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

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• 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.

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• Preparing to use GDB/DBX
  • 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

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  • To insert debugging information into the compiled code use:

    Solaris C++ compiler: CC -g Prog.C GNU C++ compiler: g++ -g Prog.C

    The -g flag will cause the compiler output additional information to help the debugger gdb or dbx to find the program location in the program SOURCE file

    The information inserted into the program is compiler/debugger dependend

    You must use the vendor X's debugger when you compile your program with vendor X's compiler...

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

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

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  • Sun Micro System software
    • dbx is Sun's debugger, use with:

    • cc = C compiler
    • CC = C++ compiler
    • f90 = Fortran 90 compiler

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• Start using the debugger...
  • Program file to debug:

    float A[10] = {1, 2, 3, 4, 5, 6, 7}; // Array (vector) float sum = 0.1234; float f(float x) { float i; i = x*x; return(i); } float g(float x) { return (1/(x*f(x)) ); } int main(int argc, char *argv[]) { int i; for (i = 1; i <= MAX; i = i + 1) { A[i-1] = g(i); } sum = 0.0; for (i = 0; i < MAX; i = i + 1) { sum = sum + A[i]; } cout << "sum = " << sum << "\n\n"; }

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  • Example Program: (Demo above code)                                                
    • Prog file: RIGHT click here and save.

    Compile with: CC -g -o debug debug.C

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  • Starting the Debugger:

    dbx Program-Name In our example: dbx debug

    You will see the debugger prompt (dbx)

    Now you can enter debugger commands

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• The help command
  • To get help on dbx/gdb :

    (dbx) help

  • To get help on a command of dbx/gdb :

    (dbx) help command

  • Examples:

    (dbx) help command summary Use `commands' to see a command summary consisting of one-line descriptions of each dbx command. Execution and Tracing cancel catch clear cont delete fix fixed handler ignore intercept next pop replay rerun restore run runargs save status step stop trace unintercept when whocatches ..... (dbx) help step step (command) step # Single step one line (step INTO calls). # Non-active threads cannot be stepped. step # Single step lines (step INTO calls) step up # ... and out of the current function (dbx) help step control step control (subtopic) The dbxenv variable `step_events' controls whether breakpoints are enabled during a step. See also `help dbxenv' under `step_events'. ...

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• The list command: list the source file
  • list:   # lists the source code at the current (stopped) location in the program

  • Example:

    (dbx 1) list 30 for (i = 1; i <= MAX; i = i + 1) <----- Stop location 31 { 32 A[i-1] = g(i); 33 } 34 35 sum = 0.0; 36 for (i = 0; i < MAX; i = i + 1) 37 { 38 sum = sum + A[i]; 39 }

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• 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:

    (dbx 2) run Running: debug (process id 1173) Reading libc_psr.so.1 sum = 1.20201 execution completed, exit code is 0

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• The runned program stopped...
  • After you issued a run, the program can stop for one of the following reasons:

    Program terminates normally In this case, the debugger is DONE NOTE: your program may still contain logical errors Program stops for user input... The debugger will continue to run the program after you have entered some input NOTE: You better have your program print some prompt message before reading the input... Otherwise.... you may sit staring at your computer wondering what the program is doing... Program has crashed (received some "signal") The debugger can help you find the error : The list command will show you the location where the error occurred The print command will show you the value of the variable etc., etc Program reached a break point A break point is: location in the program where you tell the debugger to stop when execution reaches that location. You can place (insert) break points in the program using the break command. Debugging Hint: Insert a break point at a statement before where your program had crashed Run the program When it stops at the break point, use print to examine the variables to find out why it crashes

• Order of discussion
  • There are 3 things that you need to learn in order to use a debugger :

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

  • We will discuss these subjects in the given order

   

   

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

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• Displaying 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

  • There are 2 useful printing commands in the dbx:

    print Exp1, Exp2... print the values of the expressions Exp1 , Exp2 , ... ( one time only) display Exp1, Exp2... print the values of the expressions Exp1 , Exp2 , ... whenever the program stops dump print the values of all local variables in the stopped function, (We will learn about local variables later) whereis VARNAME print the full path (filename, function) of all variables with the name VARNAME (We will learn about scoping later)

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• One time printing of variables

  • The print command is used to examine variable on an one time basis

  • Examples: print

    (dbx) stop in main (dbx) run (dbx 6) print i, i+1 i = 0 i+1 = 1 (dbx 10) print A[i], A[0], A[1] A[i] = 1.0 A[0] = 1.0 (dbx 11) print A[0..3] A[0..3] = [0] = 1.0 [1] = 2.0 [2] = 3.0 [3] = 4.0

