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<!doctype html><html lang=en dir=ltr class="blog-wrapper blog-post-page plugin-blog plugin-id-default" data-has-hydrated=false><meta charset=UTF-8><meta name=generator content="Docusaurus v3.6.0"><title data-rh=true>Captain's Cookbook: Project setup | The Old Speice Guy</title><meta data-rh=true name=viewport content="width=device-width,initial-scale=1.0"><meta data-rh=true name=twitter:card content=summary_large_image><meta data-rh=true property=og:url content=https://speice.io/2018/01/captains-cookbook-part-1><meta data-rh=true property=og:locale content=en><meta data-rh=true name=docusaurus_locale content=en><meta data-rh=true name=docusaurus_tag content=default><meta data-rh=true name=docsearch:language content=en><meta data-rh=true name=docsearch:docusaurus_tag content=default><meta data-rh=true property=og:title content="Captain's Cookbook: Project setup | The Old Speice Guy"><meta data-rh=true name=description content="A basic introduction to getting started with Cap'N Proto."><meta data-rh=true property=og:description content="A basic introduction to getting started with Cap'N Proto."><meta data-rh=true property=og:type content=article><meta data-rh=true property=article:published_time content=2018-01-16T12:00:00.000Z><link data-rh=true rel=icon href=/img/favicon.ico><link data-rh=true rel=canonical href=https://speice.io/2018/01/captains-cookbook-part-1><link data-rh=true rel=alternate href=https://speice.io/2018/01/captains-cookbook-part-1 hreflang=en><link data-rh=true rel=alternate href=https://speice.io/2018/01/captains-cookbook-part-1 hreflang=x-default><script data-rh=true type=application/ld+json>{"@context":"https://schema.org","@id":"https://speice.io/2018/01/captains-cookbook-part-1","@type":"BlogPosting","author":{"@type":"Person","name":"Bradlee Speice"},"dateModified":"2024-11-10T01:23:31.000Z","datePublished":"2018-01-16T12:00:00.000Z","description":"A basic introduction to getting started with Cap'N Proto.","headline":"Captain's Cookbook: Project setup","isPartOf":{"@id":"https://speice.io/","@type":"Blog","name":"Blog"},"keywords":[],"mainEntityOfPage":"https://speice.io/2018/01/captains-cookbook-part-1","name":"Captain's Cookbook: Project setup","url":"https://speice.io/2018/01/captains-cookbook-part-1"}</script><link rel=alternate type=application/rss+xml href=/rss.xml title="The Old Speice Guy RSS Feed"><link rel=alternate type=application/atom+xml href=/atom.xml title="The Old Speice Guy Atom Feed"><link rel=stylesheet href=https://cdn.jsdelivr.net/npm/katex@0.13.24/dist/katex.min.css integrity=sha384-odtC+0UGzzFL/6PNoE8rX/SPcQDXBJ+uRepguP4QkPCm2LBxH3FA3y+fKSiJ+AmM crossorigin><link rel=stylesheet href=/assets/css/styles.ae6ff4a3.css><script src=/assets/js/runtime~main.751b419d.js defer></script><script src=/assets/js/main.62ce6156.js defer></script><body class=navigation-with-keyboard><script>!function(){var t,e=function(){try{return new URLSearchParams(window.location.search).get("docusaurus-theme")}catch(t){}}()||function(){try{return window.localStorage.getItem("theme")}catch(t){}}();t=null!==e?e:"light",document.documentElement.setAttribute("data-theme",t)}(),function(){try{for(var[t,e]of new URLSearchParams(window.location.search).entries())if(t.startsWith("docusaurus-data-")){var a=t.replace("docusaurus-data-","data-");document.documentElement.setAttribute(a,e)}}catch(t){}}()</script><div id=__docusaurus><div role=region aria-label="Skip to main content"><a class=skipToContent_fXgn href=#__docusaurus_skipToContent_fallback>Skip to main content</a></div><nav aria-label=Main class="navbar navbar--fixed-top"><div class=navbar__inner><div class=navbar__items><button aria-label="Toggle navigation bar" aria-expanded=false class="navbar__toggle clean-btn" type=button><svg width=30 height=30 viewBox="0 0 30 30" aria-hidden=true><path stroke=currentColor stroke-linecap=round stroke-miterlimit=10 stroke-width=2 d="M4 7h22M4 15h22M4 23h22"/></svg></button><a class=navbar__brand href=/><div class=navbar__logo><img src=/img/logo.svg alt="Sierpinski Gasket" class="themedComponent_mlkZ themedCompone
<p>I've been working a lot with <a href=https://capnproto.org/ target=_blank rel="noopener noreferrer">Cap'N Proto</a> recently with Rust, but there's a real dearth of information
