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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" class="active"><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"><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="using-fam.html"><strong aria-hidden="true">10.</strong> Using Flexible Array Members</a></li><li class="chapter-item expanded "><a href="faq.html"><strong aria-hidden="true">11.</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="#generating-bindings-to-c" id="generating-bindings-to-c">Generating Bindings to C++</a></h1> |
| <p><code>bindgen</code> can handle some C++ features, but not all of them. To set |
| expectations: <code>bindgen</code> will give you the type definitions and FFI declarations |
| you need to build an API to the C++ library, but using those types in Rust will |
| be nowhere near as nice as using them in C++. You will have to manually call |
| constructors, destructors, overloaded operators, etc yourself.</p> |
| <p>When passing in header files, the file will automatically be treated as C++ if |
| it ends in <code>.hpp</code>. If it doesn't, adding <code>-x c++</code> clang args can be used to |
| force C++ mode. You probably also want to use <code>-std=c++14</code> or similar clang args |
| as well.</p> |
| <p>You pretty much <strong>must</strong> use <a href="./allowlisting.html">allowlisting</a> when working |
| with C++ to avoid pulling in all of the <code>std::.*</code> types, many of which <code>bindgen</code> |
| cannot handle. Additionally, you may want to mark other types as |
| <a href="./opaque.html">opaque</a> that <code>bindgen</code> stumbles on. It is recommended to mark |
| all of <code>std::.*</code> opaque, and to allowlist only precisely the functions and types |
| you intend to use.</p> |
| <p>You should read up on the <a href="./faq.html">FAQs</a> as well.</p> |
| <h2><a class="header" href="#supported-features" id="supported-features">Supported Features</a></h2> |
| <ul> |
| <li> |
| <p>Inheritance (for the most part; there are |
| <a href="https://github.com/rust-lang/rust-bindgen/issues/380">some outstanding bugs</a>)</p> |
| </li> |
| <li> |
| <p>Methods</p> |
| </li> |
| <li> |
| <p>Bindings to constructors and destructors (but they aren't implicitly or |
| automatically invoked)</p> |
| </li> |
| <li> |
| <p>Function and method overloading</p> |
| </li> |
| <li> |
| <p>Templates <em>without</em> specialization. You should be able to access individual |
| fields of the class or struct.</p> |
| </li> |
| </ul> |
| <h2><a class="header" href="#unsupported-features" id="unsupported-features">Unsupported Features</a></h2> |
| <p>When <code>bindgen</code> finds a type that is too difficult or impossible to translate |
| into Rust, it will automatically treat it as an opaque blob of bytes. The |
| philosophy is that</p> |
| <ol> |
| <li> |
| <p>we should always get layout, size, and alignment correct, and</p> |
| </li> |
| <li> |
| <p>just because one type uses specialization, that shouldn't cause <code>bindgen</code> to |
| give up on everything else.</p> |
| </li> |
| </ol> |
| <p>Without further ado, here are C++ features that <code>bindgen</code> does not support or |
| cannot translate into Rust:</p> |
| <ul> |
| <li> |
| <p>Inline functions and methods: see |
| <a href="./faq.html#why-isnt-bindgen-generating-bindings-to-inline-functions">"Why isn't <code>bindgen</code> generating bindings to inline functions?"</a></p> |
| </li> |
| <li> |
| <p>Template functions, methods of template classes and structs. We don't know |
| which monomorphizations exist, and can't create new ones because we aren't a |
| C++ compiler.</p> |
| </li> |
| <li> |
| <p>Anything related to template specialization:</p> |
| <ul> |
| <li>Partial template specialization</li> |
| <li>Traits templates</li> |
| <li>Substitution Failure Is Not An Error (SFINAE)</li> |
| </ul> |
| </li> |
| <li> |
| <p>Cross language inheritance, for example inheriting from a Rust struct in C++.</p> |
| </li> |
| <li> |
