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| <h1 id="a-little-c-with-your-rust"><a class="header" href="#a-little-c-with-your-rust">A little C with your Rust</a></h1> |
| <p>Using C or C++ inside of a Rust project consists of two major parts:</p> |
| <ul> |
| <li>Wrapping the exposed C API for use with Rust</li> |
| <li>Building your C or C++ code to be integrated with the Rust code</li> |
| </ul> |
| <p>As C++ does not have a stable ABI for the Rust compiler to target, it is recommended to use the <code>C</code> ABI when combining Rust with C or C++.</p> |
| <h2 id="defining-the-interface"><a class="header" href="#defining-the-interface">Defining the interface</a></h2> |
| <p>Before consuming C or C++ code from Rust, it is necessary to define (in Rust) what data types and function signatures exist in the linked code. In C or C++, you would include a header (<code>.h</code> or <code>.hpp</code>) file which defines this data. In Rust, it is necessary to either manually translate these definitions to Rust, or use a tool to generate these definitions.</p> |
| <p>First, we will cover manually translating these definitions from C/C++ to Rust.</p> |
| <h3 id="wrapping-c-functions-and-datatypes"><a class="header" href="#wrapping-c-functions-and-datatypes">Wrapping C functions and Datatypes</a></h3> |
| <p>Typically, libraries written in C or C++ will provide a header file defining all types and functions used in public interfaces. An example file may look like this:</p> |
| <pre><code class="language-C">/* File: cool.h */ |
| typedef struct CoolStruct { |
| int x; |
| int y; |
| } CoolStruct; |
| |
| void cool_function(int i, char c, CoolStruct* cs); |
| </code></pre> |
| <p>When translated to Rust, this interface would look as such:</p> |
| <pre><code class="language-rust ignore">/* File: cool_bindings.rs */ |
| #[repr(C)] |
| pub struct CoolStruct { |
| pub x: cty::c_int, |
| pub y: cty::c_int, |
| } |
| |
| extern "C" { |
| pub fn cool_function( |
| i: cty::c_int, |
| c: cty::c_char, |
| cs: *mut CoolStruct |
| ); |
| }</code></pre> |
| <p>Let's take a look at this definition one piece at a time, to explain each of the parts.</p> |
| <pre><code class="language-rust ignore">#[repr(C)] |
| pub struct CoolStruct { ... }</code></pre> |
| <p>By default, Rust does not guarantee order, padding, or the size of data included in a <code>struct</code>. In order to guarantee compatibility with C code, we include the <code>#[repr(C)]</code> attribute, which instructs the Rust compiler to always use the same rules C does for organizing data within a struct.</p> |
| <pre><code class="language-rust ignore">pub x: cty::c_int, |
| pub y: cty::c_int,</code></pre> |
| <p>Due to the flexibility of how C or C++ defines an <code>int</code> or <code>char</code>, it is recommended to use primitive data types defined in <code>cty</code>, which will map types from C to types in Rust.</p> |
| <pre><code class="language-rust ignore">extern "C" { pub fn cool_function( ... ); }</code></pre> |
| <p>This statement defines the signature of a function that uses the C ABI, called <code>cool_function</code>. By defining the signature without defining the body of the function, the definition of this function will need to be provided elsewhere, or linked into the final library or binary from a static library.</p> |
| <pre><code class="language-rust ignore"> i: cty::c_int, |
| c: cty::c_char, |
| cs: *mut CoolStruct</code></pre> |
| <p>Similar to our datatype above, we define the datatypes of the function arguments using C-compatible definitions. We also retain the same argument names, for clarity.</p> |
| <p>We have one new type here, <code>*mut CoolStruct</code>. As C does not have a concept of Rust's references, which would look like this: <code>&mut CoolStruct</code>, we instead have a raw pointer. As dereferencing this pointer is <code>unsafe</code>, and the pointer may in fact be a <code>null</code> pointer, care must be taken to ensure the guarantees typical of Rust when interacting with C or C++ code.</p> |
| <h3 id="automatically-generating-the-interface"><a class="header" href="#automatically-generating-the-interface">Automatically generating the interface</a></h3> |
| <p>Rather than manually generating these interfaces, which may be tedious and error prone, there is a tool called <a href="https://github.com/rust-lang/rust-bindgen">bindgen</a> which will perform these conversions automatically. For instructions of the usage of <a href="https://github.com/rust-lang/rust-bindgen">bindgen</a>, please refer to the <a href="https://rust-lang.github.io/rust-bindgen/">bindgen user's manual</a>, however the typical process consists of the following:</p> |
| <ol> |
| <li>Gather all C or C++ headers defining interfaces or datatypes you would like to use with Rust.</li> |
| <li>Write a <code>bindings.h</code> file, which <code>#include "..."</code>'s each of the files you gathered in step one.</li> |