on how to set up and get going quickly. In the interest of trying to get more people using this (because I think it's
fantastic), I'm going to work through a couple of examples detailing what exactly should be done to get going.</p>
<p>So, what is Cap'N Proto? It's a data serialization library. It has contemporaries with <a href=https://developers.google.com/protocol-buffers/ target=_blank rel="noopener noreferrer">Protobuf</a>
and <a href=https://google.github.io/flatbuffers/ target=_blank rel="noopener noreferrer">FlatBuffers</a>, but is better compared with FlatBuffers. The whole point behind it
is to define a schema language and serialization format such that:</p>
<ol>
<li>Applications that do not share the same base programming language can communicate</li>
<li>The data and schema you use can naturally evolve over time as your needs change</li>
</ol>
<p>Accompanying this are typically code generators that take the schemas you define for your application and give you back
code for different languages to get data to and from that schema.</p>
<p>Now, what makes Cap'N Proto different from, say, Protobuf, is that there is no serialization/deserialization step the same way
as is implemented with Protobuf. Instead, the idea is that the message itself can be loaded in memory and used directly there.</p>
<p>We're going to take a look at a series of progressively more complex projects that use Cap'N Proto in an effort to provide some
examples of what idiomatic usage looks like, and shorten the startup time needed to make use of this library in Rust projects.
If you want to follow along, feel free. If not, I've posted <a href=https://github.com/bspeice/capnp_cookbook_1 target=_blank rel="noopener noreferrer">the final result</a>
for reference.</p>
<h2 class="anchor anchorWithStickyNavbar_LWe7" id=step-1-installing-capnp>Step 1: Installing <code>capnp</code><a href=#step-1-installing-capnp class=hash-link aria-label="Direct link to step-1-installing-capnp" title="Direct link to step-1-installing-capnp">​</a></h2>
<p>The <code>capnp</code> binary itself is needed for taking the schema files you write and turning them into a format that can be used by the
code generation libraries. Don't ask me what that actually means, I just know that you need to make sure this is installed.</p>
<p>I'll refer you to <a href=https://capnproto.org/install.html target=_blank rel="noopener noreferrer">Cap'N Proto's installation instructions</a> here. As a quick TLDR though:</p>
<ul>
<li>Linux users will likely have a binary shipped by their package manager - On Ubuntu, <code>apt install capnproto</code> is enough</li>
<li>OS X users can use <a href=https://brew.sh/ target=_blank rel="noopener noreferrer">Homebrew</a> as an easy install path. Just <code>brew install capnp</code></li>
<li>Windows users are a bit more complicated. If you're using <a href=https://chocolatey.org/ target=_blank rel="noopener noreferrer">Chocolatey</a>, there's <a href=https://chocolatey.org/packages/capnproto/ target=_blank rel="noopener noreferrer">a package</a> available. If that doesn't work however, you need to download <a href=https://capnproto.org/capnproto-c++-win32-0.6.1.zip target=_blank rel="noopener noreferrer">a release zip</a> and make sure that the <code>capnp.exe</code> binary is in your <code>%PATH%</code> environment variable</li>
</ul>
<p>The way you know you're done with this step is if the following command works in your shell:</p>
<div class="language-bash codeBlockContainer_Ckt0 theme-code-block" style="--prism-background-color:hsl(230, 1%, 98%);--prism-color:hsl(230, 8%, 24%)"><div class=codeBlockContent_biex><pre tabindex=0 class="prism-code language-bash codeBlock_bY9V thin-scrollbar" style="background-color:hsl(230, 1%, 98%);color:hsl(230, 8%, 24%)"><code class=codeBlockLines_e6Vv><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain">capnp </span><span class="token function" style="color:hsl(221, 87%, 60%)">id</span><br></span></code></pre><div class=buttonGroup__atx><button type=button aria-label="Copy code to clipboard" title=Copy class=clean-btn><span class=copyButtonIcons_eSgA aria-hidden=true><svg viewBox="0 0 24 24" class=copyButtonIcon_y97N><path fill=currentColor d="M19,21H8V7H19M19,5H8A2,2 0 0,0 6,7V21A2,2 0 0,0 8,23H19A2,2 0 0,0 21,21V7A2,2 0 0,0 19,5M16,1H4A2,2 0 0,0 2,3V17H4V3H16V1Z"/></svg><svg viewBox="0 0 24 24" class=copyButtonSuccessIcon_LjdS><path fill=currentColor d=M21,7L9,19L3.5,13.5L4.91,12.09L9,16.17L19.59,5.59L21,7Z /></svg></span></button></div></div></div>
