Debugging

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

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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

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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)

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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

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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

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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)

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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

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Resources

• GDB Tutorial
• Cheat sheets: simpler and denser
• Valgrind Tutorial
• LLDB tutorial
• GDB to LLDB command map