| <li>Feed this <code>bindings.h</code> file, along with any compilation flags used to compile |
| your code into <code>bindgen</code>. Tip: use <code>Builder.ctypes_prefix("cty")</code> / |
| <code>--ctypes-prefix=cty</code> and <code>Builder.use_core()</code> / <code>--use-core</code> to make the generated code <code>#![no_std]</code> compatible.</li> |
| <li><code>bindgen</code> will produce the generated Rust code to the output of the terminal window. This output may be piped to a file in your project, such as <code>bindings.rs</code>. You may use this file in your Rust project to interact with C/C++ code compiled and linked as an external library. Tip: don't forget to use the <a href="https://crates.io/crates/cty"><code>cty</code></a> crate if your types in the generated bindings are prefixed with <code>cty</code>.</li> |
| </ol> |
| <h2 id="building-your-cc-code"><a class="header" href="#building-your-cc-code">Building your C/C++ code</a></h2> |
| <p>As the Rust compiler does not directly know how to compile C or C++ code (or code from any other language, which presents a C interface), it is necessary to compile your non-Rust code ahead of time.</p> |
| <p>For embedded projects, this most commonly means compiling the C/C++ code to a static archive (such as <code>cool-library.a</code>), which can then be combined with your Rust code at the final linking step.</p> |
| <p>If the library you would like to use is already distributed as a static archive, it is not necessary to rebuild your code. Just convert the provided interface header file as described above, and include the static archive at compile/link time.</p> |
| <p>If your code exists as a source project, it will be necessary to compile your C/C++ code to a static library, either by triggering your existing build system (such as <code>make</code>, <code>CMake</code>, etc.), or by porting the necessary compilation steps to use a tool called the <code>cc</code> crate. For both of these steps, it is necessary to use a <code>build.rs</code> script.</p> |
| <h3 id="rust-buildrs-build-scripts"><a class="header" href="#rust-buildrs-build-scripts">Rust <code>build.rs</code> build scripts</a></h3> |
| <p>A <code>build.rs</code> script is a file written in Rust syntax, that is executed on your compilation machine, AFTER dependencies of your project have been built, but BEFORE your project is built.</p> |
| <p>The full reference may be found <a href="https://doc.rust-lang.org/cargo/reference/build-scripts.html">here</a>. <code>build.rs</code> scripts are useful for generating code (such as via <a href="https://github.com/rust-lang/rust-bindgen">bindgen</a>), calling out to external build systems such as <code>Make</code>, or directly compiling C/C++ through use of the <code>cc</code> crate.</p> |
| <h3 id="triggering-external-build-systems"><a class="header" href="#triggering-external-build-systems">Triggering external build systems</a></h3> |
| <p>For projects with complex external projects or build systems, it may be easiest to use <a href="https://doc.rust-lang.org/std/process/struct.Command.html"><code>std::process::Command</code></a> to "shell out" to your other build systems by traversing relative paths, calling a fixed command (such as <code>make library</code>), and then copying the resulting static library to the proper location in the <code>target</code> build directory.</p> |
| <p>While your crate may be targeting a <code>no_std</code> embedded platform, your <code>build.rs</code> executes only on machines compiling your crate. This means you may use any Rust crates which will run on your compilation host.</p> |
| <h3 id="building-cc-code-with-the-cc-crate"><a class="header" href="#building-cc-code-with-the-cc-crate">Building C/C++ code with the <code>cc</code> crate</a></h3> |
| <p>For projects with limited dependencies or complexity, or for projects where it is difficult to modify the build system to produce a static library (rather than a final binary or executable), it may be easier to instead utilize the <a href="https://github.com/alexcrichton/cc-rs"><code>cc</code> crate</a>, which provides an idiomatic Rust interface to the compiler provided by the host.</p> |
| <p>In the simplest case of compiling a single C file as a dependency to a static library, an example <code>build.rs</code> script using the <a href="https://github.com/alexcrichton/cc-rs"><code>cc</code> crate</a> would look like this:</p> |
| <pre><code class="language-rust ignore">fn main() { |
| cc::Build::new() |
| .file("src/foo.c") |
| .compile("foo"); |
| }</code></pre> |
| <p>The <code>build.rs</code> is placed at the root of the package. Then <code>cargo build</code> will compile and execute it before the build of the package. A static archive named <code>libfoo.a</code> is generated and placed in the <code>target</code> directory.</p> |
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