Embedded Rust Basics.

If something goes wrong, the program will panic!. In embedded, we will always need to add a panic-handler.

esp-backtrace provides a panic-handler feature that handles panic!. This is why we need to use esp-backtrace as _.

Let’s access the project at exercises/panic, and modify the code to test panic!.

We will also take a look at compilation profiles. We don’t need deep knowledge about profiles, just the very basics.

cargo uses profile-settings to control compilation using defaults when unspecified. We are interested in controlling a few aspects:

• Should it be as fast as possible? As short as possible?
• Should it include information useful for debugging?

Exercise: part 1

1. In main.rs use the esp-backtrace crate.

2. Then add a panic! somewhere, e.g. after our println.

3. Run the code with cargo run; this uses the development profile.

   • It outputs debug information into the compiled binary.

4. Then run with release profile cargo run --release.

   • This profile will not output all debug information in the binary. We should find:

     Hello world!
     ====================== PANIC ======================
     panicked at examples/panic.rs:24:5:
     This is a panic
     Backtrace:
     0x4200252a
     main
         at ??:??
     

     The default --release behaviour excludes debug information and minimises the binary size; the backtrace shows the missing debug information with ??.

Exercise: part 2

We nearly always use --release (we want small binary size). If we want debug information we need to configure the release profile in .cargo/config.toml:

+[profile.release]
+debug = true

Now it will emit debug information in the ELF binary file; yet debug info isn’t flashed into the target, it is just used to display the backtrace.

• Re-run the program with --release and confirm ??:?? is now filled in.

exercises/panic/examples/panic.rs contains a solution. It can be run with: cargo run --example panic --release.

Recap

• esp-backtrace handles panics (by us or by any library).
• We can manually use panic! to exit the program.
• Configure compilation so that panics print a backtrace with all debug information.

Suggested Reading

• esp-backtrace