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| <h1 id="a-little-rust-with-your-c"><a class="header" href="#a-little-rust-with-your-c">A little Rust with your C</a></h1> |
| <p>Using Rust code inside a C or C++ project mostly consists of two parts.</p> |
| <ul> |
| <li>Creating a C-friendly API in Rust</li> |
| <li>Embedding your Rust project into an external build system</li> |
| </ul> |
| <p>Apart from <code>cargo</code> and <code>meson</code>, most build systems don't have native Rust support. |
| So you're most likely best off just using <code>cargo</code> for compiling your crate and |
| any dependencies.</p> |
| <h2 id="setting-up-a-project"><a class="header" href="#setting-up-a-project">Setting up a project</a></h2> |
| <p>Create a new <code>cargo</code> project as usual.</p> |
| <p>There are flags to tell <code>cargo</code> to emit a systems library, instead of |
| its regular rust target. |
| This also allows you to set a different output name for your library, |
| if you want it to differ from the rest of your crate.</p> |
| <pre><code class="language-toml">[lib] |
| name = "your_crate" |
| crate-type = ["cdylib"] # Creates dynamic lib |
| # crate-type = ["staticlib"] # Creates static lib |
| </code></pre> |
| <h2 id="building-a-c-api"><a class="header" href="#building-a-c-api">Building a <code>C</code> API</a></h2> |
| <p>Because C++ has no stable ABI for the Rust compiler to target, we use <code>C</code> for |
| any interoperability between different languages. This is no exception when using Rust |
| inside of C and C++ code.</p> |
| <h3 id="no_mangle"><a class="header" href="#no_mangle"><code>#[no_mangle]</code></a></h3> |
| <p>The Rust compiler mangles symbol names differently than native code linkers expect. |
| As such, any function that Rust exports to be used outside of Rust needs to be told |
| not to be mangled by the compiler.</p> |
| <h3 id="extern-c"><a class="header" href="#extern-c"><code>extern "C"</code></a></h3> |
| <p>By default, any function you write in Rust will use the |
| Rust ABI (which is also not stabilized). |
| Instead, when building outwards facing FFI APIs we need to |
| tell the compiler to use the system ABI.</p> |
| <p>Depending on your platform, you might want to target a specific ABI version, which are |
| documented <a href="https://doc.rust-lang.org/reference/items/external-blocks.html">here</a>.</p> |
| <hr /> |
| <p>Putting these parts together, you get a function that looks roughly like this.</p> |
| <pre><code class="language-rust ignore">#[no_mangle] |
| pub extern "C" fn rust_function() { |
| |
| }</code></pre> |
| <p>Just as when using <code>C</code> code in your Rust project you now need to transform data |
| from and to a form that the rest of the application will understand.</p> |
| <h2 id="linking-and-greater-project-context"><a class="header" href="#linking-and-greater-project-context">Linking and greater project context.</a></h2> |
| <p>So then, that's one half of the problem solved. |
| How do you use this now?</p> |
| <p><strong>This very much depends on your project and/or build system</strong></p> |
| <p><code>cargo</code> will create a <code>my_lib.so</code>/<code>my_lib.dll</code> or <code>my_lib.a</code> file, |
| depending on your platform and settings. This library can simply be linked |
| by your build system.</p> |
| <p>However, calling a Rust function from C requires a header file to declare |
| the function signatures.</p> |
| <p>Every function in your Rust-ffi API needs to have a corresponding header function.</p> |
| <pre><code class="language-rust ignore">#[no_mangle] |
| pub extern "C" fn rust_function() {}</code></pre> |
| <p>would then become</p> |
| <pre><code class="language-C">void rust_function(); |
| </code></pre> |
| <p>etc.</p> |
| <p>There is a tool to automate this process, |
| called <a href="https://github.com/eqrion/cbindgen">cbindgen</a> which analyses your Rust code |
| and then generates headers for your C and C++ projects from it.</p> |
| <p>At this point, using the Rust functions from C |
| is as simple as including the header and calling them!</p> |
| <pre><code class="language-C">#include "my-rust-project.h" |
| rust_function(); |
| </code></pre> |
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