Debug it

We are already inside a debugging session so let's debug our program.

After the load command, our program is stopped at its entry point. This is indicated by the "Start address 0x8000XXX" part of GDB's output. The entry point is the part of a program that a processor / CPU will execute first.

The starter project I've provided to you has some extra code that runs before the main function. At this time, we are not interested in that "pre-main" part so let's skip right to the beginning of the main function. We'll do that using a breakpoint. Issue break main at the (gdb) prompt:

Note for these gdb commands I generally won't provide a copyable code block as these are short and it's faster just to type them yourself. In addition most can be shortend. For instance b for break or s for step, see gdb quick ref for more info or use Google to find your others. In addition, you can use tab completion by typing the first few letters than one tab to complete or two tabs to see all possible commands.

Finally, help xxxx where xxxx is the comand will provide short names and other info:

(gdb) help s
step, s
Step program until it reaches a different source line.
Usage: step [N]
Argument N means step N times (or till program stops for another reason).
(gdb) break main
Breakpoint 1 at 0x80001f0: file src/05-led-roulette/src/main.rs, line 7.
Note: automatically using hardware breakpoints for read-only addresses.

Next issue a continue command:

(gdb) continue
Continuing.

Breakpoint 1, led_roulette::__cortex_m_rt_main_trampoline () at src/05-led-roulette/src/main.rs:7
7       #[entry]

Breakpoints can be used to stop the normal flow of a program. The continue command will let the program run freely until it reaches a breakpoint. In this case, until it reaches #[entry] which is a trampoline to the main function and where break main sets the breakpoint.

Note that GDB output says "Breakpoint 1". Remember that our processor can only use six of these breakpoints so it's a good idea to pay attention to these messages.

OK. Since we are stopped at #[entry] and using the disassemble /m we see the code for entry, which is a trampoline to main. What that means it sets up the stack and then invokes a subroutine call to the main function using an ARM branch and link instruction, bl.

(gdb) disassemble /m
Dump of assembler code for function main:
8       #[entry]
   0x080001ec <+0>:     push    {r7, lr}
   0x080001ee <+2>:     mov     r7, sp
=> 0x080001f0 <+4>:     bl      0x80001f6 <hello_world::__cortex_m_rt_main>
   0x080001f4 <+8>:     udf     #254    ; 0xfe

End of assembler dump.

Next we need to issue a step gdb command which will advance the program statement by statement stepping into functions/procedures. So after this first step command we're inside main and are positioned at the first executable rust statement, line 10, but it is not executed:

(gdb) step
led_roulette::__cortex_m_rt_main () at src/05-led-roulette/src/main.rs:10
10          let x = 42;

Next we'll issue a second step which executes line 10 and stops at line 11 _y = x;, again line 11 is not executed.

Note we could have pressed enter at the second (gdb) prompt and it would have reissued the previous statement, step, but for clarity in this tutorial we'll generally retype the command.

(gdb) step
11          _y = x;

As you can see, in this mode, on each step command GDB will print the current statement along along with its line number. As you'll see later in the TUI mode you'll not the see the statement in the command area.

We are now "on" the _y = x statement; that statement hasn't been executed yet. This means that x is initialized but _y is not. Let's inspect those stack/local variables using the print command, p for short:

(gdb) print x
$1 = 42
(gdb) p &x
$2 = (*mut i32) 0x20009fe0
(gdb) p _y
$3 = 536870912
(gdb) p &_y
$4 = (*mut i32) 0x20009fe4

As expected, x contains the value 42. _y, however, contains the value 536870912 (?). This is because _y has not been initialized yet, it contains some garbage value.

The command print &x prints the address of the variable x. The interesting bit here is that GDB output shows the type of the reference: *mut i32, a mutable pointer to an i32 value. Another interesting thing is that the addresses of x and _y are very close to each other: their addresses are just 4 bytes apart.

Instead of printing the local variables one by one, you can also use the info locals command:

(gdb) info locals
x = 42
_y = 536870912

OK. With another step, we'll be on top of the loop {} statement:

(gdb) step
14          loop {}

And _y should now be initialized.

(gdb) print _y
$5 = 42

If we use step again on top of the loop {} statement, we'll get stuck because the program will never pass that statement.

NOTE If you used the step or any other command by mistake and GDB gets stuck, you can get it unstuck by hitting Ctrl+C.

As introduced above the disassemble /m command can be used to disassemble the program around the line you are currently at. You might also want to set print asm-demangle on so the names are demangled, this only needs to be done once a debug session. Later this and other commands will be placed in an initialization file which will simplify starting a debug session.

