Shared Memory Programming Assignment

Shared Memory Programming Assignment

Goal: To be able to write a C, C++, or Java program using the shared memory programming constructs and simple mutex or synchronize statements.

Preparation:

Study the class notes on how to use the pthread library calls, including: pthread_create, pthread_join, pthread_exit, pthread_mutex_init, pthread_mutex_lock, and pthread_mutex_unlock.
If you want to write your code using Java then you will need to read the material on Java threads at http://java.sun.com.

Guidelines:

For this assignment you may share information about how to compile and execute programs in Unix. However, each of you is to write the code on your own and submit your own program for a grade.

What you turn in:

Place a copy of your source code into a subdirectory of your home directory on Prospero with the name "sharememATM". If you create any additional data files place them there, also. We may ask for a demonstration of your program.

Program Specifications:

This programming assignment will involve the idea of modern-day banking using an ATM (Automatic Transaction Machine). This program works on the idea that multiple users sometimes share the same bank account. For this program, you will assume that a single account will be accessed by up to four different users. Commands that a user may want to perform to the account include adding an amount to the balance, subtracting an amount from the balance, and printing the current balance. Users will act independently and concurrently on the account. Since each user may add or subtract funds from the account, your program must use appropriate synchronization tools to ensure that the balance in the account is accurate and valid.

Write a C, C++, or Java program using the pthread library that consists of code for main, a user thread, and supporting functions as needed. The code for main must do the following:

Accept a command line parameter which is the number of users (i.e., threads) that the program will handle. Ensure that the parameter is a valid integer between 1 and 4. If the command line parameter is invalid, print out a usage message and exit.
For the purpose of this exercise, the commands that each "user" will enter will come from a data file in the current directory. The main program should open a file for each user, from 1 up to 4. You may hardcode the filenames for this exercise, if you wish, or you may specify the filenames at run time with command-line arguments. Do not put prompts into your program. The filenames should default to "file1.dat", "file2.dat", "file3.dat", and "file4.dat". Sample data files are located at file1.dat, file2.dat, file3.dat, and file4.dat. The main program should open the files before any thread operations are performed.
Main should declare an initial balance for the account of $1000.
The main program should create a thread for each "user". Pass as a parameter to the thread a thread identifier (1, 2, 3, or 4). You may also optionally pass as a parameter the open file pointer of the file that represents the commands that the thread will execute. Thread 1 should read the first file specified, thread 2 should read the second file, and so on.
The main program should wait for the user threads to complete before it exits. Before exiting it should close the open files.
The final account balance should be printed before main exits.

The code for the user thread must read from the file according to its thread identifier. Until all lines in the file are read, the user thread will read the next command and perform it. Valid commands are to ADD amount, SUB amount, and GET a balance. The commands should do the following:

ADD amount should add the amount to the account balance.
SUB amount should subtract the amount from the account balance.
GET should display the current account balance. Sample ouput is:
           User 1 views account balance of $970.
           User 2 views account balance of $950.
           User 1 views account balance of $970.
           . . . and so on.

Output should only be performed for a GET command. You may assume that the files do not contain errors. When all commands from the file have been read then the thread should exit.

Grading:

For full credit, your program must run correctly on file1 only (one thread), or on file1 and file2 (two threads concurrently), or on file1 and file2 and file3 (three threads concurrently), or on all four files (four threads concurrently) on Prospero. You must follow all program specifications. Your program should be appropriately commented and formatted. This program is worth 30 points.

Extra Credit:

As specified, it is possible that the account balance can drop below zero. For up to 10 extra credit points, extend the program so that the account balance is always positive. If a user attempts to subtract money from the account in a manner that will make the balance negative, force the user to wait until the balance is large enough that the subtraction can be made safely. You may find condition variables (using pthread_cond_init, pthread_cond_lock, etc.) useful for this exercise. Try to come up with a solution that does not hang indefinitely.

Enjoy!