The structure of the program is similar to the usedb.cc program from a previous lab. The file usestock.cc stores the client program that manipulates stock databases. The class StockList implements a stock database (by storing linked lists of Stocks in a vector).
Class Structure
The class StockList stores a collection of stocks in a vector of linked lists. Each cell in the vector points to a linked list of stocks whose symbols begin with the same name, e.g., CPQ, CSG, and CVC. Stock information is stored in an instance of the class Stock, and a node of the linked list is represented by the struct StockNode. This is diagrammed below.
Compiling/Running usestock
In this section of the lab you'll copy files and then compile and run the stock database program.
First change into your cps100e subdirectory (type pwd
to verify where you are). Create a
lab6 subdirectory by typing mkdir lab6 and change
into this subdirectory (be sure to check that you're
in the lab6 subdirectory.) Now copy the files for the lab
(don't forget the . when copying).
You should see the files listed below (these are links to the files in case you use Netscape, and for users outside of Duke). This will copy two data files the program reads whose names are hard-wired into the program.
- Makefile (site specific)
- usestock.cc
- stlist.h
- stlist.cc
- stock.h
- stock.cc
- stocks.dat
- trades.dat
Be sure you're in the lab6 subdirectory, and check to see that all files are there (type ls). Then, from an xterm window (at the prompt [1] ola@teer8% or similar) compile the first version of the program by typing: make usestock.
Note that you type make usestock since the function main is in the usestock.cc file.
This should compile several files and link them together with the library libtapestry.a. Now run the program by typing: usestock at the prompt. When you run the program, you should get an error saying "segmentation fault". Your first job is to find the place in the code that causes the segmentation fault by using ddd (a debugging tool that we want you to begin to use.)
Debugging: Using ddd:
In this part of the lab you'll learn how to use ddd to find the reason that usestock crashes. You will fix the bug and check to make sure that the program runs correctly.
To start the debugger, type
You should run the program: either type run inside the command window, or click on the run button (located near the bottom lefthand corner of the source code window.) ddd will run the program and stop when the segmentation fault occurs. There will be an arrow (kind of greenish in color) on the left side of the source code window pointing at the line number that is causing the segementation fault. This information, the name of the file, and the line number is printed in the DDD command window (the command window is running the gdb debugger, ddd is a graphical front end to gdb. You can run gdb without the GUI front end if you're on a computer without the ability to display X windows.)
Displaying items with ddd
To start finding the error, first click on current in the source code window so that it is highlighted. Then click the Display() button. This should pop up the Data Window with a graphical picture of current. Note that the the value of current is shown as 0x0 which is NULL. Thus trying to dereference to get current->next generates a segmentation fault. To trace execution and find the error follow the steps below. Even if you already know what the problem is you should follow these steps to get practice with the debugger.
- Place a break point on line 184 (the while loop).
Click near the
line number so that the line number is diplayed at the bottom left of
the source code window, click on Break(), and a stop sign
should appear to the left of 184.
- Click the Run button again.
A popup window will appear saying "The program being debugged has been
started already. Start it from the beginning?". Click on Yes.
The program will run until it reaches the line of code where the break
point was set.
-
Note that the value of current in the display window is
different. Click on current so that it is highlighted,
then click on Dereference() in the dsiplaywindow. This shows
a diagram of what *current is (the node current points
to.)
- Click on stock = {...} in the
*current diagram. Then click on Show() to
show the stock. You can drag this larger box around with the mouse
button. Note that mySymbol is shown as hidden. Click on
mySymbol = {...} so that it is highlighted, then click on
Show(). This should show you the symbol for the stock.
Write
the symbol down.
- Now click on the Next button in the source code
window. This executes the program one step at a time.
Press Next but stop before executing the
statement at line 181 which causes the segmentation fault.
- You will now display the value of newNode. Highlight newNode, click on Display(), then use Dereference() and Show() to find the symbol that should be added to the list of stokcs. Write the symbol down.
Fixing the Code
The problem is that to add a new node in order, the code needs to look one node ahead. This is shown in the Tapestry book on page 569 in the function WordList::Update(). Rather than use the loop test belowYou'll need to modify the code after the loop to link newNode into the list (this will require two statements!). You should start up emacs, make the changes that are necessary (think about this!!, don't just type at random), then recompile. You can reload the source code into ddd from the Source menu. If you run the program from within ddd the newly compiled code will be loaded and run. You may need several iterations of edit, compile, debug to get the program working. You can either step through with the debugger or you can remove the break point (use Clear() to remove a break point) and try the print option when the menu of choices appears in the ddd command window. There are LOTS of stocks, but the last stock should have symbol ZRN. You can also use the option of printing stocks starting with a give letter.