CS 202 - Notes 2016-04-06

Debugging with GDB

A good starting place is Professor Johnson’s guide to GDB. This will give you a good run down on the basic commands.

We have a couple of commands that are useful to us that are not included in that guide.

nexti (ni) & stepi (si)

The next and step commands only work when debugging data is included in the file and they operate on the level of lines of C code. nexti and stepi, on the other hand, work at the instruction level, allowing us to walk through the code one instruction at a time. As with next and step, nexti will “step over” the call instruction, while stepi will follow the jump into the function.

Recommendation for assembly level debugging

• Start by getting your bearings
  • disassemble function_name is a good place to start
  • if you don’t know any function names, use “main” and look for call instructions
• Set some breakpoints
  • b function_name will put a breakpoint at the start of the named function
  • you can also use b \* address to set a breakpoint at a particular address (which you will find when you disassemble)
• I suggest using d \i $pc, to display the next instruction to be run after every call to nexti or stepi
• Look at the registers using info reg or use display or print to look at specific ones
  • Note that we use “$” instead of “%” when giving them to gdb