<h2 class="anchor anchorWithStickyNavbar_LWe7" id=step-2-starting-a-capn-proto-rust-project>Step 2: Starting a Cap'N Proto Rust project<a href=#step-2-starting-a-capn-proto-rust-project class=hash-link aria-label="Direct link to Step 2: Starting a Cap'N Proto Rust project" title="Direct link to Step 2: Starting a Cap'N Proto Rust project">​</a></h2>
<p>After the <code>capnp</code> binary is set up, it's time to actually create our Rust project. Nothing terribly complex here, just a simple</p>
<div class="language-bash codeBlockContainer_Ckt0 theme-code-block" style="--prism-background-color:hsl(230, 1%, 98%);--prism-color:hsl(230, 8%, 24%)"><div class=codeBlockContent_biex><pre tabindex=0 class="prism-code language-bash codeBlock_bY9V thin-scrollbar" style="background-color:hsl(230, 1%, 98%);color:hsl(230, 8%, 24%)"><code class=codeBlockLines_e6Vv><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token function" style="color:hsl(221, 87%, 60%)">mkdir</span><span class="token plain"> capnp_cookbook_1</span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain"></span><span class="token builtin class-name" style="color:hsl(35, 99%, 36%)">cd</span><span class="token plain"> capnp_cookbook_1</span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain"></span><span class="token function" style="color:hsl(221, 87%, 60%)">cargo</span><span class="token plain"> init </span><span class="token parameter variable" style="color:hsl(221, 87%, 60%)">--bin</span><br></span></code></pre><div class=buttonGroup__atx><button type=button aria-label="Copy code to clipboard" title=Copy class=clean-btn><span class=copyButtonIcons_eSgA aria-hidden=true><svg viewBox="0 0 24 24" class=copyButtonIcon_y97N><path fill=currentColor d="M19,21H8V7H19M19,5H8A2,2 0 0,0 6,7V21A2,2 0 0,0 8,23H19A2,2 0 0,0 21,21V7A2,2 0 0,0 19,5M16,1H4A2,2 0 0,0 2,3V17H4V3H16V1Z"/></svg><svg viewBox="0 0 24 24" class=copyButtonSuccessIcon_LjdS><path fill=currentColor d=M21,7L9,19L3.5,13.5L4.91,12.09L9,16.17L19.59,5.59L21,7Z /></svg></span></button></div></div></div>
<p>We'll put the following content into <code>Cargo.toml</code>:</p>
<div class="codeBlockContainer_Ckt0 theme-code-block" style="--prism-background-color:hsl(230, 1%, 98%);--prism-color:hsl(230, 8%, 24%)"><div class=codeBlockContent_biex><pre tabindex=0 class="prism-code language-text codeBlock_bY9V thin-scrollbar" style="background-color:hsl(230, 1%, 98%);color:hsl(230, 8%, 24%)"><code class=codeBlockLines_e6Vv><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain">[package]</span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain">name = "capnp_cookbook_1"</span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain">version = "0.1.0"</span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain">authors = ["Bradlee Speice &lt;bspeice@kcg.com>"]</span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain" style=display:inline-block></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain">[build-dependencies]</span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain">capnpc = "0.8" # 1</span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain" style=display:inline-block></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain">[dependencies]</span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain">capnp = "0.8" # 2</span><br></span></code></pre><div class=buttonGroup__atx><button type=button aria-label="Copy code to clipboard" title=Copy class=clean-btn><span class=copyButtonIcons_eSgA aria-hidden=true><svg viewBox="0 0 24 24" class=copyButtonIcon_y97N><path fill=currentColor d="M19,21H8V7H19M19,5H8A2,2 0 0,0 6,7V21A2,2 0 0,0 8,23H19A2,2 0 0,0 21,21V7A2,2 0 0,0 19,5M16,1H4A2,2 0 0,0 2,3V17H4V3H16V1Z"/></svg><svg viewBox="0 0 24 24" class=copyButtonSuccessIcon_LjdS><path fill=currentColor d=M21,7L9,19L3.5,13.5L4.91,12.09L9,16.17L19.59,5.59L21,7Z /></svg></span></button></div></div></div>
<p>This sets up:</p>
