This tutorial has both ddd and gdb. I haven't tried the ddd yet (it's interface might have changed slightly if there is a new version) but I did try the gdb which is below. You can do either or both.

Setup

For this tutorial, you will need to create a directory called debug in your cps100 directory. Type the following commands:

  cd cps100
  mkdir debug
  cd debug

You should be in your debug directory. Type pwd just to make sure.

Next you need to copy files for the tutorial Type the following (don't forget the dot):

   cp ~rodger/cps100/debug/*  .

Type the command ls to list the files. You should see the following files:

• Makefile
• arith.cpp

Part 1: Debugging with DDD

When you can't figure out what is wrong with your program, a debugger is useful for "watching" the execution of your program to see what it is doing, and to check the values of variables. In this part of the lab you'll learn how to use the debugger ddd to find errors in the program arith.cpp.

First compile arith using make. Then to start the debugger, type

You should run the program: either type run at the bottom of the console window at the (gdb) prompt, or click on the run button (in the button window) ddd will run the program. In the console it will ask for input. Type in the input and you will see that the output is not correct. Do the following:

• You can put breakpoints in your program and then run the program and it will pause at the breakpoints so you can examine the values of variables. In the source code window scroll to find the Add function, put the mouse to the left of the first assignment statement in the Add function and click to put the cursor there, then click on the "Break" button at the top of the window and a Stop sign will appear on this line.

• Put a breakpoint on the line with the first assignment statement in the Mult function.

• Now run the program again by typing run in the ddd console window, or clicking the run button. You will have to enter input. This time the program runs and halts on the line with the stop sign in the Add function. There will be an arrow on the left side of the source code window pointing at the current line number where the execution has halted. This line has not been executed yet, but is the next line to be executed.

• Click on the word newnum (anywhere in the source window) and a box will surround it. Then click on the Display() button at the top of the window. The value of newnum is shown. There is a bogus value, since this line has not been executed. Click on the step button. The line "newnum = 0" is executed, the arrow moves to the next line, and note the new value of newnum displayed in the display window. It is now 0.

• Display the value of num1. (click on num1 and then click on the display button at the top). It's value should appear above, but you might have to scroll to see it under newnum.

• Step one more time and the value of num1 is added to newnum. What is wrong with the Add function? Don't fix it yet, keep going.

• Click on the "cont" button and the program will continue execution until it reaches the next breakpoint, in the Mult function. Note the newnum displayed is disabled. Why? Because the newnum displayed was the local variable in the Add function, and since that function completed, the memory location of that newnum is no longer valid.

• Try displaying the values of newnum, num1 and num2 and stepping through the statements in the Mult function. What is wrong with this function?

• Click the cont button to finish the program. Then in the console window type quit and ddd will exit.

• Correct the errors in arith.cpp.

Summary of DDD buttons/commands

• Break: Set a break point at current line
• Clear: Clear a break point at current line.
• Print: Print the value of current variable.
• Find: Search for current text.
• Run: Start Program from beginning until it ends or hits the first break point.
• Step: Step into function. Run the next line of code. If the code is a function call go to first statement in the function call
• Next: Go to Next line. Similar to Step, but if the line is a function call, Next will run the entire function and stop at the next line of code in the current function to be excuted.
• Cont: Continue running the program from current spot until it hits the next break point or ends.

For more information, you can click on the Help option in the menu and look it up in the ddd manual.

NOTE: Occasionally code will appear in the console window from a library file, just click on "cont" to go back to your program. You may have to put in break points to focus on the parts of your program that are not working.

Part 2: Debugging with GDB

DDD is a visual debugger and sometimes takes a long time. You can also run gdb, a textual based debugger that runs faster, but doesn't have a visual interface. (ddd actually uses gdb, so the commands are similar).

To run gdb on the above file just type: gdb arith

Note, if you modified it and did the ddd tutorial, just use the cp command again to copy over it, and then the make command again to compile it.

First compile arith using make. Then to start the debugger, type

You should run the program: type run at the (gdb) prompt

Type in the input and you will see that the output is not correct. Do the following:

• You can put breakpoints in your program and then run the program and it will pause at the breakpoints so you can examine the values of variables. To list the first few lines of the program type: list 1

  Then keep pressing the return bar to see the rest of the program.

  Determine the line number for the first assignment statement in the Add function. If the number is 20 then type:

      break 20
  

  to put a break point on this line.

• Put a breakpoint on the line with the first assignment statement in the Mult function.

• Now run the program again by typing run. You will have to enter input. This time the program runs and halts on the line with the stop sign in the Add function and shows the next line to be executed. This line has not been executed yet, but is the next line to be executed.

• type: display newnum

  The value of newnum is shown. There is a bogus value, since this line has npot been executed. Type: step

  The line "newnum = 0" is executed, the next line is shown.

  Note the new value of newnum displayed in the display window. It is now 0.

• Display the value of num1.

• Step one more time and the value of num1 is added to newnum. What is wrong with the Add function? Don't fix it yet, keep going.

• Type: cont

  and the program will continue execution until it reaches the next breakpoint, in the Mult function. Note the variables displayed values are gone. Why? Because the newnum displayed was the local variable in the Add function, and since that function completed, the memory location of that newnum is no longer valid.

• Try displaying the values of newnum, num1 and num2 and stepping through the statements in the Mult function. What is wrong with this function?

• Click the cont button to finish the program. Then in the console window type quit and gdb will exit.

• Correct the errors in arith.cpp.

  Some useful gdb commands are:

  • run
  • l 25 - lists about 10 lines of code around line 25
  • l Mult - lists about 10 lines of code around the function Mult
  • break 26 - puts a breakpoint on line 26
  • display newnum - displays the current value of newnum
  • step - executes one step
  • cont - continue
  • quit - quit gdb
  • help - see a list of gdb commands

  ---

  Susan Rodger
  Last modified: Sun Sep 1 16:54:26 EDT 2002