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| <h1 id="tips-for-embedded-c-developers"><a class="header" href="#tips-for-embedded-c-developers">Tips for embedded C developers</a></h1> |
| <p>This chapter collects a variety of tips that might be useful to experienced |
| embedded C developers looking to start writing Rust. It will especially |
| highlight how things you might already be used to in C are different in Rust.</p> |
| <h2 id="preprocessor"><a class="header" href="#preprocessor">Preprocessor</a></h2> |
| <p>In embedded C it is very common to use the preprocessor for a variety of |
| purposes, such as:</p> |
| <ul> |
| <li>Compile-time selection of code blocks with <code>#ifdef</code></li> |
| <li>Compile-time array sizes and computations</li> |
| <li>Macros to simplify common patterns (to avoid function call overhead)</li> |
| </ul> |
| <p>In Rust there is no preprocessor, and so many of these use cases are addressed |
| differently. In the rest of this section we cover various alternatives to |
| using the preprocessor.</p> |
| <h3 id="compile-time-code-selection"><a class="header" href="#compile-time-code-selection">Compile-Time Code Selection</a></h3> |
| <p>The closest match to <code>#ifdef ... #endif</code> in Rust are <a href="https://doc.rust-lang.org/cargo/reference/manifest.html#the-features-section">Cargo features</a>. These |
| are a little more formal than the C preprocessor: all possible features are |
| explicitly listed per crate, and can only be either on or off. Features are |
| turned on when you list a crate as a dependency, and are additive: if any crate |
| in your dependency tree enables a feature for another crate, that feature will |
| be enabled for all users of that crate.</p> |
| <p>For example, you might have a crate which provides a library of signal |
| processing primitives. Each one might take some extra time to compile or |
| declare some large table of constants which you’d like to avoid. You could |
| declare a Cargo feature for each component in your <code>Cargo.toml</code>:</p> |
| <pre><code class="language-toml">[features] |
| FIR = [] |
| IIR = [] |
| </code></pre> |
| <p>Then, in your code, use <code>#[cfg(feature="FIR")]</code> to control what is included.</p> |
| <pre class="playground"><code class="language-rust"><span class="boring">#![allow(unused)] |
| </span><span class="boring">fn main() { |
| </span>/// In your top-level lib.rs |
| |
| #[cfg(feature="FIR")] |
| pub mod fir; |
| |
| #[cfg(feature="IIR")] |
| pub mod iir; |
| <span class="boring">}</span></code></pre> |
| <p>You can similarly include code blocks only if a feature is <em>not</em> enabled, or if |
| any combination of features are or are not enabled.</p> |
| <p>Additionally, Rust provides a number of automatically-set conditions you can |
| use, such as <code>target_arch</code> to select different code based on architecture. For |
| full details of the conditional compilation support, refer to the |
| <a href="https://doc.rust-lang.org/reference/conditional-compilation.html">conditional compilation</a> chapter of the Rust reference.</p> |
| <p>The conditional compilation will only apply to the next statement or block. If |
| a block can not be used in the current scope then the <code>cfg</code> attribute will |
| need to be used multiple times. It’s worth noting that most of the time it is |
| better to simply include all the code and allow the compiler to remove dead |
| code when optimising: it’s simpler for you and your users, and in general the |
| compiler will do a good job of removing unused code.</p> |
| <h3 id="compile-time-sizes-and-computation"><a class="header" href="#compile-time-sizes-and-computation">Compile-Time Sizes and Computation</a></h3> |
| <p>Rust supports <code>const fn</code>, functions which are guaranteed to be evaluable at |
| compile-time and can therefore be used where constants are required, such as |
| in the size of arrays. This can be used alongside features mentioned above, |
| for example:</p> |
| <pre class="playground"><code class="language-rust"><span class="boring">#![allow(unused)] |
| </span><span class="boring">fn main() { |
| </span>const fn array_size() -> usize { |
| #[cfg(feature="use_more_ram")] |
| { 1024 } |
| #[cfg(not(feature="use_more_ram"))] |
| { 128 } |
| } |
| |
| static BUF: [u32; array_size()] = [0u32; array_size()]; |
| <span class="boring">}</span></code></pre> |
| <p>These are new to stable Rust as of 1.31, so documentation is still sparse. The |
| functionality available to <code>const fn</code> is also very limited at the time of |
| writing; in future Rust releases it is expected to expand on what is permitted |
| in a <code>const fn</code>.</p> |
| <h3 id="macros"><a class="header" href="#macros">Macros</a></h3> |
