<!doctype html><htmllang=endir=ltrclass="blog-wrapper blog-post-page plugin-blog plugin-id-default"data-has-hydrated=false><metacharset=UTF-8><metaname=generatorcontent="Docusaurus v3.6.1"><titledata-rh=true>Captain's Cookbook: Project setup | The Old Speice Guy</title><metadata-rh=truename=viewportcontent="width=device-width,initial-scale=1.0"><metadata-rh=truename=twitter:cardcontent=summary_large_image><metadata-rh=trueproperty=og:urlcontent=https://speice.io/2018/01/captains-cookbook-part-1><metadata-rh=trueproperty=og:localecontent=en><metadata-rh=truename=docusaurus_localecontent=en><metadata-rh=truename=docusaurus_tagcontent=default><metadata-rh=truename=docsearch:languagecontent=en><metadata-rh=truename=docsearch:docusaurus_tagcontent=default><metadata-rh=trueproperty=og:titlecontent="Captain's Cookbook: Project setup | The Old Speice Guy"><metadata-rh=truename=descriptioncontent="A basic introduction to getting started with Cap'N Proto."><metadata-rh=trueproperty=og:descriptioncontent="A basic introduction to getting started with Cap'N Proto."><metadata-rh=trueproperty=og:typecontent=article><metadata-rh=trueproperty=article:published_timecontent=2018-01-16T12:00:00.000Z><linkdata-rh=truerel=iconhref=/img/favicon.ico><linkdata-rh=truerel=canonicalhref=https://speice.io/2018/01/captains-cookbook-part-1><linkdata-rh=truerel=alternatehref=https://speice.io/2018/01/captains-cookbook-part-1hreflang=en><linkdata-rh=truerel=alternatehref=https://speice.io/2018/01/captains-cookbook-part-1hreflang=x-default><scriptdata-rh=truetype=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><linkrel=alternatetype=application/rss+xmlhref=/rss.xmltitle="The Old Speice Guy RSS Feed"><linkrel=alternatetype=application/atom+xmlhref=/atom.xmltitle="The Old Speice Guy Atom Feed"><linkrel=stylesheethref=/katex/katex.min.css><linkrel=stylesheethref=/assets/css/styles.16c3428d.css><scriptsrc=/assets/js/runtime~main.29a27dcf.jsdefer></script><scriptsrc=/assets/js/main.d461af80.jsdefer></script><bodyclass=navigation-with-keyboard><script>!function(){vart,e=function(){try{returnnewURLSearchParams(window.location.search).get("docusaurus-theme")}catch(t){}}()||function(){try{returnwindow.localStorage.getItem("theme")}catch(t){}}();t=null!==e?e:"light",document.documentElement.setAttribute("data-theme",t)}(),function(){try{for(var[t,e]ofnewURLSearchParams(window.location.search).entries())if(t.startsWith("docusaurus-data-")){vara=t.replace("docusaurus-data-","data-");document.documentElement.setAttribute(a,e)}}catch(t){}}()</script><divid=__docusaurus><divrole=regionaria-label="Skip to main content"><aclass=skipToContent_fXgnhref=#__docusaurus_skipToContent_fallback>Skip to main content</a></div><navaria-label=Mainclass="navbar navbar--fixed-top"><divclass=navbar__inner><divclass=navbar__items><buttonaria-label="Toggle navigation bar"aria-expanded=falseclass="navbar__toggle clean-btn"type=button><svgwidth=30height=30viewBox="0 0 30 30"aria-hidden=true><pathstroke=currentColorstroke-linecap=roundstroke-miterlimit=10stroke-width=2d="M4 7h22M4 15h22M4 23h22"/></svg></button><aclass=navbar__brandhref=/><divclass=navbar__logo><imgsrc=/img/logo.svgalt="Sierpinski Gasket"class="themedComponent_mlkZ themedComponent--light_NVdE"><imgsrc=/img/logo-dark.svgalt="Sierpinski Gasket"class="themedComponent_mlkZ themedComponent--dark_xIcU"></div><bcl
<p>I've been working a lot with <ahref=https://capnproto.org/target=_blankrel="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 <ahref=https://developers.google.com/protocol-buffers/target=_blankrel="noopener noreferrer">Protobuf</a>
