Project 1 Process Programming

Programming Problems 155
3.21 The Collatz conjecture concerns what happens when we take any
positive integer n and apply the following algorithm:
n =
 n/2, if n is even
3 × n + 1, if n is odd
The conjecture states that when this algorithm is continually applied,
all positive integers will eventually reach 1. For example, if n = 35, the
sequence is
35, 106, 53, 160, 80, 40, 20, 10, 5, 16, 8, 4, 2, 1
Write a C program using the fork() system call that generates this
sequence in the child process. The starting number will be provided
from the command line. For example, if 8 is passed as a parameter on
the command line, the child process will output 8, 4, 2, 1. Because the
parent and child processes have their own copies of the data, it will be
necessary for the child to output the sequence. Have the parent invoke
the wait() call to wait for the child process to complete before exiting
the program. Perform necessary error checking to ensure that a positive
integer is passed on the command line.
3.22 In Exercise 3.21, the child process must output the sequence of numbers
generated from the algorithm specified by the Collatz conjecture because
the parent and child have their own copies of the data. Another
approach to designing this program is to establish a shared-memory
object between the parent and child processes. This technique allows the
child to write the contents of the sequence to the shared-memory object.
The parent can then output the sequence when the child completes.
Because the memory is shared, any changes the child makes will be
reflected in the parent process as well.
This program will be structured using POSIX shared memory as
described in Section 3.5.1. The parent process will progress through the
following steps:
a. Establish the shared-memory object (shm open(), ftruncate(),
and mmap()).
b. Create the child process and wait for it to terminate.
c. Output the contents of shared memory.
d. Remove the shared-memory object.
One area of concern with cooperating processes involves synchronization issues. In this exercise, the parent and child processes must be
coordinated so that the parent does not output the sequence until the
child finishes execution. These two processes will be synchronized using
the wait() system call: the parent process will invoke wait(), which
will suspend it until the child process exits.
3.23 Section 3.6.1 describes port numbers below 1024 as being well known—
that is, they provide standard services. Port 17 is known as the quote-of-
Programming Projects 157
copied and the name of the copied file. The program will then create
an ordinary pipe and write the contents of the file to be copied to the
pipe. The child process will read this file from the pipe and write it to
the destination file. For example, if we invoke the program as follows:
filecopy input.txt copy.txt
the file input.txt will be written to the pipe. The child process will
read the contents of this file and write it to the destination file copy.txt.
You may write this program using either UNIX or Windows pipes.
Programming Projects
Project 1—UNIX Shell and History Feature
This project consists of designing a C program to serve as a shell interface
that accepts user commands and then executes each command in a separate
process. This project can be completed on any Linux, UNIX, or Mac OS X system.
A shell interface gives the user a prompt, after which the next command
is entered. The example below illustrates the prompt osh and the user’s
next command: cat prog.c. (This command displays the file prog.c on the
terminal using the UNIX cat command.)
osh cat prog.c
One technique for implementing a shell interface is to have the parent process
first read what the user enters on the command line (in this case, cat
prog.c), and then create a separate child process that performs the command.
Unless otherwise specified, the parent process waits for the child to exit
before continuing. This is similar in functionality to the new process creation
illustrated in Figure 3.10. However, UNIX shells typically also allow the child
process to run in the background, or concurrently. To accomplish this, we add
an ampersand (&) at the end of the command. Thus, if we rewrite the above
command as
osh cat prog.c &
the parent and child processes will run concurrently.
The separate child process is created using the fork() system call, and the
user’s command is executed using one of the system calls in the exec() family
(as described in Section 3.3.1).
A C program that provides the general operations of a command-line shell
is supplied in Figure 3.36. The main() function presents the prompt osh-
and outlines the steps to be taken after input from the user has been read. The
main() function continually loops as long as should run equals 1; when the
user enters exit at the prompt, your program will set should run to 0 and
terminate.
This project is organized into two parts: (1) creating the child process and
executing the command in the child, and (2) modifying the shell to allow a
history feature.
158 Chapter 3 Processes
#include <stdio.h
#include <unistd.h
#define MAX LINE 80 /* The maximum length command */
int main(void)
{
char *args[MAX LINE/2 + 1]; /* command line arguments */
int should run = 1; /* flag to determine when to exit program */
while (should run) {
printf("osh");
fflush(stdout);
/**
* After reading user input, the steps are:
* (1) fork a child process using fork()
* (2) the child process will invoke execvp()
* (3) if command included &, parent will invoke wait()
*/
}
return 0;
}
Figure 3.36 Outline of simple shell.
Part I— Creating a Child Process
The first task is to modify the main() function in Figure 3.36 so that a child
process is forked and executes the command specified by the user. This will
require parsing what the user has entered into separate tokens and storing the
tokens in an array of character strings (args in Figure 3.36). For example, if the
user enters the command ps -ael at the osh prompt, the values stored in the
args array are:
args[0] = "ps"
args[1] = "-ael"
args[2] = NULL
This args array will be passed to the execvp() function, which has the
following prototype:
execvp(char *command, char *params[]);
Here, command represents the command to be performed and params stores the
parameters to this command. For this project, the execvp() function should
be invoked as execvp(args[0], args). Be sure to check whether the user
included an & to determine whether or not the parent process is to wait for the
child to exit.
Programming Projects 159
Part II—Creating a History Feature
The next task is to modify the shell interface program so that it provides
a history feature that allows the user to access the most recently entered
commands. The user will be able to access up to 10 commands by using the
feature. The commands will be consecutively numbered starting at 1, and
the numbering will continue past 10. For example, if the user has entered 35
commands, the 10 most recent commands will be numbered 26 to 35.
The user will be able to list the command history by entering the command
history
at the osh prompt. As an example, assume that the history consists of the
commands (from most to least recent):
ps, ls -l, top, cal, who, date
The command history will output:
6 ps
5 ls -l
4 top
3 cal
2 who
1 date
Your program should support two techniques for retrieving commands
from the command history:
1. When the user enters !!, the most recent command in the history is
executed.
2. When the user enters a single ! followed by an integer N, the Nth
command in the history is executed.
Continuing our example from above, if the user enters !!, the ps command
will be performed; if the user enters !3, the command cal will be executed.
Any command executed in this fashion should be echoed on the user’s screen.
The command should also be placed in the history buffer as the next command.
The program should also manage basic error handling. If there are
no commands in the history, entering !! should result in a message “No
commands in history.” If there is no command corresponding to the number
entered with the single !, the program should output "No such command in
history."
Project 2—Linux Kernel Module for Listing Tasks
In this project, you will write a kernel module that lists all current tasks in a
Linux system. Be sure to review the programming project in Chapter 2, which
deals with creating Linux kernel modules, before you begin this project. The
project can be completed using the Linux virtual machine provided with this
text.