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  • Examples: dump

    (dbx) stop in main (dbx) run (dbx) dump argv = 0xffbfedc4 argc = 1 i = 0 (argv, argc and i are LOCAL variables in function "main") (dbx) stop in f (dbx) cont (dbx) dump x = 1.0 i = -NaN.0 (i have not been given a value yet...) (x and i are LOCAL variables in function "f")

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• Locating variables of a given name: whereis
  • In C++, different variables can be named with the SAME name

  • Example: the variable i is defined in functions main() and f()

    float A[10] = {1, 2, 3, 4, 5, 6, 7}; // Array (vector) float sum = 0.1234; float f(float x) { float i; i = x*x; return(i); } float g(float x) { return (1/(x*f(x)) ); } int main(int argc, char *argv[]) { int i; for (i = 1; i <= MAX; i = i + 1) { A[i-1] = g(i); } sum = 0.0; for (i = 0; i < MAX; i = i + 1) { sum = sum + A[i]; } cout << "sum = " << sum << "\n\n"; }

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  • You can use the debugger's whereis command to find variables of a given name:

    (dbx) stop in main (dbx) run (dbx) whereis i variable: `debug`debug.C`f(float)`i variable: `debug`debug.C`main`i

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• 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:

    (dbx) stop in main (dbx) run (dbx) display i, A[0..5] (dbx) run stopped in main at line 30 in file "debug.C" 30 for (i = 1; i <= MAX; i = i + 1) i = 0 A[0..5] = [0] = 1.0 [1] = 2.0 [2] = 3.0 [3] = 4.0 [4] = 5.0 [5] = 6.0 (dbx) next <-- do this repeatedly and see changes in A[] .... i = 2 A[0..5] = [0] = 1.0 [1] = 2.0 [2] = 3.0 [3] = 4.0 [4] = 5.0 .... i = 3 [0] = 1.0 [1] = 0.125 [2] = 3.0 [3] = 4.0 [4] = 5.0 [5] = 6.0

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• Managing the Display List
  • The following commands are used to manage the display list:

    display ( without any arguments): Print the list of expressions being displayed undisplay NUMBER Undo the `display' command numbered NUMBER NOTE: Find the NUMBER by using the display command

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  • Example:

    (dbx) display (1) i (2) A[0..5] (dbx) undisplay 1 (dbx) display (2) A[0..5]

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  Looking for errors in your program: setting breaking points and single stepping your program

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• Break Point
  • breakpoint:

    a location in the program where the debugger will (temporarily) stop execution

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  • 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, next and cont commands to control program execution

      These commands allows you to execute one statement or one function at a time to find errors

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• Setting Breakpoints in DBX
  • 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

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• 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]

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

    You may want to delete or disable some breakpoints

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  • 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...

   

   

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  Controlling program execution: next, cont

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• 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:

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

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  • cont

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

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  • 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

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  • 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
    step up	finish the current function and stop

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• Customizing DBX
  • When you start to debug your programs with DBX, you will find that typing in those long debug commands to be very annoying.

  • Every serious programmer will control his/her environment

    To quote George Bernard Shaw:

    The reasonable man adapts himself to the world; the unreasonable one persists in trying to adapt the world to himself. Therefore, all progress depends on the unreasonable man.                                                 George Bernard Shaw                                                 Irish dramatist & socialist (1856 - 1950)

  • Renaming BDX commands:

    dalias NAME DEFINITION	Define NAME to be an alias for DEFINITION. DEFINITION may contain white space. A semicolon or newline terminates the definition.
    dalias NAME	List the definition for the name NAME
    dalias	List the definitions for all aliases

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  • Using Kshell style renaming commands:

    If you defined:

    kalias alias=kalias

    then DBX will use KShell style alias syntax

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  • Sample dbx startup file
    • Here is an example of aliases of a .dbxrc startup file: click here

    • You can create this file using the command:

      (dbx) help .dbxrc > sample-dbxrc

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• Final notes..
  • The GNU's "gdb" debugger has similar features and commands are "dbx"

    Once you know what a command does, it very easy to learn a different debugger.

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  • There is an integrated development environment (IDE) available in Solaris: sunstudio

    It allows the user to edit, compile and debug programs through a GUI (graphic user interface).

    However, it is extremely heavy weight (i.e., slooooow).

  • After you can fix some bugs in your source code, you should re-compile your program again to be debugged.

    You DO NOT have to exit DBX. When you run the program from DBX, it will reload the new version.

    You may need to change some break points (especially the break points that uses line numbers)