CSS 430
Program 1: System Calls and Shell
Duedate: See the syllabus

0. Attention

Please note that you must independently work on all programming assignments except the final project. The instructor keeps the code of all the former students in his database, compares your code with the database contents, and will never tolerate any academic dishonesty. In fact, some students have been sent to the vice chancellor's office when their plagiarism were found.

1. Purpose

2. Shell

Command interpretation

1. Displaying a prompt to indicate that it is ready to accept a next command from its user,
2. Reading a line of keyboard input as a command, and
3. Spawning and having a new process execute the user command.

   uw1-320-00%

   $

1. The shell locates an executable file whose name is specified in the first string given from a keyboard input.
2. It creates a child process by duplicating itself.
3. The duplicated shell overloads its process image with the executable file.
4. The overloaded process receives all the remaining strings given from a keyboard input, and starts a command execution.

   uw1-320-00% a.out a b c

The shell has some built-in commands that rather changes its current status than executes a user program. (Note that user programs are distinguished as external commands from the shell built-in commands.) For instance,

   uw1-320-00% cd public_html

The shell can receive two or more commands at a time from a keyboard input. The symbols ';' and '&' are used as a delimiter specifying the end of each single command. If a command is delimited by ';', the shell spawns a new process to execute this command, waits for the process to be terminated, and thereafter continues to interpret the next command. If a command is delimited by '&', the shell goes forward and interprets the next command without waiting for the completion of the current command.

   uw1-320-00% who & ls & date

1. Displaying a prompt to show that it is ready to accept a new line input from the keyboard,
2. Reading a keyboard input,
3. Repeating the following interpretation till reaching the end of input line:
   • Changing its current working status if a command is built-in, otherwise
   • Spawning a new process and having it execute this external command.
   • Waiting for the process to be terminated if the command is delimited by ';'.

I/O redirection and pipeline

   uw1-320-00% a.out < file1 > file2

   uw1-320-00% command1 | command2 | command3

   uw1-320-00% who | wc -l

Shell implementation techniques

3. ThreadOS

Through this series of assignments, you are to implement and/or to enhance some portions of ThreadOS. ThreadOS loads Java programs that have been derived from the Thread class, manages them as user processes, and provides them with some basic operating system services. Those services include thread spawn, thread termination, disk operations, and even standard input/output. ThreadOS receives all service requests as a form of interrupt from each user thread, handles them, and returns a status value to the interrupting thread.

Structure

To start ThreadOS, simply type:

   uw1-320-00% java Boot
   ThreadOS ver 1.0:
   Type ? for help
   threadOS: a new thread (thread=Thread[Thread-3,2,main] tid=0 pid=-1)
   -->

Note that Loader is not a shell. It simply launches and waits for the completion of a user program (which may behave as a shell). From ThreadOS' point of view, there is no distinction between Loader and the other user programs.

User Programs

   public class PingPong extends Thread {
      private String word;
      private int loop;
      public PingPong( String[] args ) {
         word = args[0];
         loop = Integer.parseInt( args[1] );
      }
      public void run( ) {
         while ( true ) {
            System.out.print( word + " " );
            for (int i = 0; i < loop; i++ );
         }
      }
   }

public class ThreadDriver {
   public static void main( String[] args ) {
     String args[2];

     args[0] = "ping"; args[1] = "10000";
     new PingPong( args ).start( );

     args[0] = "PING"; args[1] = "90000";
     new PingPong( args ).start( );
   }
}

ThreadOS Loader actually takes care of this thread-instantiating part of main function. Once you invoke ThreadOS, Loader waits for a l command, say "l PingPong ping 10000". Then, it will load your PingPong class into the memory, instantiate its object, pass a String array including ping and 10000 as arguments to this thread, and wait for its termination. Note that general Java threads can receive any type of and any number of arguments, however ThreadOS restricts its user programs to receive only a String array as their argument.

Java itself provides various classes and methods that invoke real OS system calls such as System.out.println and sleep. Since ThreadOS is an operating systems simulator, user programs running on ThreadOS are prohibited to use such real OS system calls. Prohibited classes include but are not limited to:

   SysLib.cout( word + " " );

   SysLib.exit( );

   public class PingPong extends Thread {
      private String word;
      private int loop;
      public PingPong( String[] args ) {
         word = args[0];
         loop = Integer.parseInt( args[1] );
      }
      public void run( ) {
         for ( int j = 0; j < 100; j++ )
            SysLib.cout( word + " " );
            for (int i = 0; i < loop; i++ );
         }
         SysLib.cout( "\n" );
         SysLib.exit( );
      }
   }

   Kernel.interrupt( int interruptRequestVector, 
                     int trapNumber, 
                     int parameter, 
                     Object args );

   ps -A | grep argv[1] | wc -l

   uw1-320-00% java Boot
   ThreadOS ver 1.0:
   Type ? for help
   -->l Shell
   l Shell
   threadOS: a new thread (thread=Thread[Thread-6,2,main] tid=1 pid=0)
   shell[1]% TestProg1 & TestProg2 &
   ... A concurrent execution of TestProg1 and TestProg2
   ...  
   shell[2]% TestProg1 ; TestProg2
   ... A sequential execution of TestProg1 and TestProg2
   ...
   shell[3]% exit
   -->q
   uw1-320-00%

   shell[1]%

Shell[2] PingPong abc 10000000 & PingPong xyz 10000000 & PingPong 123 10000000 &
Shell[1] PingPong abc 10000000 ; PingPong xyz 10000000 ; PingPong 123 10000000 ;