---

GDB: a tool to locate errors in programs... don't leave home without it

---

• What is a "Debugger"
  • An error in a computer program is commonly called a bug

  • A debugger is a computer program that helps the programmer find and remove bugs

  • The key in eliminating bugs is to locate the error

  • We will study the popular (public available) debugger GDB and the Solaris debugger DBX which have similar features.

  ---

  ---

• Preparing to use GDB
  • A debugger will work best with its associated compiler :

    The compiler needs to add debugging information to the compiled program The debugger uses the debugging information

    The compiler and debugger must agree on the coding of the information to work properly !!!

    ---

  • To insert debugging information into the compiled code use:

    GNU C compiler: gcc -g Prog.c

    The -g flag will cause the compiler output additional information to help the GNU debugger gdb to debug the program.

    The information inserted into the program is compiler/debugger dependend

    You must use the gdb debugger when you compile your program with the gcc compiler...

    ---

  • GNU software
    • gdb is GNU's debugger, use with:

    • gcc = C compiler
    • g++ = C++ compiler
    • g95 = Fortran 95 compiler

    ---

    ---

  • Starting the Debugger:

    gdb Program-Name Example: gdb main GNU gdb (Ubuntu/Linaro 7.2-1ubuntu11) 7.2 Copyright (C) 2010 Free Software Foundation, Inc. License GPLv3+: GNU GPL version 3 or later This is free software: you are free to change and redistribute it. There is NO WARRANTY, to the extent permitted by law. Type "show copying" and "show warranty" for details. This GDB was configured as "x86_64-linux-gnu". For bug reporting instructions, please see: ... Reading symbols from main...done. (gdb)

    You will see the debugger prompt (gdb)

    Now you can enter debugger commands

  ---

  ---

• The help command
  • To list all commands of gdb :

    (dbx) help all Command class: aliases ni -- Step one instruction rc -- Continue program being debugged but run it in reverse rni -- Step backward one instruction rsi -- Step backward exactly one instruction si -- Step one instruction exactly stepping -- Specify single-stepping behavior at a tracepoint tp -- Set a tracepoint at specified line or function tty -- Set terminal for future runs of program being debugged where -- Print backtrace of all stack frames ws -- Specify single-stepping behavior at a tracepoint Command class: breakpoints awatch -- Set a watchpoint for an expression break -- Set breakpoint at specified line or function catch -- Set catchpoints to catch events catch assert -- Catch failed Ada assertions catch catch -- Catch an exception catch exception -- Catch Ada exceptions catch exec -- Catch calls to exec ---Type to continue, or q to quit---

  • To get help on a command of gdb :

    (dbx) help command Example: (gdb) help print Print value of expression EXP. Variables accessible are those of the lexical environment of the selected stack frame, plus all those whose scope is global or an entire file. $NUM gets previous value number NUM. $ and $$ are the last two values. $$NUM refers to NUM'th value back from the last one. Names starting with $ refer to registers (with the values they would have if the program were to return to the stack frame now selected, restoring all registers saved by frames farther in) or else to debugger "convenience" variables (any such name not a known register). Use assignment expressions to give values to convenience variables. {TYPE}ADREXP refers to a datum of data type TYPE, located at address ADREXP. @ is a binary operator for treating consecutive data objects anywhere in memory as an array. FOO@NUM gives an array whose first element is FOO, whose second element is stored in the space following where FOO is stored, etc. FOO must be an expression whose value resides in memory. EXP may be preceded with /FMT, where FMT is a format letter but no count or size letter (see "x" command).

  ---

  ---

  ---

  ---

• The run command: Run the program from the start
  • The run program takes some arguments:

    run [program-arguments]

  • The program will start executing from the beginning (main function)

  • Example:

    (gdb) run 50000 Starting program: main 50000 Program received signal SIGSEGV, Segmentation fault. 0x0000000000400a69 in setBit (n=16384) at primes2.c:41 41 p->bits[j] = p->bits[j] | mask; (gdb)

    The message tells you that there was an error at line 41 in the primes2.c file.

  ---

  ---

  ---

  ---

• The list command: list the source file
  • list:   # lists some lines before and after the stopped location in the source code of the program

  • Example:

    (gdb) list 36 c++; 37 } 38 int j = n/32; 39 int k = n%32; 40 int mask = 1 << k; 41 p->bits[j] = p->bits[j] | mask; 42 } 43 44 int testBitIs0(int n) 45 {

    I don't use the list command, I open a second window with an editor to see the full source code.