and <ahref=https://google.github.io/flatbuffers/target=_blankrel="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 <ahref=https://github.com/bspeice/capnp_cookbook_1target=_blankrel="noopener noreferrer">the final result</a>
for reference.</p>
<h2class="anchor anchorWithStickyNavbar_LWe7"id=step-1-installing-capnp>Step 1: Installing <code>capnp</code><ahref=#step-1-installing-capnpclass=hash-linkaria-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 <ahref=https://capnproto.org/install.htmltarget=_blankrel="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 <ahref=https://brew.sh/target=_blankrel="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 <ahref=https://chocolatey.org/target=_blankrel="noopener noreferrer">Chocolatey</a>, there's <ahref=https://chocolatey.org/packages/capnproto/target=_blankrel="noopener noreferrer">a package</a> available. If that doesn't work however, you need to download <ahref=https://capnproto.org/capnproto-c++-win32-0.6.1.ziptarget=_blankrel="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>
<h2class="anchor anchorWithStickyNavbar_LWe7"id=step-2-starting-a-capn-proto-rust-project>Step 2: Starting a Cap'N Proto Rust project<ahref=#step-2-starting-a-capn-proto-rust-projectclass=hash-linkaria-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>
<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>
<h2class="anchor anchorWithStickyNavbar_LWe7"id=step-3-writing-a-basic-schema>Step 3: Writing a basic schema<ahref=#step-3-writing-a-basic-schemaclass=hash-linkaria-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>
<p>Pretty easy, we've now got structure for an object we'll be able to quickly encode in a binary format.</p>
<h2class="anchor anchorWithStickyNavbar_LWe7"id=step-4-setting-up-the-build-process>Step 4: Setting up the build process<ahref=#step-4-setting-up-the-build-processclass=hash-linkaria-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>
<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>
<h2class="anchor anchorWithStickyNavbar_LWe7"id=step-5-running-the-build>Step 5: Running the build<ahref=#step-5-running-the-buildclass=hash-linkaria-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>
<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 <ahref=https://github.com/bspeice/capnp_cookbook_1/blob/master/target/debug/build/capnp_cookbook_1-c6e2990393c32fe6/out/point_capnp.rstarget=_blankrel="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 <ahref=https://github.com/bspeice/capnp_cookbook_1/blob/master/target/debug/build/capnp_cookbook_1-c6e2990393c32fe6/out/point_capnp.rs#L90target=_blankrel="noopener noreferrer"><code>Builder</code> impl</a> for <code>point</code> defines <ahref=https://github.com/bspeice/capnp_cookbook_1/blob/master/target/debug/build/capnp_cookbook_1-c6e2990393c32fe6/out/point_capnp.rs#L105target=_blankrel="noopener noreferrer"><code>get_x()</code></a>, <ahref=https://github.com/bspeice/capnp_cookbook_1/blob/master/target/debug/build/capnp_cookbook_1-c6e2990393c32fe6/out/point_capnp.rs#L109target=_blankrel="noopener noreferrer"><code>set_x()</code></a>, <ahref=https://github.com/bspeice/capnp_cookbook_1/blob/master/target/debug/build/capnp_cookbook_1-c6e2990393c32fe6/out/point_capnp.rs#L113target=_blankrel="noopener noreferrer"><code>get_y()</code></a>, and <ahref=https://github.com/bspeice/capnp_cookbook_1/blob/master/target/debug/build/capnp_cookbook_1-c6e2990393c32fe6/out/point_capnp.rs#L117target=_blankrel="noopener noreferrer"><code>set_y()</code></a> methods.