<ol>
<li>The Rust code generator (CAPNProto Compiler)</li>
<li>The Cap'N Proto runtime library (CAPNProto runtime)</li>
</ol>
<p>We've now got everything prepared that we need for writing a Cap'N Proto project.</p>
<h2 class="anchor anchorWithStickyNavbar_LWe7" id=step-3-writing-a-basic-schema>Step 3: Writing a basic schema<a href=#step-3-writing-a-basic-schema class=hash-link aria-label="Direct link to Step 3: Writing a basic schema" title="Direct link to Step 3: Writing a basic schema">​</a></h2>
<p>We're going to start with writing a pretty trivial data schema that we can extend later. This is just intended to make sure
you get familiar with how to start from a basic project.</p>
<p>First, we're going to create a top-level directory for storing the schema files in:</p>
<div class="language-bash codeBlockContainer_Ckt0 theme-code-block" style="--prism-background-color:hsl(230, 1%, 98%);--prism-color:hsl(230, 8%, 24%)"><div class=codeBlockContent_biex><pre tabindex=0 class="prism-code language-bash codeBlock_bY9V thin-scrollbar" style="background-color:hsl(230, 1%, 98%);color:hsl(230, 8%, 24%)"><code class=codeBlockLines_e6Vv><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token comment" style="color:hsl(230, 4%, 64%)"># Assuming we're starting from the `capnp_cookbook_1` directory created earlier</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain" style=display:inline-block></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain"></span><span class="token function" style="color:hsl(221, 87%, 60%)">mkdir</span><span class="token plain"> schema</span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain"></span><span class="token builtin class-name" style="color:hsl(35, 99%, 36%)">cd</span><span class="token plain"> schema</span><br></span></code></pre><div class=buttonGroup__atx><button type=button aria-label="Copy code to clipboard" title=Copy class=clean-btn><span class=copyButtonIcons_eSgA aria-hidden=true><svg viewBox="0 0 24 24" class=copyButtonIcon_y97N><path fill=currentColor d="M19,21H8V7H19M19,5H8A2,2 0 0,0 6,7V21A2,2 0 0,0 8,23H19A2,2 0 0,0 21,21V7A2,2 0 0,0 19,5M16,1H4A2,2 0 0,0 2,3V17H4V3H16V1Z"/></svg><svg viewBox="0 0 24 24" class=copyButtonSuccessIcon_LjdS><path fill=currentColor d=M21,7L9,19L3.5,13.5L4.91,12.09L9,16.17L19.59,5.59L21,7Z /></svg></span></button></div></div></div>
<p>Now, we're going to put the following content in <code>point.capnp</code>:</p>
<div class="codeBlockContainer_Ckt0 theme-code-block" style="--prism-background-color:hsl(230, 1%, 98%);--prism-color:hsl(230, 8%, 24%)"><div class=codeBlockContent_biex><pre tabindex=0 class="prism-code language-text codeBlock_bY9V thin-scrollbar" style="background-color:hsl(230, 1%, 98%);color:hsl(230, 8%, 24%)"><code class=codeBlockLines_e6Vv><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain">@0xab555145c708dad2;</span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain" style=display:inline-block></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain">struct Point {</span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain"> x @0 :Int32;</span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain"> y @1 :Int32;</span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain">}</span><br></span></code></pre><div class=buttonGroup__atx><button type=button aria-label="Copy code to clipboard" title=Copy class=clean-btn><span class=copyButtonIcons_eSgA aria-hidden=true><svg viewBox="0 0 24 24" class=copyButtonIcon_y97N><path fill=currentColor d="M19,21H8V7H19M19,5H8A2,2 0 0,0 6,7V21A2,2 0 0,0 8,23H19A2,2 0 0,0 21,21V7A2,2 0 0,0 19,5M16,1H4A2,2 0 0,0 2,3V17H4V3H16V1Z"/></svg><svg viewBox="0 0 24 24" class=copyButtonSuccessIcon_LjdS><path fill=currentColor d=M21,7L9,19L3.5,13.5L4.91,12.09L9,16.17L19.59,5.59L21,7Z /></svg></span></button></div></div></div>
<p>Pretty easy, we've now got structure for an object we'll be able to quickly encode in a binary format.</p>
<h2 class="anchor anchorWithStickyNavbar_LWe7" id=step-4-setting-up-the-build-process>Step 4: Setting up the build process<a href=#step-4-setting-up-the-build-process class=hash-link aria-label="Direct link to Step 4: Setting up the build process" title="Direct link to Step 4: Setting up the build process">​</a></h2>
