Remote Debugging with GDB
DevkitPro comes with gdb, the gnu debugger, which has been configured to allow remote debugging over the USB Gecko. You compile a stub of code into your wii program and run the debugger on your pc. Firstly include the code which establishes the connection:
You can then manually insert breakpoint into your code with the line:
Your wii code must be compiled with the option -g linked with the option -ldb, included before other libraries such as -logc.
Start your wii code using normal means such as the homebrew channel. There are three debugging commands available with devkitPro are:
- powerpc-gekko-gdb: the basic text version of gdb driven from the command line
- powerpc-gekko-gdbtui: a version of gdb which includes a text window showing the source code of your application
- powerpc-gekko-insight: a custom version of Insight, a full GUI built on top of gdb, available as a separate package within devkitPro
Assuming that you have started the text based gdb, at the command prompt type:
target remote /dev/ttyUSB0
Tell gdb where to find symbol information by typing
You can then insert other break points manually by typing for example (assuming you are in the same directory as the source file):
If sources are in another directory, you can tell gdb where with the command:
You can then continue running the program by typing:
Particularly if you have hardcoded a breakpoint with the _break() function, it is useful to be able to run until the current function exits with the command:
Examine the value of variables by typing
The above examples are confirmed to work under linux but may require some modification to work under windows/msys (particularly /dev/ttyUSB0).
When faced with a crash in your Homebrew, often you'll see a code dump with an address and some machine code. Here's my trick to track that back to a line of C++ code.
For example if your homebrew game crashes it might show something like this:
CODE DUMP: 800084ac: 809F0020 2F840000 ... 800084bc: ... 800084cc: ...
The 800084ac is the memory address in hex of where the crash occurred. 809F0020 is the machine code for the offending instruction.
Using gdb, you can find out where the relevant code is with the command:
gdb info line *0x800084ac
This should work even if you don't have a USB gecko, as it is only using the symbol information stored in the .elf file.
- Step 1:
In your makefile change the CXXFLAGS line to the following:
CXXFLAGS = -save-temps -Xassembler -aln=$@.lst $(CFLAGS)
The "-save-temps" will save the assembly language file, which can be interesting. The "-Xassembler -aln=$@.lst" creates a list file which contains the assembly and the machine code. Now recompile your entire project. Note, this just affects C++ code.
- Step 2:
Look at the map file that was built. The mapfile is on by default in the Wii template makefile. Typically it's in the build subdirectory and called something.map. Look in that mapfile for the nearest memory address that doesn't go over the one found in the CODE DUMP. Here is an example:
This tells me that the crash was 72 bytes into the ShooterView::Render() function. Now to find the line number in Render()
- Step 3:
Look at the list file for the relevant function. Here's an example:
473 .globl _ZN11ShooterView6RenderER17BibGraphicsDevice 474 .type _ZN11ShooterView6RenderER17BibGraphicsDevice, @function 475 _ZN11ShooterView6RenderER17BibGraphicsDevice: 476 .LFB1465: 477 .loc 1 158 0 478 .LVL20: 479 02d0 9421FF00 stwu 1,-256(1)
The function names are mangled because this is C++ code. See http://en.wikipedia.org/wiki/Name_mangling#Name_mangling_in_C.2B.2B The address of the first instruction of Render() is at 02d0. This is also line 158 in the file (".loc 1 158 0"). To find the error location, just look at 0x2d0 + 72 = 0x318. See below:
.loc 1 168 0 528 0314 809F0020 lwz 4,32(31) 529 0318 2F840000 cmpwi 7,4,0
This shows machine address 0x318 has the proper machine code and the nearest .loc statement says the problem is at line 168 of the ShooterView.cpp. For more info on the assembler output see the manual here: http://sourceware.org/binutils/docs-2.18/as/index.html