In comparison, the <ahref=https://github.com/bspeice/capnp_cookbook_1/blob/master/target/debug/build/capnp_cookbook_1-c6e2990393c32fe6/out/point_capnp.rs#L38target=_blankrel="noopener noreferrer"><code>Reader</code> impl</a> only defines <ahref=https://github.com/bspeice/capnp_cookbook_1/blob/master/target/debug/build/capnp_cookbook_1-c6e2990393c32fe6/out/point_capnp.rs#L47target=_blankrel="noopener noreferrer"><code>get_x()</code></a> and <ahref=https://github.com/bspeice/capnp_cookbook_1/blob/master/target/debug/build/capnp_cookbook_1-c6e2990393c32fe6/out/point_capnp.rs#L51target=_blankrel="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>
<h2class="anchor anchorWithStickyNavbar_LWe7"id=step-6-making-a-point>Step 6: Making a point<ahref=#step-6-making-a-pointclass=hash-linkaria-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 <ahref=https://github.com/bspeice/capnp_cookbook_1/blob/master/src/main.rstarget=_blankrel="noopener noreferrer"><code>src/main.rs</code></a>:</p>
<divclass="language-rust codeBlockContainer_Ckt0 theme-code-block"style="--prism-background-color:hsl(230, 1%, 98%);--prism-color:hsl(230, 8%, 24%)"><divclass=codeBlockContent_biex><pretabindex=0class="prism-code language-rust codeBlock_bY9V thin-scrollbar"style="background-color:hsl(230, 1%, 98%);color:hsl(230, 8%, 24%)"><codeclass=codeBlockLines_e6Vv><spanclass=token-linestyle="color:hsl(230, 8%, 24%)"><spanclass="token comment"style="color:hsl(230, 4%, 64%)">// Note that we use `capnp` here, NOT `capnpc`</span><spanclass="token plain"></span><br></span><spanclass=token-linestyle="color:hsl(230, 8%, 24%)"><spanclass="token plain"></span><spanclass="token keyword"style="color:hsl(301, 63%, 40%)">extern</span><spanclass="token plain"></span><spanclass="token keyword"style="color:hsl(301, 63%, 40%)">crate</span><spanclass="token plain"></span><spanclass="token module-declaration namespace">capnp</span><spanclass="token punctuation"style="color:hsl(119, 34%, 47%)">;</span><spanclass="token plain"></span><br></span><spanclass=token-linestyle="color:hsl(230, 8%, 24%)"><spanclass="token plain"style=display:inline-block></span><br></span><spanclass=token-linestyle="color:hsl(230, 8%, 24%)"><spanclass="token plain"></span><spanclass="token comment"style="color:hsl(230, 4%, 64%)">// We create a module here to define how we are to access the code</span><spanclass="token plain"></span><br></span><spanclass=token-linestyle="color:hsl(230, 8%, 24%)"><spanclass="token plain"></span><spanclass="token comment"style="color:hsl(230, 4%, 64%)">// being included.</span><spanclass="token plain"></span><br></span><spanclass=token-linestyle="color:hsl(230, 8%, 24%)"><spanclass="token plain"></span><spanclass="token keyword"style="color:hsl(301, 63%, 40%)">pub</span><spanclass="token plain"></span><spanclass="token keyword"style="color:hsl(301, 63%, 40%)">mod</span><spanclass="token plain"></span><spanclass="token module-declaration namespace">point_capnp</span><spanclass="token plain"></span><spanclass="token punctuation"style="color:hsl(119, 34%, 47%)">{</span><spanclass="token plain"></span><br></span><spanclass=token-linestyle="color:hsl(230, 8%, 24%)"><spanclass="token plain"></span><spanclass="token comment"style="color:hsl(230, 4%, 64%)">// The environment variable OUT_DIR is set by Cargo, and</span><spanclass="token plain"></span><br></span><spanclass=token-linestyle="color:hsl(230, 8%, 24%)"><spanclass="token plain"></span><spanclass="token comment"style="color:hsl(230, 4%, 64%)">// is the location of all the code that was built as part</span><spanclass="token plain"></span><br></span><spanclass=token-linestyle="color:hsl(230, 8%, 24%)"><spanclass="token plain"></span><spanclass="token comment"style="color:hsl(230, 4%, 64%)">// of the codegen step.</span><spanclass="token plain"></span><br></span><spanclass=token-linestyle="color:hsl(230, 8%, 24%)"><spanclass="token plain"></span><spanclass="token comment"style="color:hsl(230, 4%, 64%)">// point_capnp.rs is the actual file to include</span><spanclass="token plain"></span><br></span><spanclass=token-linestyle="color:hsl(230, 8%, 24%)"><spanclass="token plain"></span><spanclass="token macro property"style="color:hsl(5, 74%, 59%)">include!</span><spanclass="token punctuation"style="color:hsl(119, 34%, 47%)">(</span><spanclass="token macro property"style="color:hsl(5, 74%, 59%)">concat!</span><spanclass="token punctuation"style="color:hsl(119, 34%, 47%)">(</span><spanclass="token macro property"style="color:hsl(5, 74%, 59%)">env!</span><spanclass="token punctuation"style="color:hsl(119, 34%, 47%)">(</span><spanclass="token string"style="color:hsl(119, 34%, 47%)">"OUT_DIR"</span><spanclass="token punctuation"style="color:hsl(119, 34%, 47%)">)</span><spanclass="token punctuation"style="color:hsl(119, 34%, 47%)">,</span><spanclass="token plain"></span><spanclass="token string"style="color:hsl(119, 34%, 47%)">"/point_capnp.rs"</span><spanclass="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
