Panic

If something goes very wrong, a program will panic!.

esp-backtrace provides a panic! implementation through the panic-handler feature.

"Handling panic!" here means to print what was executed up to that point (the backtrace), and probably stops code execution as well.

We could use our own panic-handler instead, and no backtrace at all. For example, this one:

use core::panic::PanicInfo;
use core::sync::atomic::{self, Ordering};

#[inline(never)]
#[panic_handler]
fn panic(_info: &PanicInfo) -> ! {
    loop {
        atomic::compiler_fence(Ordering::SeqCst);
    }
}

Warning

We need to use esp-backtrace as _; so that this handler is included in the final binary.

In the exercise below, we configure a profile. Profiles are configurations cargo uses to control compilation. When unspecified, cargo sets sensible defaults for us.

Exercise 1

0. Access the project at exercises/panic.

1. Add all the boilerplate code described earlier.

2. Add a panic! within main.

   • But remember, it's for irrecoverable errors!

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.

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

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

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

Exercise 2

1. Edit the .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.

2. Try another option:

   +[profile.dev]
   +opt-level = "s"
   

   which turns size-optimisation on, and has debug info by default. Run it with cargo run (no --release flag since we want to run the dev profile.)

Recap

• Using esp-backtrace.
• Using panic! to exit the program on error.
• Profiles:
  • Tweak debug information on backtrace.
  • Optimise binary size.

Suggested Reading

• esp-backtrace.