<p>Now it's time to actually set up the build process to make sure that Cap'N Proto generates the Rust code we'll eventually be using.
This is typically done through a <code>build.rs</code> file to invoke the schema compiler.</p>
<p>In the same folder as your <code>Cargo.toml</code> file, please put the following content in <code>build.rs</code>:</p>
<div class="language-rust codeBlockContainer_Ckt0 theme-code-block" style="--prism-background-color:hsl(230, 1%, 98%);--prism-color:hsl(230, 8%, 24%)"><div class=codeBlockContent_biex><pre tabindex=0 class="prism-code language-rust codeBlock_bY9V thin-scrollbar" style="background-color:hsl(230, 1%, 98%);color:hsl(230, 8%, 24%)"><code class=codeBlockLines_e6Vv><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token keyword" style="color:hsl(301, 63%, 40%)">extern</span><span class="token plain"> </span><span class="token keyword" style="color:hsl(301, 63%, 40%)">crate</span><span class="token plain"> </span><span class="token module-declaration namespace">capnpc</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">;</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain" style=display:inline-block></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain"></span><span class="token keyword" style="color:hsl(301, 63%, 40%)">fn</span><span class="token plain"> </span><span class="token function-definition function" style="color:hsl(221, 87%, 60%)">main</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><span class="token plain"> </span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">{</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain"> </span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">::</span><span class="token namespace">capnpc</span><span class="token namespace punctuation" style="color:hsl(119, 34%, 47%)">::</span><span class="token class-name" style="color:hsl(35, 99%, 36%)">CompilerCommand</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">::</span><span class="token function" style="color:hsl(221, 87%, 60%)">new</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain"> </span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">.</span><span class="token function" style="color:hsl(221, 87%, 60%)">src_prefix</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token string" style="color:hsl(119, 34%, 47%)">"schema"</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><span class="token plain"> </span><span class="token comment" style="color:hsl(230, 4%, 64%)">// 1</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain"> </span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">.</span><span class="token function" style="color:hsl(221, 87%, 60%)">file</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token string" style="color:hsl(119, 34%, 47%)">"schema/point.capnp"</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><span class="token plain"> </span><span class="token comment" style="color:hsl(230, 4%, 64%)">// 2</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain"> </span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">.</span><span class="token function" style="color:hsl(221, 87%, 60%)">run</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">.</span><span class="token function" style="color:hsl(221, 87%, 60%)">expect</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span clas
<p>This sets up the protocol compiler (<code>capnpc</code> from earlier) to compile the schema we've built so far.</p>
<ol>
<li>Because Cap'N Proto schema files can re-use types specified in other files, the <code>src_prefix()</code> tells the compiler
where to look for those extra files at.</li>
<li>We specify the schema file we're including by hand. In a much larger project, you could presumably build the <code>CompilerCommand</code>
dynamically, but we won't worry too much about that one for now.</li>
</ol>
<h2 class="anchor anchorWithStickyNavbar_LWe7" id=step-5-running-the-build>Step 5: Running the build<a href=#step-5-running-the-build class=hash-link aria-label="Direct link to Step 5: Running the build" title="Direct link to Step 5: Running the build">​</a></h2>
<p>If you've done everything correctly so far, you should be able to actually build the project and see the auto-generated code.
