a "tracked" shared
|
|
reference</li>
|
|
</ol>
|
|
<p>Consider an alternative, where the string is actually copied:</p>
|
|
<pre><code class="language-rust ignore">pub mod unsafe_module {
|
|
|
|
// other module content
|
|
|
|
pub extern "C" fn mylib_log(msg: *const libc::c_char, level: libc::c_int) {
|
|
// DO NOT USE THIS CODE.
|
|
// IT IS UGLY, VERBOSE, AND CONTAINS A SUBTLE BUG.
|
|
|
|
let level: crate::LogLevel = match level { /* ... */ };
|
|
|
|
let msg_len = unsafe { /* SAFETY: strlen is what it is, I guess? */
|
|
libc::strlen(msg)
|
|
};
|
|
|
|
let mut msg_data = Vec::with_capacity(msg_len + 1);
|
|
|
|
let msg_cstr: std::ffi::CString = unsafe {
|
|
// SAFETY: copying from a foreign pointer expected to live
|
|
// for the entire stack frame into owned memory
|
|
std::ptr::copy_nonoverlapping(msg, msg_data.as_mut(), msg_len);
|
|
|
|
msg_data.set_len(msg_len + 1);
|
|
|
|
std::ffi::CString::from_vec_with_nul(msg_data).unwrap()
|
|
}
|
|
|
|
let msg_str: String = unsafe {
|
|
match msg_cstr.into_string() {
|
|
Ok(s) => s,
|
|
Err(e) => {
|
|
crate::log_error("FFI string conversion failed");
|
|
return;
|
|
}
|
|
}
|
|
};
|
|
|
|
crate::log(&msg_str, level);
|
|
}
|
|
}</code></pre>
|
|
<p>This code in inferior to the original in two respects:</p>
|
|
<ol>
|
|
<li>There is much more <code>unsafe</code> code, and more importantly, more invariants it
|
|
must uphold.</li>
|
|
<li>Due to the extensive arithmetic required, there is a bug in this version
|
|
that cases Rust <code>undefined behaviour</code>.</li>
|
|
</ol>
|
|
<p>The bug here is a simple mistake in pointer arithmetic: the string was copied,
|
|
all <code>msg_len</code> bytes of it. However, the <code>NUL</code> terminator at the end was not.</p>
|
|
<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.
|
|
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>
|
|
<p>Like many such issues, this would be difficult issue to track down.
|
|
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>
|
|
<h2 id="disadvantages"><a class="header" href="#disadvantages">Disadvantages</a></h2>
|
|
<p>None?</p>
|
|
|
|
</main>
|
|
|
|
<nav class="nav-wrapper" aria-label="Page navigation">
|
|
<!-- Mobile navigation buttons -->
|
|
<a rel="prev" href="../../idioms/ffi/errors.html" class="mobile-nav-chapters previous" title="Previous chapter" aria-label="Previous chapter" aria-keyshortcuts="Left">
|
|
<i class="fa fa-angle-left"></i>
|
|
</a>
|
|
|
|
<a rel="next" href="../../idioms/ffi/passing-strings.html" class="mobile-nav-chapters next" title="Next chapter" aria-label="Next chapter" aria-keyshortcuts="Right">
|
|
<i class="fa fa-angle-right"></i>
|
|
</a>
|
|
|
|
<div style="clear: both"></div>
|
|
</nav>
|
|
</div>
|
|
</div>
|
|
|
|
<nav class="nav-wide-wrapper" aria-label="Page navigation">
|
|
<a rel="prev" href="../../idioms/ffi/errors.html" class="nav-chapters previous" title="Previous chapter" aria-label="Previous chapter" aria-keyshortcuts="Left">
|
|
<i class="fa fa-angle-left"></i>
|
|
</a>
|
|
|
|
<a rel="next" href="../../idioms/ffi/passing-strings.html" class="nav-chapters next" title="Next chapter" aria-label="Next chapter" aria-keyshortcuts="Right">
|
|
<i class="fa fa-angle-right"></i>
|
|
</a>
|
|
</nav>
|
|
|
|
</div>
|
|
|
|
|
|
|
|
|
|
<script>
|
|
window.playground_copyable = true;
|
|
</script>
|
|
|
|
|
|
<script src="../../elasticlunr.min.js"></script>
|
|
<script src="../../mark.min.js"></script>
|
|
<script src="../../searcher.js"></script>
|
|
|
|
<script src="../../clipboard.min.js"></script>
|
|
<script src="../../highlight.js"></script>
|
|
<script src="../../book.js"></script>
|
|
|
|
<!-- Custom JS scripts -->
|
|
|
|
|
|
</body>
|
|
</html>
|