| <p>Rust provides an extremely powerful <a href="https://doc.rust-lang.org/book/ch19-06-macros.html">macro system</a>. While the C preprocessor |
| operates almost directly on the text of your source code, the Rust macro system |
| operates at a higher level. There are two varieties of Rust macro: <em>macros by |
| example</em> and <em>procedural macros</em>. The former are simpler and most common; they |
| look like function calls and can expand to a complete expression, statement, |
| item, or pattern. Procedural macros are more complex but permit extremely |
| powerful additions to the Rust language: they can transform arbitrary Rust |
| syntax into new Rust syntax.</p> |
| <p>In general, where you might have used a C preprocessor macro, you probably want |
| to see if a macro-by-example can do the job instead. They can be defined in |
| your crate and easily used by your own crate or exported for other users. Be |
| aware that since they must expand to complete expressions, statements, items, |
| or patterns, some use cases of C preprocessor macros will not work, for example |
| a macro that expands to part of a variable name or an incomplete set of items |
| in a list.</p> |
| <p>As with Cargo features, it is worth considering if you even need the macro. In |
| many cases a regular function is easier to understand and will be inlined to |
| the same code as a macro. The <code>#[inline]</code> and <code>#[inline(always)]</code> <a href="https://doc.rust-lang.org/reference/attributes.html#inline-attribute">attributes</a> |
| give you further control over this process, although care should be taken here |
| as well — the compiler will automatically inline functions from the same crate |
| where appropriate, so forcing it to do so inappropriately might actually lead |
| to decreased performance.</p> |
| <p>Explaining the entire Rust macro system is out of scope for this tips page, so |
| you are encouraged to consult the Rust documentation for full details.</p> |
| <h2 id="build-system"><a class="header" href="#build-system">Build System</a></h2> |
| <p>Most Rust crates are built using Cargo (although it is not required). This |
| takes care of many difficult problems with traditional build systems. However, |
| you may wish to customise the build process. Cargo provides <a href="https://doc.rust-lang.org/cargo/reference/build-scripts.html"><code>build.rs</code> |
| scripts</a> for this purpose. They are Rust scripts which can interact with the |
| Cargo build system as required.</p> |
| <p>Common use cases for build scripts include:</p> |
| <ul> |
| <li>provide build-time information, for example statically embedding the build |
| date or Git commit hash into your executable</li> |
| <li>generate linker scripts at build time depending on selected features or other |
| logic</li> |
| <li>change the Cargo build configuration</li> |
| <li>add extra static libraries to link against</li> |
| </ul> |
| <p>At present there is no support for post-build scripts, which you might |
| traditionally have used for tasks like automatic generation of binaries from |
| the build objects or printing build information.</p> |
| <h3 id="cross-compiling"><a class="header" href="#cross-compiling">Cross-Compiling</a></h3> |
| <p>Using Cargo for your build system also simplifies cross-compiling. In most |
| cases it suffices to tell Cargo <code>--target thumbv6m-none-eabi</code> and find a |
| suitable executable in <code>target/thumbv6m-none-eabi/debug/myapp</code>.</p> |
| <p>For platforms not natively supported by Rust, you will need to build <code>libcore</code> |
| for that target yourself. On such platforms, <a href="https://github.com/japaric/xargo">Xargo</a> can be used as a stand-in |
| for Cargo which automatically builds <code>libcore</code> for you.</p> |
| <h2 id="iterators-vs-array-access"><a class="header" href="#iterators-vs-array-access">Iterators vs Array Access</a></h2> |
| <p>In C you are probably used to accessing arrays directly by their index:</p> |
| <pre><code class="language-c">int16_t arr[16]; |
| int i; |
| for(i=0; i<sizeof(arr)/sizeof(arr[0]); i++) { |
| process(arr[i]); |
| } |
| </code></pre> |
| <p>In Rust this is an anti-pattern: indexed access can be slower (as it needs to |
| be bounds checked) and may prevent various compiler optimisations. This is an |
| important distinction and worth repeating: Rust will check for out-of-bounds |
| access on manual array indexing to guarantee memory safety, while C will |
| happily index outside the array.</p> |
| <p>Instead, use iterators:</p> |
| <pre><code class="language-rust ignore">let arr = [0u16; 16]; |
| for element in arr.iter() { |
| process(*element); |
| }</code></pre> |
| <p>Iterators provide a powerful array of functionality you would have to implement |