Run a <code>cargo build</code> command, and if you don't see <code>cargo</code> complaining, you're doing just fine!</p>
<p>So where exactly does the generated code go to? I think it's critically important for people to be able to see what the generated
code looks like, because you need to understand what you're actually programming against. The short answer is: the generated code lives
somewhere in the <code>target/</code> directory.</p>
<p>The long answer is that you're best off running a <code>find</code> command to get the actual file path:</p>
<div class="language-bash codeBlockContainer_Ckt0 theme-code-block" style="--prism-background-color:hsl(230, 1%, 98%);--prism-color:hsl(230, 8%, 24%)"><div class=codeBlockContent_biex><pre tabindex=0 class="prism-code language-bash codeBlock_bY9V thin-scrollbar" style="background-color:hsl(230, 1%, 98%);color:hsl(230, 8%, 24%)"><code class=codeBlockLines_e6Vv><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token comment" style="color:hsl(230, 4%, 64%)"># Assuming we're running from the capnp_cookbook_1 project folder</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain"></span><span class="token function" style="color:hsl(221, 87%, 60%)">find</span><span class="token plain"> </span><span class="token builtin class-name" style="color:hsl(35, 99%, 36%)">.</span><span class="token plain"> </span><span class="token parameter variable" style="color:hsl(221, 87%, 60%)">-name</span><span class="token plain"> point_capnp.rs</span><br></span></code></pre><div class=buttonGroup__atx><button type=button aria-label="Copy code to clipboard" title=Copy class=clean-btn><span class=copyButtonIcons_eSgA aria-hidden=true><svg viewBox="0 0 24 24" class=copyButtonIcon_y97N><path fill=currentColor d="M19,21H8V7H19M19,5H8A2,2 0 0,0 6,7V21A2,2 0 0,0 8,23H19A2,2 0 0,0 21,21V7A2,2 0 0,0 19,5M16,1H4A2,2 0 0,0 2,3V17H4V3H16V1Z"/></svg><svg viewBox="0 0 24 24" class=copyButtonSuccessIcon_LjdS><path fill=currentColor d=M21,7L9,19L3.5,13.5L4.91,12.09L9,16.17L19.59,5.59L21,7Z /></svg></span></button></div></div></div>
<p>Alternately, if the <code>find</code> command isn't available, the path will look something like:</p>
<div class="codeBlockContainer_Ckt0 theme-code-block" style="--prism-background-color:hsl(230, 1%, 98%);--prism-color:hsl(230, 8%, 24%)"><div class=codeBlockContent_biex><pre tabindex=0 class="prism-code language-text codeBlock_bY9V thin-scrollbar" style="background-color:hsl(230, 1%, 98%);color:hsl(230, 8%, 24%)"><code class=codeBlockLines_e6Vv><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain">./target/debug/build/capnp_cookbook_1-c6e2990393c32fe6/out/point_capnp.rs</span><br></span></code></pre><div class=buttonGroup__atx><button type=button aria-label="Copy code to clipboard" title=Copy class=clean-btn><span class=copyButtonIcons_eSgA aria-hidden=true><svg viewBox="0 0 24 24" class=copyButtonIcon_y97N><path fill=currentColor d="M19,21H8V7H19M19,5H8A2,2 0 0,0 6,7V21A2,2 0 0,0 8,23H19A2,2 0 0,0 21,21V7A2,2 0 0,0 19,5M16,1H4A2,2 0 0,0 2,3V17H4V3H16V1Z"/></svg><svg viewBox="0 0 24 24" class=copyButtonSuccessIcon_LjdS><path fill=currentColor d=M21,7L9,19L3.5,13.5L4.91,12.09L9,16.17L19.59,5.59L21,7Z /></svg></span></button></div></div></div>
<p>See if there are any paths in your target directory that look similar.</p>
<p>Now, the file content looks pretty nasty. I've included an example <a href=https://github.com/bspeice/capnp_cookbook_1/blob/master/target/debug/build/capnp_cookbook_1-c6e2990393c32fe6/out/point_capnp.rs target=_blank rel="noopener noreferrer">here</a>
if you aren't following along at home. There are a couple things I'll try and point out though so you can get an idea of how
the schema we wrote for the "Point" message is tied to the generated code.</p>
<p>First, the Cap'N Proto library splits things up into <code>Builder</code> and <code>Reader</code> structs. These are best thought of the same way
