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<ol class="chapter"><li><a href="../intro/index.html"><strong aria-hidden="true">1.</strong> Introduction</a></li><li><ol class="section"><li><a href="../intro/hardware.html"><strong aria-hidden="true">1.1.</strong> Hardware</a></li><li><a href="../intro/no-std.html"><strong aria-hidden="true">1.2.</strong> no_std</a></li><li><a href="../intro/tooling.html"><strong aria-hidden="true">1.3.</strong> Tooling</a></li><li><a href="../intro/install.html"><strong aria-hidden="true">1.4.</strong> Installation</a></li><li><ol class="section"><li><a href="../intro/install/linux.html"><strong aria-hidden="true">1.4.1.</strong> Linux</a></li><li><a href="../intro/install/macos.html"><strong aria-hidden="true">1.4.2.</strong> MacOS</a></li><li><a href="../intro/install/windows.html"><strong aria-hidden="true">1.4.3.</strong> Windows</a></li><li><a href="../intro/install/verify.html"><strong aria-hidden="true">1.4.4.</strong> Verify Installation</a></li></ol></li></ol></li><li><a href="../start/index.html"><strong aria-hidden="true">2.</strong> Getting started</a></li><li><ol class="section"><li><a href="../start/qemu.html"><strong aria-hidden="true">2.1.</strong> QEMU</a></li><li><a href="../start/hardware.html"><strong aria-hidden="true">2.2.</strong> Hardware</a></li><li><a href="../start/registers.html"><strong aria-hidden="true">2.3.</strong> Memory-mapped Registers</a></li><li><a href="../start/semihosting.html" class="active"><strong aria-hidden="true">2.4.</strong> Semihosting</a></li><li><a href="../start/panicking.html"><strong aria-hidden="true">2.5.</strong> Panicking</a></li><li><a href="../start/exceptions.html"><strong aria-hidden="true">2.6.</strong> Exceptions</a></li><li><a href="../start/interrupts.html"><strong aria-hidden="true">2.7.</strong> Interrupts</a></li><li><a href="../start/io.html"><strong aria-hidden="true">2.8.</strong> IO</a></li></ol></li><li><a href="../peripherals/index.html"><strong aria-hidden="true">3.</strong> Peripherals</a></li><li><ol class="section"><li><a href="../peripherals/a-first-attempt.html"><strong aria-hidden="true">3.1.</strong> A first attempt in Rust</a></li><li><a href="../peripherals/borrowck.html"><strong aria-hidden="true">3.2.</strong> The Borrow Checker</a></li><li><a href="../peripherals/singletons.html"><strong aria-hidden="true">3.3.</strong> Singletons</a></li></ol></li><li><a href="../static-guarantees/index.html"><strong aria-hidden="true">4.</strong> Static Guarantees</a></li><li><ol class="section"><li><a href="../static-guarantees/typestate-programming.html"><strong aria-hidden="true">4.1.</strong> Typestate Programming</a></li><li><a href="../static-guarantees/state-machines.html"><strong aria-hidden="true">4.2.</strong> Peripherals as State Machines</a></li><li><a href="../static-guarantees/design-contracts.html"><strong aria-hidden="true">4.3.</strong> Design Contracts</a></li><li><a href="../static-guarantees/zero-cost-abstractions.html"><strong aria-hidden="true">4.4.</strong> Zero Cost Abstractions</a></li></ol></li><li><a href="../portability/index.html"><strong aria-hidden="true">5.</strong> Portability</a></li><li><a href="../concurrency/index.html"><strong aria-hidden="true">6.</strong> Concurrency</a></li><li><a href="../collections/index.html"><strong aria-hidden="true">7.</strong> Collections</a></li><li><a href="../c-tips/index.html"><strong aria-hidden="true">8.</strong> Tips for embedded C developers</a></li><li><a href="../interoperability/index.html"><strong aria-hidden="true">9.</strong> Interoperability</a></li><li><ol class="section"><li><a href="../interoperability/c-with-rust.html"><strong aria-hidden="true">9.1.</strong> A little C with your Rust</a></li><li><a href="../interoperability/rust-with-c.html"><strong aria-hidden="true">9.2.</strong> A little Rust with your C</a></li></ol></li><li><a href="../unsorted/index.html"><strong aria-hidden="true">10.</strong> Unsorted topics</a></li><li><ol class="section"><li><a href="../unsorted/speed-vs-size.html"><strong aria-hidden="true">10.1.</strong> Optimizations: The speed size tradeoff</a></li></ol></li></ol>
