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| <ol class="chapter"><li class="chapter-item expanded "><a href="introduction.html"><strong aria-hidden="true">1.</strong> Introduction</a></li><li class="chapter-item expanded "><a href="requirements.html"><strong aria-hidden="true">2.</strong> Requirements</a></li><li class="chapter-item expanded "><a href="library-usage.html"><strong aria-hidden="true">3.</strong> Library Usage with build.rs</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="tutorial-0.html"><strong aria-hidden="true">3.1.</strong> Tutorial</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="tutorial-1.html"><strong aria-hidden="true">3.1.1.</strong> Add bindgen as a Build Dependency</a></li><li class="chapter-item expanded "><a href="tutorial-2.html"><strong aria-hidden="true">3.1.2.</strong> Create a wrapper.h Header</a></li><li class="chapter-item expanded "><a href="tutorial-3.html"><strong aria-hidden="true">3.1.3.</strong> Create a build.rs File</a></li><li class="chapter-item expanded "><a href="tutorial-4.html"><strong aria-hidden="true">3.1.4.</strong> Include the Generated Bindings in src/lib.rs</a></li><li class="chapter-item expanded "><a href="tutorial-5.html"><strong aria-hidden="true">3.1.5.</strong> Write a Sanity Test</a></li><li class="chapter-item expanded "><a href="tutorial-6.html"><strong aria-hidden="true">3.1.6.</strong> Publish Your Crate!</a></li></ol></li><li class="chapter-item expanded "><a href="non-system-libraries.html"><strong aria-hidden="true">3.2.</strong> Bindings for non-system libraries</a></li></ol></li><li class="chapter-item expanded "><a href="command-line-usage.html"><strong aria-hidden="true">4.</strong> Command Line Usage</a></li><li class="chapter-item expanded "><a href="customizing-generated-bindings.html"><strong aria-hidden="true">5.</strong> Customizing the Generated Bindings</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="allowlisting.html"><strong aria-hidden="true">5.1.</strong> Allowlisting</a></li><li class="chapter-item expanded "><a href="blocklisting.html"><strong aria-hidden="true">5.2.</strong> Blocklisting</a></li><li class="chapter-item expanded "><a href="opaque.html"><strong aria-hidden="true">5.3.</strong> Treating a Type as an Opaque Blob of Bytes</a></li><li class="chapter-item expanded "><a href="replacing-types.html"><strong aria-hidden="true">5.4.</strong> Replacing One Type with Another</a></li><li class="chapter-item expanded "><a href="nocopy.html"><strong aria-hidden="true">5.5.</strong> Preventing the Derivation of Copy and Clone</a></li><li class="chapter-item expanded "><a href="nodebug.html"><strong aria-hidden="true">5.6.</strong> Preventing the Derivation of Debug</a></li><li class="chapter-item expanded "><a href="nodefault.html"><strong aria-hidden="true">5.7.</strong> Preventing the Derivation of Default</a></li><li class="chapter-item expanded "><a href="must-use-types.html"><strong aria-hidden="true">5.8.</strong> Annotating types with #[must-use]</a></li><li class="chapter-item expanded "><a href="visibility.html"><strong aria-hidden="true">5.9.</strong> Field visibility</a></li><li class="chapter-item expanded "><a href="code-formatting.html"><strong aria-hidden="true">5.10.</strong> Code formatting</a></li></ol></li><li class="chapter-item expanded "><a href="cpp.html"><strong aria-hidden="true">6.</strong> Generating Bindings to C++</a></li><li class="chapter-item expanded "><a href="objc.html"><strong aria-hidden="true">7.</strong> Generating Bindings to Objective-c</a></li><li class="chapter-item expanded "><a href="using-unions.html" class="active"><strong aria-hidden="true">8.</strong> Using Unions</a></li><li class="chapter-item expanded "><a href="using-bitfields.html"><strong aria-hidden="true">9.</strong> Using Bitfields</a></li><li class="chapter-item expanded "><a href="faq.html"><strong aria-hidden="true">10.</strong> FAQ</a></li></ol> |
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| <h1 class="menu-title">The bindgen User Guide</h1> |
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| <h1><a class="header" href="#using-the-union-types-generated-by-bindgen" id="using-the-union-types-generated-by-bindgen">Using the Union Types Generated by Bindgen</a></h1> |
| <p><strong>NOTE</strong>: Rust 1.19 stabilized the <code>union</code> type (see Rust issue <a href="https://github.com/rust-lang/rust/issues/32836">#32836</a>).</p> |
| <p>You can pass the <code>--rust-target</code> option to tell <code>bindgen</code> to target a specific version of Rust. |
| By default, <code>bindgen</code> will target the latest stable Rust. |
| The <code>--rust-target</code> option accepts a specific stable version (such as "1.0" or "1.19") or "nightly".</p> |
| <p><strong>NOTE</strong>: The <code>--unstable-rust</code> option is deprecated; use <code>--rust-target nightly</code> instead.</p> |