| manually in C, such as chaining, zipping, enumerating, finding the min or max, |
| summing, and more. Iterator methods can also be chained, giving very readable |
| data processing code.</p> |
| <p>See the <a href="https://doc.rust-lang.org/book/ch13-02-iterators.html">Iterators in the Book</a> and <a href="https://doc.rust-lang.org/core/iter/trait.Iterator.html">Iterator documentation</a> for more details.</p> |
| <h2 id="references-vs-pointers"><a class="header" href="#references-vs-pointers">References vs Pointers</a></h2> |
| <p>In Rust, pointers (called <a href="https://doc.rust-lang.org/book/ch19-01-unsafe-rust.html#dereferencing-a-raw-pointer"><em>raw pointers</em></a>) exist but are only used in specific |
| circumstances, as dereferencing them is always considered <code>unsafe</code> – Rust |
| cannot provide its usual guarantees about what might be behind the pointer.</p> |
| <p>In most cases, we instead use <em>references</em>, indicated by the <code>&</code> symbol, or |
| <em>mutable references</em>, indicated by <code>&mut</code>. References behave similarly to |
| pointers, in that they can be dereferenced to access the underlying values, but |
| they are a key part of Rust’s ownership system: Rust will strictly enforce that |
| you may only have one mutable reference <em>or</em> multiple non-mutable references to |
| the same value at any given time.</p> |
| <p>In practice this means you have to be more careful about whether you need |
| mutable access to data: where in C the default is mutable and you must be |
| explicit about <code>const</code>, in Rust the opposite is true.</p> |
| <p>One situation where you might still use raw pointers is interacting directly |
| with hardware (for example, writing a pointer to a buffer into a DMA peripheral |
| register), and they are also used under the hood for all peripheral access |
| crates to allow you to read and write memory-mapped registers.</p> |
| <h2 id="volatile-access"><a class="header" href="#volatile-access">Volatile Access</a></h2> |
| <p>In C, individual variables may be marked <code>volatile</code>, indicating to the compiler |
| that the value in the variable may change between accesses. Volatile variables |
| are commonly used in an embedded context for memory-mapped registers.</p> |
| <p>In Rust, instead of marking a variable as <code>volatile</code>, we use specific methods |
| to perform volatile access: <a href="https://doc.rust-lang.org/core/ptr/fn.read_volatile.html"><code>core::ptr::read_volatile</code></a> and |
| <a href="https://doc.rust-lang.org/core/ptr/fn.write_volatile.html"><code>core::ptr::write_volatile</code></a>. These methods take a <code>*const T</code> or a <code>*mut T</code> |
| (<em>raw pointers</em>, as discussed above) and perform a volatile read or write.</p> |
| <p>For example, in C you might write:</p> |
| <pre><code class="language-c">volatile bool signalled = false; |
| |
| void ISR() { |
| // Signal that the interrupt has occurred |
| signalled = true; |
| } |
| |
| void driver() { |
| while(true) { |
| // Sleep until signalled |
| while(!signalled) { WFI(); } |
| // Reset signalled indicator |
| signalled = false; |
| // Perform some task that was waiting for the interrupt |
| run_task(); |
| } |
| } |
| </code></pre> |
| <p>The equivalent in Rust would use volatile methods on each access:</p> |
| <pre><code class="language-rust ignore">static mut SIGNALLED: bool = false; |
| |
| #[interrupt] |
| fn ISR() { |
| // Signal that the interrupt has occurred |
| // (In real code, you should consider a higher level primitive, |
| // such as an atomic type). |
| unsafe { core::ptr::write_volatile(&mut SIGNALLED, true) }; |
| } |
| |
| fn driver() { |
| loop { |
| // Sleep until signalled |
| while unsafe { !core::ptr::read_volatile(&SIGNALLED) } {} |
| // Reset signalled indicator |
| unsafe { core::ptr::write_volatile(&mut SIGNALLED, false) }; |
| // Perform some task that was waiting for the interrupt |
| run_task(); |
| } |
| }</code></pre> |
| <p>A few things are worth noting in the code sample:</p> |
| <ul> |
| <li>We can pass <code>&mut SIGNALLED</code> into the function requiring <code>*mut T</code>, since |
| <code>&mut T</code> automatically converts to a <code>*mut T</code> (and the same for <code>*const T</code>)</li> |
| <li>We need <code>unsafe</code> blocks for the <code>read_volatile</code>/<code>write_volatile</code> methods, |
| since they are <code>unsafe</code> functions. It is the programmer’s responsibility |
| to ensure safe use: see the methods’ documentation for further details.</li> |
| </ul> |
| <p>It is rare to require these functions directly in your code, as they will |
| usually be taken care of for you by higher-level libraries. For memory mapped |