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<h1 class="menu-title">The Embedded Rust Book</h1>
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<a class="header" href="#semihosting" id="semihosting"><h1>Semihosting</h1></a>
<p>Semihosting is a mechanism that lets embedded devices do I/O on the host and is
mainly used to log messages to the host console. Semihosting requires a debug
session and pretty much nothing else (no extra wires!) so it's super convenient
to use. The downside is that it's super slow: each write operation can take
several milliseconds depending on the hardware debugger (e.g. ST-Link) you use.</p>
<p>The <a href="https://crates.io/crates/cortex-m-semihosting"><code>cortex-m-semihosting</code></a> crate provides an API to do semihosting operations
on Cortex-M devices. The program below is the semihosting version of "Hello,
world!":</p>
<pre><code class="language-rust ignore">#![no_main]
#![no_std]
extern crate panic_halt;
use cortex_m_rt::entry;
use cortex_m_semihosting::hprintln;
#[entry]
fn main() -> ! {
hprintln!("Hello, world!").unwrap();
loop {}
}
</code></pre>
<p>If you run this program on hardware you'll see the "Hello, world!" message
within the OpenOCD logs.</p>
<pre><code class="language-console">$ openocd
(..)
Hello, world!
(..)
</code></pre>
<p>You do need to enable semihosting in OpenOCD from GDB first:</p>
<pre><code class="language-console">(gdb) monitor arm semihosting enable
semihosting is enabled
</code></pre>
<p>QEMU understands semihosting operations so the above program will also work with
<code>qemu-system-arm</code> without having to start a debug session. Note that you'll
need to pass the <code>-semihosting-config</code> flag to QEMU to enable semihosting
support; these flags are already included in the <code>.cargo/config</code> file of the
template.</p>
<pre><code class="language-console">$ # this program will block the terminal
$ cargo run
Running `qemu-system-arm (..)
Hello, world!
</code></pre>
<p>There's also an <code>exit</code> semihosting operation that can be used to terminate the
QEMU process. Important: do <strong>not</strong> use <code>debug::exit</code> on hardware; this function
can corrupt your OpenOCD session and you will not be able to debug more programs
until you restart it.</p>
<pre><code class="language-rust ignore">#![no_main]
#![no_std]
extern crate panic_halt;
use cortex_m_rt::entry;
use cortex_m_semihosting::debug;
#[entry]
fn main() -> ! {
let roses = "blue";
if roses == "red" {
debug::exit(debug::EXIT_SUCCESS);
} else {
debug::exit(debug::EXIT_FAILURE);
}
loop {}
}
</code></pre>
<pre><code class="language-console">$ cargo run
Running `qemu-system-arm (..)
$ echo $?
1
</code></pre>
<p>One last tip: you can set the panicking behavior to <code>exit(EXIT_FAILURE)</code>. This
will let you write <code>no_std</code> run-pass tests that you can run on QEMU.</p>
<p>For convenience, the <code>panic-semihosting</code> crate has an "exit" feature that when
enabled invokes <code>exit(EXIT_FAILURE)</code> after logging the panic message to the host
stderr.</p>
<pre><code class="language-rust ignore">#![no_main]
#![no_std]
extern crate panic_semihosting; // features = ["exit"]
use cortex_m_rt::entry;
use cortex_m_semihosting::debug;
#[entry]
fn main() -> ! {
let roses = "blue";
assert_eq!(roses, "red");
loop {}
}
</code></pre>
<pre><code class="language-console">$ cargo run
Running `qemu-system-arm (..)
panicked at 'assertion failed: `(left == right)`
left: `"blue"`,
right: `"red"`', examples/hello.rs:15:5
$ echo $?
1
</code></pre>
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