Debugging C Code on the RP2040 Using gdb: Hints

3 minute read

Where I provide hints on using gdb to debug code on the RP2040 (Pi Pico board).

Sources

gdb Hints

  1. gdb recommended in its screen feedback to use “target extended-remote :3333” instead of the Guide’s “target remote localhost:3333”. I found that this seemed to solve some of these inconsistencies, so I updated my instructions. After a bit of reading “extended-remote” does seem to be the right instruction.
  2. If you find your self executing a lengthy command quite a bit (such as “monitor reset init”), use “define” to create a shorter command for it. If you put it into the .gdbinit file (as I have), it will remember from session to session.
  3. gdb can be incredibly easy to use. Norman Matloff’s tutorial is great for illustrating the commands needed. I recommend reading and practicing with it, its well worth it!
  4. Start with using gdb to replace printf statements. Run code, have it stop by using a “breakpoint function name” or “breakpoint line number”, then use “print” or “display” to get the values of variables. This will work the same as inserting print statements, however, it doesn’t affect your code.
  5. It took me less time to learn gdb, and write these two tutorials than the time I spent attempting to debug Visual Studio Code. Your mileage may vary, however, a simple gdb/code editor setup is fast and easy to learn and use.
  6. If your code ends, you will see “exit(status=0)….”. To restart the program, simply issue a “mri” (a command from .gdbinit) which will reset the processor. Press “c” and the program will begin running again.
  7. gdb operates in at least two modes:
    • command, indicated by (gdb) prompt. This is where you will enter all of the commands to interact with gdb
    • execution, indicated by a lack of a (gdb) prompt. This means the program is running and hitting a breakpoint or watchpoint will return you back to command mode. Or you can hit Ctrl-C which will interrupt the process and put into command mode.
  8. Enter the command layout src or start by using gdb –tui and the window will divide into two horizontal windows. The top window will show 20 lines of source and the bottom window will continue to be the command window. This is a great way to view a breakpoint. For example, set a breakpoint on a function name, then “n(ext)” through the code to view execution. Once you have entered “n”, you can simply hit the return key and it will advance using the next command.
  9. Note found elsewhere, I’ll link to it when I find it: “remember to use -g only for the image you pass to gdb, not the firmware you load into the device (if it can grok elf). Also another very critical prerequisite for C programs is -O0 (which is usually default). But if you use -Os or something, you can get very unpredictable debugging results. This btw. makes debugging some code on avrs for example almost impossible, as they often rely on -Os to fit the image in there. But.. it is very possible and comfy to debug simple optimized C programs in assembly, if you know roughly what to expect. Use objdump -dS on the optimized .elf file with debugging info and you’ll roughly see what assembly code was produced for what C code.”

avr-gdb Hints

  1. Check the version number on avr-gdb as of December 2021, it is 11.1. On Ubuntu, the latest version is 8.1 from 2018 which had quite a few bugs. Attempted to build it manually (which failed), so I spun up a Manjaro (Arch Linux) instance to get the latest. Crazy, I know.
  2. Start avarice in a separate window using, then launch gdb in another window:
    avarice -g -w -d -P atmega328p :3333 	# window 1
    avr-gdb -tui							# window 2
  3. If avarice crashes on startup, cycle power on both the debugger (Dragon) and the target (Uno)

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