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| <h1 id="semihosting"><a class="header" href="#semihosting">Semihosting</a></h1> |
| <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] |
| |
| use panic_halt as _; |
| |
| 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-text">$ 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.toml</code> file of the |
| template.</p> |
| <pre><code class="language-text">$ # 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] |
| |
| use panic_halt as _; |
| |
| 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-text">$ cargo run |
| Running `qemu-system-arm (..) |
| |
| $ echo $? |
| 1 |
| </code></pre> |
| <p>One last tip: you can set the panicking behavior to <code>exit(EXIT_FAILURE)</code>. This |
| will let you write <code>no_std</code> run-pass tests that you can run on QEMU.</p> |
| <p>For convenience, the <code>panic-semihosting</code> crate has an "exit" feature that when |
| enabled invokes <code>exit(EXIT_FAILURE)</code> after logging the panic message to the host |
| stderr.</p> |
| <pre><code class="language-rust ignore">#![no_main] |
| #![no_std] |
| |
| use panic_semihosting as _; // features = ["exit"] |
| |
| use cortex_m_rt::entry; |
| use cortex_m_semihosting::debug; |
| |
| #[entry] |
| fn main() -> ! { |
| let roses = "blue"; |
| |
| assert_eq!(roses, "red"); |
| |
| loop {} |
| }</code></pre> |
| <pre><code class="language-text">$ 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> |
| <p><strong>NOTE</strong>: To enable this feature on <code>panic-semihosting</code>, edit your |
| <code>Cargo.toml</code> dependencies section where <code>panic-semihosting</code> is specified with:</p> |
| <pre><code class="language-toml">panic-semihosting = { version = "VERSION", features = ["exit"] } |
| </code></pre> |
| <p>where <code>VERSION</code> is the version desired. For more information on dependencies |
| features check the <a href="https://doc.rust-lang.org/cargo/reference/specifying-dependencies.html"><code>specifying dependencies</code></a> section of the Cargo book.</p> |
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