(gdb) set print asm-demangle on
(gdb) disassemble /m
Dump of assembler code for function _ZN12led_roulette18__cortex_m_rt_main17h51e7c3daad2af251E:
8       fn main() -> ! {
   0x080001f6 <+0>:     sub     sp, #8
   0x080001f8 <+2>:     movs    r0, #42 ; 0x2a

9           let _y;
10          let x = 42;
   0x080001fa <+4>:     str     r0, [sp, #0]

11          _y = x;
   0x080001fc <+6>:     str     r0, [sp, #4]

12
13          // infinite loop; just so we don't leave this stack frame
14          loop {}
=> 0x080001fe <+8>:     b.n     0x8000200 <led_roulette::__cortex_m_rt_main+10>
   0x08000200 <+10>:    b.n     0x8000200 <led_roulette::__cortex_m_rt_main+10>

End of assembler dump.

See the fat arrow => on the left side? It shows the instruction the processor will execute next.

Also, as mentioned above if you were to execute the step command GDB gets stuck because it is executing a branch instruction to itself and never gets past it. So you need to use Ctrl+C to regain control. An alternative is to use the stepi(si) GDB command, which steps one asm instruction, and GDB will print the address and line number of the statement the processor will execute next and it won't get stuck.

(gdb) stepi
0x08000194      14          loop {}

(gdb) si
0x08000194      14          loop {}

One last trick before we move to something more interesting. Enter the following commands into GDB:

(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) continue
Continuing.

Breakpoint 1, led_roulette::__cortex_m_rt_main_trampoline () at src/05-led-roulette/src/main.rs:7
7       #[entry]

(gdb) disassemble /m
Dump of assembler code for function main:
7       #[entry]
   0x080001ec <+0>:     push    {r7, lr}
   0x080001ee <+2>:     mov     r7, sp
=> 0x080001f0 <+4>:     bl      0x80001f6 <led_roulette::__cortex_m_rt_main>
   0x080001f4 <+8>:     udf     #254    ; 0xfe

End of assembler dump.

We are now back at the beginning of #[entry]!

monitor reset halt will reset the microcontroller and stop it right at the beginning of the program. The continue command will then let the program run freely until it reaches a breakpoint, in this case it is the breakpoint at #[entry].

This combo is handy when you, by mistake, skipped over a part of the program that you were interested in inspecting. You can easily roll back the state of your program back to its very beginning.

The fine print: This reset command doesn't clear or touch RAM. That memory will retain its values from the previous run. That shouldn't be a problem though, unless your program behavior depends of the value of uninitialized variables but that's the definition of Undefined Behavior (UB).

We are done with this debug session. You can end it with the quit command.

(gdb) quit
A debugging session is active.

        Inferior 1 [Remote target] will be detached.

Quit anyway? (y or n) y
Detaching from program: $PWD/target/thumbv7em-none-eabihf/debug/led-roulette, Remote target
Ending remote debugging.

For a nicer debugging experience, you can use GDB's Text User Interface (TUI). To enter into that mode enter one of the following commands in the GDB shell:

(gdb) layout src
(gdb) layout asm
(gdb) layout split

NOTE Apologies to Windows users, the GDB shipped with the GNU ARM Embedded Toolchain may not support this TUI mode :-(.

Below is an example of setting up for a layout split by executing the follow commands. As you can see we've dropped passing the --target parameter:

$ cargo run
(gdb) target remote :3333
(gdb) load
(gdb) set print asm-demangle on
(gdb) set style sources off
(gdb) break main
(gdb) continue

Here is a command line with the above commands as -ex parameters to save you some typing, shortly we'll be providing an easier way to execute the initial set of commands:

cargo run -- -q -ex 'target remote :3333' -ex 'load' -ex 'set print asm-demangle on' -ex 'set style sources off' -ex 'b main' -ex 'c' target/thumbv7em-none-eabihf/debug/led-roulette

And below is the result:

GDB session layout split

Now we'll scroll the top source window down so we see the entire file and execute layout split and then step:

GDB session layout split

Then we'll execute a few info locals and step's:

(gdb) info locals
(gdb) step
(gdb) info locals
(gdb) step
(gdb) info locals

GDB session layout split

At any point you can leave the TUI mode using the following command:

(gdb) tui disable

GDB session layout split

NOTE If the default GDB CLI is not to your liking check out gdb-dashboard. It uses Python to turn the default GDB CLI into a dashboard that shows registers, the source view, the assembly view and other things.

Don't close OpenOCD though! We'll use it again and again later on. It's better just to leave it running. If you want to learn more about what GDB can do, check out the section How to use GDB.

What's next? The high level API I promised.