| <p>Automatically calling copy and/or move constructors or destructors. Supporting |
| this isn't possible with Rust's move semantics.</p> |
| </li> |
| <li> |
| <p>Exceptions: if a function called through a <code>bindgen</code>-generated interface |
| raises an exception that is not caught by the function itself, this will |
| generate undefined behaviour. See |
| <a href="https://github.com/rust-lang/rust-bindgen/issues/1208">the tracking issue for exceptions</a> |
| for more details.</p> |
| </li> |
| <li> |
| <p>Many C++ specific aspects of calling conventions. For example in the Itanium abi types that are |
| "<a href="https://itanium-cxx-abi.github.io/cxx-abi/abi.html#non-trivial">non trivial for the purposes of calls</a>" |
| should be passed by pointer, even if they are otherwise eligible to be passed in a register. |
| Similarly in both the Itanium and MSVC ABIs such types are returned by "hidden parameter", much like |
| large structs in C that would not fit into a register. This also applies to types with any base classes |
| in the MSVC ABI (see <a href="https://learn.microsoft.com/en-us/cpp/build/x64-calling-convention?view=msvc-170#return-values">x64 calling convention</a>). |
| Because bindgen does not know about these rules generated interfaces using such types are currently invalid.</p> |
| </li> |
| </ul> |
| <h2><a class="header" href="#constructor-semantics" id="constructor-semantics">Constructor semantics</a></h2> |
| <p><code>bindgen</code> will generate a wrapper for any class constructor declared in the |
| input headers. For example, this headers file</p> |
| <pre><code class="language-c++">class MyClass { |
| public: |
| MyClass(); |
| void method(); |
| }; |
| </code></pre> |
| <p>Will produce the following code:</p> |
| <pre><code class="language-rust ignore">#[repr(C)] |
| #[derive(Debug, Copy, Clone)] |
| pub struct MyClass { |
| pub _address: u8, |
| } |
| extern "C" { |
| #[link_name = "\u{1}_ZN7MyClass6methodEv"] |
| pub fn MyClass_method(this: *mut MyClass); |
| } |
| extern "C" { |
| #[link_name = "\u{1}_ZN7MyClassC1Ev"] |
| pub fn MyClass_MyClass(this: *mut MyClass); |
| } |
| impl MyClass { |
| #[inline] |
| pub unsafe fn method(&mut self) { |
| MyClass_method(self) |
| } |
| #[inline] |
| pub unsafe fn new() -> Self { |
| let mut __bindgen_tmp = ::std::mem::MaybeUninit::uninit(); |
| MyClass_MyClass(__bindgen_tmp.as_mut_ptr()); |
| __bindgen_tmp.assume_init() |
| } |
| } |
| </code></pre> |
| <p>This <code>MyClass::new</code> Rust method can be used as a substitute for the <code>MyClass</code> |
| C++ constructor. However, the address of the value from inside the method will |
| be different than from the outside. This is because the <code>__bindgen_tmp</code> value |
| is moved when the <code>MyClass::new</code> method returns.</p> |
| <p>In contrast, the C++ constructor will not move the value, meaning that the |
| address of the value will be the same inside and outside the constructor. |
| If the original C++ relies on this semantic difference somehow, you should use the |
| <code>MyClass_MyClass</code> binding directly instead of the <code>MyClass::new</code> method.</p> |
| <p>In other words, the Rust equivalent for the following C++ code</p> |
| <pre><code class="language-c++">MyClass instance = MyClass(); |
| instance.method(); |
| </code></pre> |
| <p>is not this</p> |
| <pre><code class="language-rust ignore">let instance = MyClass::new(); |
| instance.method(); |
| </code></pre> |
| <p>but this</p> |
| <pre><code class="language-rust ignore">let instance = std::mem::MaybeUninit::<MyClass>::uninit(); |
| MyClass_MyClass(instance.as_mut_ptr()); |
| instance.assume_init_mut().method(); |
| </code></pre> |
| <p>You can easily verify this fact if you provide a implementation for <code>MyClass</code> |
| and <code>method</code> that prints the <code>this</code> pointer address. However, you can |
| ignore this fact if you know that the original C++ code does not rely on the |
| instance address in its internal logic.</p> |
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