gdb: GNU Debugger

1
gdb GNU Debugger

• Lecturer Prof. Andrzej (AJ) Bieszczad
• Email andrzej_at_csun.edu
• Phone [phone]

UNIX for Programmers and Users Third Edition,
Prentice-Hall, GRAHAM GLASS, KING ABLES Slides
partially adapted from Kumoh National University
of Technology (Korea) and NYU
2
GDB, the GNU Debugger

• Text-based, invoked with
• gdb ltprogramfilegt ltcorefilegtltpidgt
• Argument descriptions
• ltprogramfilegt executable program file
• ltcorefilegt core dump of program (crash
  records)
• ltpidgt process id of already running program
• Compile ltprogramfilegt with g for debug info (cc
  -g ltprogramfilegt)
• Use -xdb for compatibility with xdb
• corefiles very useful in post mortem analysis
  (figuring out what went wrong why did it crash?)
• Starting with pid very useful for programs that
  misbehave non-deterministically (scenario
  difficult to repeat problem exhibited
  sporadically field troubleshooting)

3
gdb Example

• Get reversefirst.c from
• http//www.csun.edu/andrzej/COMP421-F04/source/ch
  12/reversefirst.c
• cc -g reversefirst.c -o reverse
• gdb reverse
• HP gdb 3.2 for PA-RISC 1.1 or 2.0 (narrow), HP-UX
  11.00.
• ltgt
• (gdb) help
• ltgt
• Type "help" followed by a class name for a list
  of commands in that class.
• Type "help" followed by command name for full
  documentation.
• Command name abbreviations are allowed if
  unambiguous.
• (gdb)

4
Basic GDB Commands

• General Commands
• file ltfilegt selects ltfilegt as the program to
  debug
• run ltargsgt runs selected program with
  arguments ltargsgt
• attach ltpidgt attach gdb to a running process
  ltpidgt
• kill kills the process being debugged
• quit quits the gdb program
• help lttopicgt accesses the internal help
  documentation
• Stepping and Continuing
• continue continue execution (after a stop)
• step step one line, entering called functions
• next step one line, without entering
  functions
• finish finish the function and print the return
  value

5
GDB Breakpoints

• Useful breakpoint commands
• break ltwheregt sets breakpoints. ltwheregt can
  be a number of things, including a
  hex address, a function name, a
  line number, or a relative line offset
• rwatch ltexprgt sets a watchpoint, which will
  break when ltexprgt is written to or read
• info breakpoints prints out a listing of all
  breakpoints
• clear ltwheregt clears a breakpoint at ltwheregt
• delete ltnumsgt deletes breakpoints by number

6
Playing with Data in GDB

• Commands for looking around
• list ltwheregt prints out source code at ltwheregt
• search ltregexpgt searches source code for ltregexpgt
• backtrace ltngt prints a backtrace ltngt levels
  deep
• info ltwhatgt prints out info on ltwhatgt
  (like local variables or function args)
• print ltexprgt prints out the evaluation of
  ltexprgt
• Commands for altering data and control path
• set ltnamegt ltexprgt sets variables or arguments
• return ltexprgt returns ltexprgt from current
  function
• jump ltwheregt jumps execution to ltwheregt

7
gdb - Example

• (gdb) list 1 ---gt can use l for list
• 1 / REVERSE.C /
• 2
• 3 include ltstdio.hgt
• 4
• 5 / Function Prototype /
• 6 void reverse ()
• 7
• 8 /
  /
• 9
• 10 main ()
• (gdb) l ---gt same as list continues from
  the previous
• 11
• 12
• 13 char str 100 / Buffer to hold reversed
  string /
• 14
• 15 reverse ("cat", str) / Reverse the string
  "cat" /
• 16 printf ("reverse (\"cat\") s\n", str) /
  Display /
• 17 reverse ("noon", str) / Reverse the string
  "noon" /

