Parts of an Embedded Program

We will look at the elements that distinguish an embedded Rust program from a desktop program.

✅ Open the stm32-code folder in VS Code.

# or use "File > Open Folder" in VS Code
code stm32-code

✅ Then open the standalone-app/src/bin/standalone-hello.rs file.

Attributes

In the file, you will find the following attributes:

#![no_std]

The #![no_std] language attribute indicates that the program will not make use of the standard library, the std crate. Instead it will use the core library, a subset of the standard library that does not depend on an underlying operating system (OS).

#![no_main]

The #![no_main] language attribute indicates that the program will use a custom entry point instead of the default fn main() { .. } one.

#[entry]

The #[entry] macro attribute marks the custom entry point of the program. The entry point must be a divergent function whose return type is the never type !. The function is not allowed to return; therefore the program is not allowed to terminate. The macro comes from the cortex-m-rt crate and is not part of the Rust language.

#[panic_handler]

Rust requires we supply a function to be executed whenever the system panics. We mark our function with this attribute and the compiler automatically links it into the panic handling infrastructure in libcore.

If we remove this function, or change the function signature, we get a compile error. You are allowed to put the panic handler in a library crate, but here we’ve just written a short one so you can see it. We’ll talk more about the panic handler later.