| peripherals, the peripheral access crates will implement volatile access |
| automatically, while for concurrency primitives there are better abstractions |
| available (see the <a href="../concurrency/index.html">Concurrency chapter</a>).</p> |
| <h2 id="packed-and-aligned-types"><a class="header" href="#packed-and-aligned-types">Packed and Aligned Types</a></h2> |
| <p>In embedded C it is common to tell the compiler a variable must have a certain |
| alignment or a struct must be packed rather than aligned, usually to meet |
| specific hardware or protocol requirements.</p> |
| <p>In Rust this is controlled by the <code>repr</code> attribute on a struct or union. The |
| default representation provides no guarantees of layout, so should not be used |
| for code that interoperates with hardware or C. The compiler may re-order |
| struct members or insert padding and the behaviour may change with future |
| versions of Rust.</p> |
| <pre class="playground"><code class="language-rust">struct Foo { |
| x: u16, |
| y: u8, |
| z: u16, |
| } |
| |
| fn main() { |
| let v = Foo { x: 0, y: 0, z: 0 }; |
| println!("{:p} {:p} {:p}", &v.x, &v.y, &v.z); |
| } |
| |
| // 0x7ffecb3511d0 0x7ffecb3511d4 0x7ffecb3511d2 |
| // Note ordering has been changed to x, z, y to improve packing.</code></pre> |
| <p>To ensure layouts that are interoperable with C, use <code>repr(C)</code>:</p> |
| <pre class="playground"><code class="language-rust">#[repr(C)] |
| struct Foo { |
| x: u16, |
| y: u8, |
| z: u16, |
| } |
| |
| fn main() { |
| let v = Foo { x: 0, y: 0, z: 0 }; |
| println!("{:p} {:p} {:p}", &v.x, &v.y, &v.z); |
| } |
| |
| // 0x7fffd0d84c60 0x7fffd0d84c62 0x7fffd0d84c64 |
| // Ordering is preserved and the layout will not change over time. |
| // `z` is two-byte aligned so a byte of padding exists between `y` and `z`.</code></pre> |
| <p>To ensure a packed representation, use <code>repr(packed)</code>:</p> |
| <pre class="playground"><code class="language-rust">#[repr(packed)] |
| struct Foo { |
| x: u16, |
| y: u8, |
| z: u16, |
| } |
| |
| fn main() { |
| let v = Foo { x: 0, y: 0, z: 0 }; |
| // References must always be aligned, so to check the addresses of the |
| // struct's fields, we use `std::ptr::addr_of!()` to get a raw pointer |
| // instead of just printing `&v.x`. |
| let px = std::ptr::addr_of!(v.x); |
| let py = std::ptr::addr_of!(v.y); |
| let pz = std::ptr::addr_of!(v.z); |
| println!("{:p} {:p} {:p}", px, py, pz); |
| } |
| |
| // 0x7ffd33598490 0x7ffd33598492 0x7ffd33598493 |
| // No padding has been inserted between `y` and `z`, so now `z` is unaligned.</code></pre> |
| <p>Note that using <code>repr(packed)</code> also sets the alignment of the type to <code>1</code>.</p> |
| <p>Finally, to specify a specific alignment, use <code>repr(align(n))</code>, where <code>n</code> is |
| the number of bytes to align to (and must be a power of two):</p> |
| <pre class="playground"><code class="language-rust">#[repr(C)] |
| #[repr(align(4096))] |
| struct Foo { |
| x: u16, |
| y: u8, |
| z: u16, |
| } |
| |
| fn main() { |
| let v = Foo { x: 0, y: 0, z: 0 }; |
| let u = Foo { x: 0, y: 0, z: 0 }; |
| println!("{:p} {:p} {:p}", &v.x, &v.y, &v.z); |
| println!("{:p} {:p} {:p}", &u.x, &u.y, &u.z); |
| } |
| |
| // 0x7ffec909a000 0x7ffec909a002 0x7ffec909a004 |
| // 0x7ffec909b000 0x7ffec909b002 0x7ffec909b004 |
| // The two instances `u` and `v` have been placed on 4096-byte alignments, |
| // evidenced by the `000` at the end of their addresses.</code></pre> |
| <p>Note we can combine <code>repr(C)</code> with <code>repr(align(n))</code> to obtain an aligned and |
| C-compatible layout. It is not permissible to combine <code>repr(align(n))</code> with |
| <code>repr(packed)</code>, since <code>repr(packed)</code> sets the alignment to <code>1</code>. It is also not |
| permissible for a <code>repr(packed)</code> type to contain a <code>repr(align(n))</code> type.</p> |
| <p>For further details on type layouts, refer to the <a href="https://doc.rust-lang.org/reference/type-layout.html">type layout</a> chapter of the |
| Rust Reference.</p> |
| <h2 id="other-resources"><a class="header" href="#other-resources">Other Resources</a></h2> |
| <ul> |
| <li>In this book: |
| <ul> |
| <li><a href="../interoperability/c-with-rust.html">A little C with your Rust</a></li> |
| <li><a href="../interoperability/rust-with-c.html">A little Rust with your C</a></li> |
| </ul> |
| </li> |
| <li><a href="https://docs.rust-embedded.org/faq.html">The Rust Embedded FAQs</a></li> |
| <li><a href="http://blahg.josefsipek.net/?p=580">Rust Pointers for C Programmers</a></li> |
| <li><a href="https://github.com/diwic/reffers-rs/blob/master/docs/Pointers.md">I used to use pointers - now what?</a></li> |
| </ul> |
| |
| </main> |
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