2.2 Hardware: References to old example code

[cocoacrumbs]

The embedded rust book in section 2.2 discusses the following example code:

#[entry]
fn main() -> ! {
    hprintln!("Hello, world!").unwrap();

    // exit QEMU
    // NOTE do not run this on hardware; it can corrupt OpenOCD state
    // debug::exit(debug::EXIT_SUCCESS);

    loop {}
}

But when we reach the Debugging part of the section, we do see this text:

Breakpoint 1, main () at examples/hello.rs:15
15          let mut stdout = hio::hstdout().unwrap();

This does not correspond to the example code given earlier. I looked a bit in the history of the example code (https://github.com/rust-embedded/cortex-m-quickstart) and the example code was modified in 2018 already to use the hprintln! macro (commit #9c6b290, "use hprint macros", "japaric committed on Nov 10, 2018").

The second deviation between the book and what I see happening is when setting the break point on fn main():

(gdb) break main

The text suggests the break point is placed on the first line of the main function:

Breakpoint 1, main () at examples/hello.rs:15
15          let mut stdout = hio::hstdout().unwrap();

This is not the same as I see happening in my setup (rustc 1.49.0 (e1884a8e3 2020-12-29)):

Breakpoint 1, hello::__cortex_m_rt_main_trampoline () at examples/hello.rs:11
11  #[entry]

Thus just before the fn main(). When issuing a next gdb command (as the book suggests), gdb will simply step over the fn main() function and then basically hangs because of the endless loop () inside fn main().

When using the step gdb command it's possible to step inside fn main()' and then execute thehprintln!` command on its own as intended by the text.

I guess the above deviations might be due more recent Rust implementations?

[neilpate]

I also just ran through this same issue and was similarly confused by this. This was my first exposure to gdb, so changing the text to mention being able to step would be helpful.

[eldruin]

Sorry about that. The whole ecosystem has been evolving. We would be glad for PRs that update the instructions.

[neilpate]

OK, great. I am currently playing around with this in my spare time, so I will try and get a PR done this week.

[neilpate]

I have created a PR to address this. https://github.com/rust-embedded/book/pull/323

[eldruin]

Fixed in #323. Thank you!