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Debugging

  • To follow along:
    • Connect to stu via SSH
    • git clone https://github.com/lam2mo/uug-debug.git

Definitions

  • Software defect: an error in code that produces incorrect behavior
    • Colloquially called “bugs”
    • Mismatch between user expectations and machine behavior
    • Proximate cause (symptom) vs. root cause (defect)
  • Debugging: working from the former towards the latter
    • Basically: the process of continually asking “why is this happening?”
    • One of the most important practical skills in programming

Definitions

  • Debugger (e.g., gdb): a program that examines another running program
    • Execute the program step-by-step (by line or instruction)
    • Examine the contents of memory at any point
    • Add breakpoints and watchpoints
  • Debuggers are more useful with extra information from the compiler
    • In gcc, compile with the -g option to enable this
    • It’s also useful to disable optimization (-O0)

Running GDB

  • gdb ./program - launch GDB on program
    • (include --tui for "graphical" interface)
  • help <cmd> - get description of a command
  • run <args> - begin/restart execution
  • start <args> - begin/restart execution and pause at main
  • quit - exit GDB

Process Control

  • break <func> - set a breakpoint at the beginning of a function
  • break <file>:<line> - set a breakpoint at a specific line of code
  • watch <loc> - pause when a specific variable or memory location changes
  • continue - resume execution (until a breakpoint, watchpoint, or segfault)
  • next - run one line of code then pause (skips over function calls)
  • step - run one line of code then pause (descends into functions)
  • nexti / stepi - like next and step but run one machine instruction
  • finish - run until current function returns

Process State

  • print <expr> - print current value of a variable or expression
  • print /x <expr> - same as above but print in hexadecimal
  • ptype <expr> - print the type of a variable or expression
  • display <expr> - print value after every pause
  • info reg - print values of all machine registers
  • info locals - print values of all local variables
  • backtrace - print stack trace (list of active functions on the stack)
    • (up and down to cycle through function call sites)

Memory Examination

  • x/<N><u><f> <addr> - examine memory at given address
    • N is the number of units to display
    • u is the unit type:
      • b (byte)
      • h (half word - 2 bytes)
      • w (word - 4 bytes)
      • g ("giant" word - 8 bytes)
    • f is the format (anyprintf format or i for machine instructions)
      • x for hexadecimal is particularly useful here

Resources