Rust separates <code>mut</code> from non-<code>mut</code> code. <code>Builder</code>s are <code>mut</code> versions of your message, and <code>Reader</code>s are immutable versions.</p>
<p>For example, the <a href=https://github.com/bspeice/capnp_cookbook_1/blob/master/target/debug/build/capnp_cookbook_1-c6e2990393c32fe6/out/point_capnp.rs#L90 target=_blank rel="noopener noreferrer"><code>Builder</code> impl</a> for <code>point</code> defines <a href=https://github.com/bspeice/capnp_cookbook_1/blob/master/target/debug/build/capnp_cookbook_1-c6e2990393c32fe6/out/point_capnp.rs#L105 target=_blank rel="noopener noreferrer"><code>get_x()</code></a>, <a href=https://github.com/bspeice/capnp_cookbook_1/blob/master/target/debug/build/capnp_cookbook_1-c6e2990393c32fe6/out/point_capnp.rs#L109 target=_blank rel="noopener noreferrer"><code>set_x()</code></a>, <a href=https://github.com/bspeice/capnp_cookbook_1/blob/master/target/debug/build/capnp_cookbook_1-c6e2990393c32fe6/out/point_capnp.rs#L113 target=_blank rel="noopener noreferrer"><code>get_y()</code></a>, and <a href=https://github.com/bspeice/capnp_cookbook_1/blob/master/target/debug/build/capnp_cookbook_1-c6e2990393c32fe6/out/point_capnp.rs#L117 target=_blank rel="noopener noreferrer"><code>set_y()</code></a> methods.
In comparison, the <a href=https://github.com/bspeice/capnp_cookbook_1/blob/master/target/debug/build/capnp_cookbook_1-c6e2990393c32fe6/out/point_capnp.rs#L38 target=_blank rel="noopener noreferrer"><code>Reader</code> impl</a> only defines <a href=https://github.com/bspeice/capnp_cookbook_1/blob/master/target/debug/build/capnp_cookbook_1-c6e2990393c32fe6/out/point_capnp.rs#L47 target=_blank rel="noopener noreferrer"><code>get_x()</code></a> and <a href=https://github.com/bspeice/capnp_cookbook_1/blob/master/target/debug/build/capnp_cookbook_1-c6e2990393c32fe6/out/point_capnp.rs#L51 target=_blank rel="noopener noreferrer"><code>get_y()</code></a> methods.</p>
<p>So now we know that there are some <code>get</code> and <code>set</code> methods available for our <code>x</code> and <code>y</code> coordinates;
but what do we actually do with those?</p>
<h2 class="anchor anchorWithStickyNavbar_LWe7" id=step-6-making-a-point>Step 6: Making a point<a href=#step-6-making-a-point class=hash-link aria-label="Direct link to Step 6: Making a point" title="Direct link to Step 6: Making a point">​</a></h2>
<p>So we've install Cap'N Proto, gotten a project set up, and can generate schema code now. It's time to actually start building
Cap'N Proto messages! I'm going to put the code you need here because it's small, and put some extra long comments inline. This code
should go in <a href=https://github.com/bspeice/capnp_cookbook_1/blob/master/src/main.rs target=_blank rel="noopener noreferrer"><code>src/main.rs</code></a>:</p>
<div class="language-rust codeBlockContainer_Ckt0 theme-code-block" style="--prism-background-color:hsl(230, 1%, 98%);--prism-color:hsl(230, 8%, 24%)"><div class=codeBlockContent_biex><pre tabindex=0 class="prism-code language-rust codeBlock_bY9V thin-scrollbar" style="background-color:hsl(230, 1%, 98%);color:hsl(230, 8%, 24%)"><code class=codeBlockLines_e6Vv><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token comment" style="color:hsl(230, 4%, 64%)">// Note that we use `capnp` here, NOT `capnpc`</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain"></span><span class="token keyword" style="color:hsl(301, 63%, 40%)">extern</span><span class="token plain"> </span><span class="token keyword" style="color:hsl(301, 63%, 40%)">crate</span><span class="token plain"> </span><span class="token module-declaration namespace">capnp</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">;</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain" style=display:inline-block></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain"></span><span class="token comment" style="color:hsl(230, 4%, 64%)">// We create a module here to define how we are to access the code</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain"></span><span class="token comment" style="color:hsl(230, 4%, 64%)">// being included.