GDB quick start

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Getting Started

  • Type "gdb <program>" to debug a program binary
  • Compile with -g (to get debugging symbols) (keep -O) to allow gdb to have more info about the program when debugging. But it will work without -g.
  • At the prompt type "run" to run it under gdb's control.
  • To debug an already running program, type "attach <PID>"
  • By default gdb can only attach to child processes. To allow attaching to processes that aren't gdb's children, do the following:
 sudo -i
 echo 0 > /proc/sys/kernel/yama/ptrace_scope
 exit    # to become a regular user again

(if you try doing the attach without doing this, you'll get an error in gdb telling you about this file)

Using GDB

  • For complex and interactive programs, connect in two windows/terminals
    • run the program you want to watch in one window
    • in the other, find out its pid (eg using ps aux | grep)
  • run gdb on the binary, then attach the PID ("attach <PID>")
  • set a breakpoint (probably at a function) so execution stops at a point of interest
  • do "tui enable" to get a litle text-mode interface that shows you code
  • note gdb will only follow one process at a time
    • so you have to decide whether you want to follow the parent or child on fork
    • by default, follows the parent
    • "set follow-fork-mode child" to follow child
  • remember that gdb has extensive help and command completion
    • tab is your friend!
  • Commands:
 n = next statement
 c = continue until next breakpoint/signal/program termination
 s = next statement, but going into functions
 print = view state of variables
 x = examine memory
 b = breakpoint (by line or function name)
 catch syscall = see every system call entered and exited
                (like strace, but you can see the backtrace
                 for each call)