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<ol class="chapter"><li class="affix"><a href="../../introduction.html">Introduction</a></li><li><a href="../../editions/index.html"><strong aria-hidden="true">1.</strong> What are editions?</a></li><li><ol class="section"><li><a href="../../editions/creating-a-new-project.html"><strong aria-hidden="true">1.1.</strong> Creating a new project</a></li><li><a href="../../editions/transitioning-an-existing-project-to-a-new-edition.html"><strong aria-hidden="true">1.2.</strong> Transitioning an existing project to a new edition</a></li></ol></li><li><a href="../../rust-2015/index.html"><strong aria-hidden="true">2.</strong> Rust 2015</a></li><li><a href="../../rust-2018/index.html"><strong aria-hidden="true">3.</strong> Rust 2018</a></li><li><ol class="section"><li><a href="../../rust-2018/edition-changes.html"><strong aria-hidden="true">3.1.</strong> 2018-Specific Changes</a></li><li><a href="../../rust-2018/module-system/index.html"><strong aria-hidden="true">3.2.</strong> Module 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<a class="header" href="#choosing-alignment-with-the-repr-attribute" id="choosing-alignment-with-the-repr-attribute"><h1>Choosing alignment with the repr attribute</h1></a>
<p><img src="https://img.shields.io/badge/Minimum%20Rust%20Version-1.25-brightgreen.svg" alt="Minimum Rust version: 1.25" /></p>
<p>From <a href="https://en.wikipedia.org/wiki/Data_structure_alignment">Wikipedia</a>:</p>
<blockquote>
<p>The CPU in modern computer hardware performs reads and writes to memory
most efficiently when the data is naturally aligned, which generally means
that the data address is a multiple of the data size. Data alignment refers
to aligning elements according to their natural alignment. To ensure natural
alignment, it may be necessary to insert some padding between structure
elements or after the last element of a structure.</p>
</blockquote>
<p>The <code>#[repr]</code> attribute has a new parameter, <code>align</code>, that sets the alignment of your struct:</p>
<pre><pre class="playpen"><code class="language-rust">
# #![allow(unused_variables)]
#fn main() {
struct Number(i32);
assert_eq!(std::mem::align_of::<Number>(), 4);
assert_eq!(std::mem::size_of::<Number>(), 4);
#[repr(align(16))]
struct Align16(i32);
assert_eq!(std::mem::align_of::<Align16>(), 16);
assert_eq!(std::mem::size_of::<Align16>(), 16);
#}</code></pre></pre>
<p>If you’re working with low-level stuff, control of these kinds of things can
be very important!</p>
<p>The alignment of a type is normally not worried about as the compiler will
"do the right thing" of picking an appropriate alignment for general use
cases. There are situations, however, where a nonstandard alignment may be
desired when operating with foreign systems. For example these sorts of
situations tend to necessitate or be much easier with a custom alignment:</p>
<ul>
<li>Hardware can often have obscure requirements such as "this structure is
aligned to 32 bytes" when it in fact is only composed of 4-byte values. While
this can typically be manually calculated and managed, it's often also useful
to express this as a property of a type to get the compiler to do a little
extra work instead.</li>
<li>C compilers like <code>gcc</code> and <code>clang</code> offer the ability to specify a custom
alignment for structures, and Rust can much more easily interoperate with
these types if Rust can also mirror the request for a custom alignment (e.g.
passing a structure to C correctly is much easier).</li>
<li>Custom alignment can often be used for various tricks here and there and is
often convenient as "let's play around with an implementation" tool. For
example this can be used to statically allocate page tables in a kernel or
create an at-least cache-line-sized structure easily for concurrent
programming.</li>
</ul>
<p>The purpose of this feature is to provide a lightweight annotation to alter
the compiler-inferred alignment of a structure to enable these situations
much more easily.</p>
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