# The challenge

You are now well armed to face a challenge! Your task will be to implement the application I showed you at the beginning of this chapter.

Here's the GIF again:

Also, this may help:

This is a timing diagram. It indicates which LED is on at any given instant of time and for how long each LED should be on. On the X axis we have the time in milliseconds. The timing diagram shows a single period. This pattern will repeat itself every 800 ms. The Y axis labels each LED with a cardinal point: North, East, etc. As part of the challenge you'll have to figure out how each element in the LEDS array maps to these cardinal points (hint: cargo doc --open ;-)).

Before you attempt this challenge, let me give you one last tip. Our GDB sessions always involve entering the same commands at the beginning. We can use a .gdbinit file to execute some commands right after GDB is started. This way you can save yourself the effort of having to enter them manually on each GDB session.

Place this .gdbinit file at the root of the Cargo project, right next to the Cargo.toml:

target remote :3333
break main
continue


With that in place, you should now be able to start a gdb session that will automatically flash the program and jump to the beginning of main:

$arm-none-eabi-gdb -q target/thumbv7em-none-eabihf/debug/led-roulette Reading symbols from target/thumbv7em-none-eabihf/debug/led-roulette...done. (..) Loading section .text, size 0x2014 lma 0x8000000 Start address 0x8000194, load size 8212 Transfer rate: 15 KB/sec, 8212 bytes/write. Breakpoint 1 at 0x80001e6: file$PWD/src/main.rs, line 12.
Note: automatically using hardware breakpoints for read-only addresses.

Breakpoint 1, led_roulette::main () at $PWD/src/main.rs:12 12 pub fn main() -> ! { (gdb)  But if that doesn't work and, instead, you get this: $ arm-none-eabi-gdb -q target/thumbv7em-none-eabihf/debug/led-roulette
Reading symbols from target/thumbv7em-none-eabihf/debug/led-roulette...done.
warning: File "$PWD/.gdbinit" auto-loading has been declined by your auto-load safe-path' set to "$debugdir:$datadir/auto-load". To enable execution of this file add add-auto-load-safe-path$PWD/.gdbinit
line to your configuration file "$HOME/.gdbinit". To completely disable this security protection add set auto-load safe-path / line to your configuration file "$HOME/.gdbinit".
"Auto-loading safe path" section in the GDB manual.  E.g., run from the shell:


You'll have to do a few extra steps. It's definitively worth it though.

## *nix

This command should do the trick.

\$ echo 'set auto-load safe-path /' > ~/.gdbinit


The project local .gdbinit should work now.

## Windows

AFAIK, Windows doesn't set a %HOME% env variable by default so you'll have to add that variable to your environment first. I recommend you set it to %USERPROFILE% (e.g. C:\Users\japaric).

Then you have to create a .gdbinit file in %HOME% (e.g. C:\Users\japaric\.gdbinit) with these contents:

set auto-load safe-path /


The project local .gdbinit` should work now.