The panic! macro also sends its output to the ITM!

Change the main function to look like this:

fn main() -> ! {
    panic!("Hello, world!");

Before running one other suggestion, I find it inconvenient to have to confirm when quitting gdb. Add the following file in your home directory ~/.gdbinit so that it quits immediately:

$ cat ~/.gdbinit
define hook-quit
  set confirm off

OK, now use cargo run and it stops at the first line of fn main():

$ cargo run
   Compiling hello-world v0.2.0 (~/embedded-discovery/src/06-hello-world)
    Finished dev [unoptimized + debuginfo] target(s) in 0.11s
     Running `arm-none-eabi-gdb -q -x ../openocd.gdb ~/embedded-discovery/target/thumbv7em-none-eabihf/debug/hello-world`
Reading symbols from ~/embedded-discovery/target/thumbv7em-none-eabihf/debug/hello-world...
hello_world::__cortex_m_rt_main () at ~/embedded-discovery/src/06-hello-world/src/
10          panic!("Hello, world!");
Loading section .vector_table, size 0x194 lma 0x8000000
Loading section .text, size 0x20fc lma 0x8000194
Loading section .rodata, size 0x554 lma 0x8002290
Start address 0x08000194, load size 10212
Transfer rate: 17 KB/sec, 3404 bytes/write.
Breakpoint 1 at 0x80001f0: file ~/embedded-discovery/src/06-hello-world/src/, line 8.
Note: automatically using hardware breakpoints for read-only addresses.
Breakpoint 2 at 0x8000222: file ~/.cargo/registry/src/, line 570.
Breakpoint 3 at 0x800227a: file ~/.cargo/registry/src/, line 560.

Breakpoint 1, hello_world::__cortex_m_rt_main_trampoline () at ~/embedded-discovery/src/06-hello-world/src/
8       #[entry]
hello_world::__cortex_m_rt_main () at ~/embedded-discovery/src/06-hello-world/src/
10          panic!("Hello, world!");

We'll use short command names to save typing, enter c then the Enter or Return key:

(gdb) c

If all is well you'll see some new output in the itmdump terminal.

$ # itmdump terminal
panicked at 'Hello, world!', src/06-hello-world/src/

Then type Ctrl-c which breaks out of a loop in the runtime:

Program received signal SIGINT, Interrupt.
0x0800115c in panic_itm::panic (info=0x20009fa0) at ~/.cargo/registry/src/
57	        atomic::compiler_fence(Ordering::SeqCst);

Ultimately, panic! is just another function call so you can see it leaves behind a trace of function calls. This allows you to use backtrace or just bt and to see call stack that caused the panic:

(gdb) bt
#0  panic_itm::panic (info=0x20009fa0) at ~/.cargo/registry/src/
#1  0x080005c2 in core::panicking::panic_fmt () at library/core/src/
#2  0x0800055a in core::panicking::panic () at library/core/src/
#3  0x08000210 in hello_world::__cortex_m_rt_main () at src/06-hello-world/src/
#4  0x080001f4 in hello_world::__cortex_m_rt_main_trampoline () at src/06-hello-world/src/

Another thing we can do is catch the panic before it does the logging. So we'll do several things; reset to the beginning, disable breakpoint 1, set a new breakpoint at rust_begin_unwind, list the break points and then continue:

(gdb) monitor reset halt
Unable to match requested speed 1000 kHz, using 950 kHz
Unable to match requested speed 1000 kHz, using 950 kHz
adapter speed: 950 kHz
target halted due to debug-request, current mode: Thread 
xPSR: 0x01000000 pc: 0x08000194 msp: 0x2000a000

(gdb) disable 1

(gdb) break rust_begin_unwind 
Breakpoint 4 at 0x800106c: file ~/.cargo/registry/src/, line 47.

(gdb) info break
Num     Type           Disp Enb Address    What
1       breakpoint     keep n   0x080001f0 in hello_world::__cortex_m_rt_main_trampoline 
                                           at ~/prgs/rust/tutorial/embedded-discovery/src/06-hello-world/src/
        breakpoint already hit 1 time
2       breakpoint     keep y   0x08000222 in cortex_m_rt::DefaultHandler_ 
                                           at ~/.cargo/registry/src/
3       breakpoint     keep y   0x0800227a in cortex_m_rt::HardFault_ 
                                           at ~/.cargo/registry/src/
4       breakpoint     keep y   0x0800106c in panic_itm::panic 
                                           at ~/.cargo/registry/src/

(gdb) c

Breakpoint 4, panic_itm::panic (info=0x20009fa0) at ~/.cargo/registry/src/
47          interrupt::disable();

You'll notice that nothing got printed on the itmdump console this time. If you resume the program using continue then a new line will be printed.

In a later section we'll look into other simpler communication protocols.

Finally, enter the q command to quit and it quits immediately without asking for confirmation:

(gdb) q
Detaching from program: ~/prgs/rust/tutorial/embedded-discovery/target/thumbv7em-none-eabihf/debug/hello-world, Remote target
Ending remote debugging.
[Inferior 1 (Remote target) detached]

As an even shorter sequence you can type Ctrl-d, which eliminates one keystroke!

NOTE In this case the (gdb) prompt is overwritten with quit)

Detaching from program: ~/prgs/rust/tutorial/embedded-discovery/target/thumbv7em-none-eabihf/debug/hello-world, Remote target
Ending remote debugging.
[Inferior 1 (Remote target) detached]