Debugging (1)

Intro

• Programs fail.
• How do we find out why they failed?
• How do we find out where they failed?
• A variety of techniques
  • Manual static analysis of code
  • "Print statements"
  • Debuggers
  • Automatic code analyzers
  • ...
• A successful programmer knows how to use her debuggers.
• Note: A successful programmer also knows how to make the program reveal its bugs (through experiments and unit testing).

An Example

• My favorite example is binary search.
• While I am tempted to write a general binary search, let's start with a simple one.
  • The one I have provided is intentionally buggy.
• We'll look at the interface.
• You'll tell me what infrastructure we'd build to test or experiment with it.
• We'll then look at some code that I've written.

GDB Basics

• My favorite C debugger is GDB. (There's another one that I loved while I was a graduate student, but it seems to be gone.)
• You start gdb by typing gdb. (Duh.)
• You insert breakpoints with b line or b function.
• You start the program with run command-line-arguments.
• Once you've hit a breakpoint, you can
  • Do one statement with n
  • Continue to the next breakpoint with c
  • Step into a subroutine with s
  • List lines with l
  • Print a variable with p
  • Print an array with p *A@len
  • Find the call stack with bt