Programming Practice: Debugging

• Testing: design/imply test to check correct operation & expose bugs
• Debugging: process of identify and solve run-time behavior of the software
• Prevent bug
  • defensive programming
  • hard
    • multiple, complex interaction
    • infinite possible input
  • program defensively
    • knowing the pro/cons of language use
    • idiom
    • pre-condition/post-condition
    • use tools
• Bug Terminology
  • reproducible is better than transient ones
  • Bohr bug: repeat bug under some unknown but well-defined conditions
  • Heisen bug: bug may disappears or alert when try to find it
• Common type of bug
  • syntex bug
    • typing error
    • misunderstanding of symbol meaning
  • arithmetic bug
    • /0
    • overflow/underflow
      • variable value larger than maximum data type can hold
    • floating point
      • lost value when convert from a float to int
    • logic bug
      • infinite loop
      • infinite recursion
      • off-by-one error
        • use >= rather than >(depend by case)
    • resource bug
      • null pointer
      • do not free dynamic memory
      • re-use of freed memory
      • confusing different memory type(static, local,dynamic)
      • array overrun
      • buffer overflow
        • often source of Heisen bug
        • major security vulnerability
    • side-effect bug
      • datatype needed not defined
      • do not have deallocator for dynamic variable
• Debug techniques
  • DEBUG flag and printf function
    • add flag and printf to place bug may occur
    • catch the ling bug occur
    • pro:
      • simple
    • con:
      • only useful with good placement
      • not flexible
      • may cause a lot output
  • wolf-fence
    • use a printf function to show which part bug occur, add printf until reach line bug occur
    • do not work if bug not in main file
  • logging
    • use print function to print relevant data for each function before and after
    • find if an error occur when not print or print wrong data
  • interactive debugger: ex. gdb
    • compile with -g for symbol and other information
    • stack trace
      • use debugger to find
        • where crash occur
        • what core made, order, and parameter
      • trace execution
        • use watch point and break pint to check through the code what happen by variable and memory location change
      • next command: execute next line include any thing happened
      • step command: execute until another line reached
      • display command: show expression each time program stop
      • watch command: set a watch point for expression
      • pro
        • use on program hard to add printf/DEBUG
        • interactive debugger to show what happen, not what programmer think happen
• Core file
  • containing an image of process`s memory at the time of termination
  • often produced as the action due to a signal
  • name : core.PID