• How you can use a debugger to help you find errors
  • There are 3 things that a debugger can do for you to help you find errors in your program:

    Check variables with the debugger Set up break points Control program execution (step through your program)

  • We will discuss these subjects in the given order

   

   

  ---

  ---

  Examining the state of the program: examining the values of variables

  ---

  ---

• Print the values of variables
  • The key to find out why a program has a bug is the values of the variables at the moment before the program crashes

  • The print command is used to examine the content of a variable

    ---

  • Print a variable:

    Program received signal SIGSEGV, Segmentation fault. 0x0000000000400a69 in setBit (n=16384) at primes2.c:41 41 p->bits[j] = p->bits[j] | mask; (Crashed - some variable must have strange values) (gdb) print j $2 = 512 *** j is too large !!! (gdb) print p $3 = (seg *) 0x4924924924f27d24 (gdb) print p->bits[j] Cannot access memory at address 0x4924924924f28524

  ---

  ---

  ---

  ---

• Repeated monitoring of variables
  • The display command is used to print the value of variables each time the program stops

    You can monitor the changes of these variables easily to find the cause of error

  • Examples:

    (gdb) display j 1: j = 512

  ---

  ---

  ---

  ---

  ---

  ---

  Advanced technique to look for errors in your program: setting breaking points and single stepping your program

  ---

  ---

• Break Point
  • breakpoint:

    is a location in the program where the debugger will (temporarily) stop execution (You tell the debugger to stop running the program when it has reached a break point !!!)

    ---

  • After the program has stopped, you can:

    use the displaying commands to examine values of the variables (check if they have the correct value !) use step and next commands to control program execution These commands allows you to execute one statement or one function at a time (program will then stop !). You can find errors by examining the variable after each statement or function call. Use the cont command to contiue program execution In this case, the program will run until it: Teminates Crashes Or stop at a break point location

  ---

  ---

• Setting Breakpoints in GDB
  • You can set a breakpoint at:

    start of a function, end of a function at a statement in a source file when some condition is satisfied

  ---

• Stop at the start of a function

  stop in FUNCTIONNAME   Example: stop in main	make program stop when it reaches the start of the function FUNCTIONNAME
  stop returns	stops the program when the current (stopped) function has returned.
  stop returns FUNCTION   Example: stop returns f	stops the program when the function FUNCTION has returned.
  stop at LINENUMBER   Example: stop at 20	sets a breakpoint at the statement on line LINENUMBER of the current (stopped) file (the current file is the file that contains the statement where the program has stopped)
  stop at FILE:LINE   Example: stop at debug.C:20	sets a breakpoint at the statement on line LINENUMBER of the file FILENAME
  stop change VARIABLE   Example: stop change A[9]	stops the program when the value of variable VARIABLE is changed
  stop if CONDITION   Example: stop if i >= 90 Example: stop if sum >= 1.2	stops the program whenever the CONDITION is satisfied

  • Experimient: ----------------------------------------------
    • Try any of the stop command with the "debug.C" file.

    • Before you type the run command, display the following:
      • display i
      • display sum
      • display A[0..12]

  ---

  ---

• Managing Breakpoints in GDB
  • Someone, you may over-do things and set TOO MANY breakpoints :-)

    You may want to delete or disable some breakpoints

    ---

  • Commands to manage breakpoints:

    status	list all breakpoints and the associated number of each break point. NOTE: the current stopped break point location is marked with an asterix (*)
    delete N	delete the breakpoint associated with number N
    delete all	delete all breakpoints...
    handler -disable N	Disable breakpoint N NOTE: A disabled break point is marked as [N]
    handler -enable N	Enable breakpoint N NOTE: An enabled break point is marked as (N)

  Debugging HINT: do not delete breakpoints, just disable them; you may never know if you need them again...

   

   

  ---

  ---

  Controlling program execution: next, cont

  ---

  ---

• Controlling the execution of a running program from a break point
  • Recall that:

    run [program-arguments]

    always run the program run from the beginning

  • After stopping at a breakpoint, you have 3 different ways to preceed with the execution of the program:

    ---

  • When the program is stopped at a breakpoint, you can control the program execution with one of the following commands:
    • step
    • next
    • cont

    ---

  • cont

    cont	Continue the execution from the current break point Program may stop again at the next breakpoint....

    ---

  • next

    next	execute the next source line and stop If next statement contains one or more function calls, the entire function call will be executed When the program stops, it will print all variables given in the display commands NOTE: the next command will NOT step into a function
    next NUMBER	execute the next NUMBER source lines and stop

    ---

  • cont

    step	execute the next statement line and stop If next statement contains one or more function calls, the first function is "stepped into" and stops. When the program stops, it will print all variables given in the display commands) NOTE: The step command will STEP INTO a function call
    step NUMBER	execute the next NUMBER statement lines and stop