</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain"></span><span class="token keyword" style="color:hsl(301, 63%, 40%)">pub</span><span class="token plain"> </span><span class="token keyword" style="color:hsl(301, 63%, 40%)">mod</span><span class="token plain"> </span><span class="token module-declaration namespace">point_capnp</span><span class="token plain"> </span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">{</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain"> </span><span class="token comment" style="color:hsl(230, 4%, 64%)">// The environment variable OUT_DIR is set by Cargo, and</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain"> </span><span class="token comment" style="color:hsl(230, 4%, 64%)">// is the location of all the code that was built as part</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain"> </span><span class="token comment" style="color:hsl(230, 4%, 64%)">// of the codegen step.</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain"> </span><span class="token comment" style="color:hsl(230, 4%, 64%)">// point_capnp.rs is the actual file to include</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain"> </span><span class="token macro property" style="color:hsl(5, 74%, 59%)">include!</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token macro property" style="color:hsl(5, 74%, 59%)">concat!</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token macro property" style="color:hsl(5, 74%, 59%)">env!</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token string" style="color:hsl(119, 34%, 47%)">"OUT_DIR"</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">,</span><span class="token plain"> </span><span class="token string" style="color:hsl(119, 34%, 47%)">"/point_capnp.rs"</span><span class="token punctuation"
<p>And with that, we've now got a functioning project. Here's the content I'm planning to go over next as we build up
some practical examples of Cap'N Proto in action:</div></article><nav class="pagination-nav docusaurus-mt-lg" aria-label="Blog post page navigation"><a class="pagination-nav__link pagination-nav__link--prev" href=/2016/11/pca-audio-compression><div class=pagination-nav__sublabel>Older post</div><div class=pagination-nav__label>PCA audio compression</div></a><a class="pagination-nav__link pagination-nav__link--next" href=/2018/01/captains-cookbook-part-2><div class=pagination-nav__sublabel>Newer post</div><div class=pagination-nav__label>Captain's Cookbook: Practical usage</div></a></nav></main><div class="col col--2"><div class="tableOfContents_bqdL thin-scrollbar"><ul class="table-of-contents table-of-contents__left-border"><li><a href=#step-1-installing-capnp class="table-of-contents__link toc-highlight">Step 1: Installing <code>capnp</code></a><li><a href=#step-2-starting-a-capn-proto-rust-project class="table-of-contents__link toc-highlight">Step 2: Starting a Cap'N Proto Rust project</a><li><a href=#step-3-writing-a-basic-schema class="table-of-contents__link toc-highlight">Step 3: Writing a basic schema</a><li><a href=#step-4-setting-up-the-build-process class="table-of-contents__link toc-highlight">Step 4: Setting up the build process</a><li><a href=#step-5-running-the-build class="table-of-contents__link toc-highlight">Step 5: Running the build</a><li><a href=#step-6-making-a-point class="table-of-contents__link toc-highlight">Step 6: Making a point</a></ul></div></div></div></div></div><footer class=footer><div class="container container-fluid"><div class="footer__bottom text--center"><div class=footer__copyright>Copyright © 2024 Bradlee Speice</div></div></div></footer></div>
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