8
gdb - Example

• (gdb) break main ---gt set a breakpoint in
  function main
• Breakpoint 1 at 0x29f4 file reversefirst.c, line
  15.
• (gdb) break 16 ---gt set a breakpoint in line 16
  (current source)
• Breakpoint 2 at 0x2a0c file reversefirst.c, line
  16.
• (gdb) break reverse ---gt set a breakpoint in
  function reverse
• Breakpoint 3 at 0x2a80 file reversefirst.c, line
  33.
• (gdb) info break ---gt display information on
  breakpoints
• Num Type Disp Enb Address What
• 1 breakpoint keep y 0x000029f4 in main at
  reversefirst.c15
• 2 breakpoint keep y 0x00002a0c in main at
  reversefirst.c16
• 3 breakpoint keep y 0x00002a80 in reverse
  at reversefirst.c33
• (gdb) run
• Starting program /home/usersNN/userID/reverse/rev
  erse
• Breakpoint 1, main () at reversefirst.c15
• 15 reverse ("cat", str) / Reverse the string
  "cat" /
• (gdb) ontinue ---gt you can use c as well
• Breakpoint 3, reverse (before0x40001028 "cat",
  after0x7f7f0958 "")

9
gdb - Example

• (gdb) backtrace ---gt show the execution stack
• 0 reverse (before0x40001028 "cat",
  after0x7f7f0958 "") at reversefirst.c33
• 1 0x2a0c in main () at reversefirst.c15
• (gdb) l
• 28
• 29 int i
• 30 int j
• 31 int len
• 32
• 33 len strlen (before)
• 34
• 35 for (j len - 1, i 0 j gt 0 j--, i) /
  Reverse loop /
• 36 afteri beforej
• 37
  
• (gdb) next ---gt execute next line
• 35 for (j len - 1, i 0 j gt 0 j--, i) /
  Reverse loop /
• (gdb) n ---gt same as next
• 36 afteri beforej
• (gdb) _

10
gdb - Example

• (gdb) print afteri ---gt display data
  (expression)
• 1 0 '\000'
• (gdb) p beforej ---gt same as print
• 2 116 't'
• (gdb) _
• (gdb) n
• 35 for (j len - 1, i 0 j gt 0 j--, i) /
  Reverse loop /
• (gdb) p after ---gt print
• 4 0x7f7f0958 "t"
• (gdb) p before
• 5 0x40001028 "cat"
• (gdb) c
• Continuing.
• Breakpoint 2, main () at reversefirst.c16
• printf ("reverse (\"cat\") s\n", str) /
  Display /
• (gdb) _

11
gdb - Example

• (gdb) n
• reverse ("cat") tac
• 17 reverse ("noon", str) / Reverse the string
  "noon" /
• (gdb) s
• Breakpoint 3, reverse (before0x40001030 "noon",
  after0x7f7f0958 "tac")
• at reversefirst.c33
• 33 len strlen (before)
• (gdb) return 0
• Make reverse return now? (y or n) y
• 0 main () at reversefirst.c18
• 18 printf ("reverse (\"noon\") s\n", str) /
  Display /
• (gdb) p str
• 4 "tac", '\000' ltrepeats 96 timesgt
• (gdb) n
• reverse ("noon") tac ---gt this is the output
  from the program
• 19
• (gdb) quit
• _

12
gdb/C Challenge

• The reverse program from the next slide is buggy.
• Copy the files (reverse1.c and reverse1.h) to
  your local file system then, compile and run
  them.
• Do you see the problem?
• Can you fix it easily?
• gdb will help
• Use gdb to examine program behavior, fix the bug
  and call me to show the working version.

13
gdb/C Challenge

• / REVERSE1.C /
• include ltstdio.hgt
• include "reverse1.h"
• /
  /
• int reverse (str)
• char str
• int i
• int len
• char c
• len strlen (str)
• for (i 0 i lt len/2 i) / Reverse loop /
• c stri
• (strI) strlen-i-1
• (strlen-i-1) c
• int main()