| <p>In general, most interactions with unions (either reading or writing) are unsafe, meaning you must surround union accesses in an <code>unsafe {}</code> block.</p> |
| <p>For this discussion, we will use the following C type definitions:</p> |
| <pre><code class="language-c">typedef struct { |
| int32_t a; |
| int32_t b; |
| } alpha_t; |
| |
| typedef struct { |
| uint32_t c; |
| uint16_t d; |
| uint16_t e; |
| uint8_t f; |
| } beta_t; |
| |
| typedef union { |
| alpha_t alfa; |
| beta_t bravo; |
| } greek_t; |
| </code></pre> |
| <h2><a class="header" href="#relevant-bindgen-options" id="relevant-bindgen-options">Relevant Bindgen Options</a></h2> |
| <h3><a class="header" href="#library" id="library">Library</a></h3> |
| <ul> |
| <li><a href="https://docs.rs/bindgen/latest/bindgen/struct.Builder.html#method.rust_target"><code>bindgen::Builder::rust_target()</code></a> <!-- Update when live --></li> |
| <li><a href="https://docs.rs/bindgen/latest/bindgen/struct.Builder.html#method.derive_default"><code>bindgen::Builder::derive_default()</code></a></li> |
| </ul> |
| <h3><a class="header" href="#command-line" id="command-line">Command Line</a></h3> |
| <ul> |
| <li><code>--rust-target</code></li> |
| <li><code>--with-derive-default</code></li> |
| </ul> |
| <h2><a class="header" href="#which-union-type-will-bindgen-generate" id="which-union-type-will-bindgen-generate">Which union type will Bindgen generate?</a></h2> |
| <p>Bindgen can emit one of two Rust types that correspond to C unions:</p> |
| <ul> |
| <li>Rust's <code>union</code> builtin (only available in Rust >= 1.19, including nightly)</li> |
| <li>Bindgen's <code>BindgenUnion</code> (available for all Rust targets)</li> |
| </ul> |
| <p>Bindgen uses the following logic to determine which Rust union type to emit:</p> |
| <ul> |
| <li>If the Rust target is >= 1.19 (including nightly) AND each field of the union can derive <code>Copy</code>, then generate a <code>union</code> builtin.</li> |
| <li>Otherwise, generate a <code>BindgenUnion</code>.</li> |
| </ul> |
| <h2><a class="header" href="#using-the-union-builtin" id="using-the-union-builtin">Using the <code>union</code> builtin</a></h2> |
| <p>When using the <code>union</code> builtin type, there are two choices for initialization:</p> |
| <ol> |
| <li>Zero</li> |
| <li>With a specific variant</li> |
| </ol> |
| <pre><code class="language-rust ignore">mod bindings_builtin_union; |
| |
| fn union_builtin() { |
| // Initialize the union to zero |
| let x = bindings_builtin_union::greek_t::default(); |
| |
| // If `--with-derive-default` option is not used, the following may be used |
| // to initialize the union to zero: |
| let x = unsafe { std::mem::zeroed::<bindings_builtin_union::greek_t>() }; |
| |
| // Or, it is possible to initialize exactly one variant of the enum: |
| let x = bindings_builtin_union::greek_t { |
| alfa: bindings_builtin_union::alpha_t { |
| a: 1, |
| b: -1, |
| }, |
| }; |
| |
| unsafe { |
| println!("{:?}", z.alfa); // alpha_t { a: 1, b: -1 } |
| println!("{:?}", z.bravo); // beta_t { c: 1, d: 65535, e: 65535, f: 127 } |
| } |
| } |
| </code></pre> |
| <h2><a class="header" href="#using-the-bindgenunion-type" id="using-the-bindgenunion-type">Using the <code>BindgenUnion</code> type</a></h2> |
| <p>If the target Rust version does not support the new <code>union</code> type or there is a field that cannot derive <code>Copy</code>, then bindgen will provide union-like access to a <code>struct</code>.</p> |
| <p>Interacting with these unions is slightly different than the new <code>union</code> types. |
| You must access union variants through a reference.</p> |
| <pre><code class="language-rust ignore">mod bindings; |
| |
| fn bindgenunion() { |
| // `default()` or `zeroed()` may still be used with Bindgen's Union types |
| let mut x = bindings::greek_t::default(); |
| |
| // This will not work: |
| // let x = bindings::greek_t { |
| // alfa: bindings::alpha_t { |
| // a: 1, |
| // b: -1, |
| // }, |
| // }; |
| |
| // Instead, access the field through `.as_ref()` and `.as_mut()` helpers: |
| unsafe { |
| *x.alfa.as_mut() = bindings::alpha_t { |
| a: 1, |
| b: -1, |
| }; |
| |
| println!("{:?}", x.alfa.as_ref()); // alpha_t { a: 1, b: -1 } |
| println!("{:?}", x.bravo.as_ref()); // beta_t { c: 1, d: 65535, e: 65535, f: 0 } |
| } |
| </code></pre> |
| <p>If you attempt to access a <code>BindgenUnion</code> field directly, you will see errors like this:</p> |
| <pre><code class="language-text">error[E0308]: mismatched types |
| --> src/main.rs:44:15 |
| | |
| 44 | alfa: bindings::alpha_t { |
| | _______________^ |
| 45 | | a: 1, |
| 46 | | b: -1, |
| 47 | | }, |
| | |_________^ expected struct `bindings::__BindgenUnionField`, found struct `bindings::alpha_t` |
| | |
| = note: expected type `bindings::__BindgenUnionField<bindings::alpha_t>` |
| found type `bindings::alpha_t` |
| </code></pre> |
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