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Operating
Systems
CSI3131
Assignment 1
Process/thread creation and inter-process communication
You must submit your assignment on-line with Virtual Campus. This is the only method
by which we accept assignment submissions. Do not send any assignment by email. We
will not accept them. We are not able to enter a mark if the assignment is not submitted
on Virtual Campus! The deadline date is firm since you cannot submit an assignment
passed the deadline. You are responsible for the proper submission of your assignments
and you cannot appeal for having failed to do so. A mark of 0 will be assigned to any
missing assignment.
Assignments must be done individually. Any team work, and any work copied from
a source external to the student (including solutions of past year assignments) will be
considered as an academic fraud and will be forwarded to the Faculty of
Engineering for imposition of sanctions. Hence, if you are judged to be guilty of an
academic fraud, you should expect, at the very least, to obtain an F for this course.
Note that we will use sophisticated software to compare your assignments (with
other student’s and with other sources…). This implies that you must take all the
appropriate measures to make sure that others cannot copy your assignment (hence,
do not leave your workstation unattended).
Goals
Practise:
1. process creation in Linux using fork() and exec() system calls
2. thread creation in Linux with the pthread API
3. inter-process communication using pipes
Posted: Jan 28, 2019
Due: Feb 11, 2019, 23:59
Description (Please read the complete assignment document before starting.)
Two programs have been provided: user.c and server.c. Your task shall be to
complete these two programs such that the execution of the program user creates four
processes as follows:

1. User process: this is the original process invoked when user is run. It is
responsible for creating the other three processes after which is simulates User
that exchanges messages with Bot (simulated by the Bot process).
2. Bot process: this is a child process of the User process that simulates Bot who
exchanges messages with User.
3. User Server process: This process (that runs the server program) provides a
server service that services the User process. It shall use one thread to read text
messages from its standard input which is then send to a communications channel
to the Bot process. A second thread is used to send to its standard output any text
messages received from the Bot process over a communications channel.
4. Bot Server process: This process (that runs the server program) provides server
service that services the Bot process. It shall use one thread to read text
messages from its standard input which is then send to a communications channel
to the User process. A second thread is used to send any text messages received
from the User process over a communications channel to its standard output.
5. Message Flow: User sends the first message and Bot responds with two
messages(two options).User selects one and Bot sends Another two options and
so on…
The user program is designed to test the server program by simulating a User and a
Bot that exchange a sequence of text messages. Four pipes are used as shown below: two
pipes allow the User and Bot processes to send messages to the standard inputs of the
User Server and Bot Server processes respectively. Two other pipes are used as the
communication channel between the Servers. The User process that spawns the Server
processes creates the pipes and passes the file descriptors of the communication channel
pipes to the server programs (see section To complete the assignment and comments
in the provided source code templates)..
The pipe identifiers and process identifiers (PIDS) shown in the above diagram are
specific to a run and correspond to the identifiers shown in the output of item 4 in the
section “Background Information”. All standard error file descriptors (2) and the standard
outputs of the Server processes are connected to the terminal. The path of messages from
Bot process is shown (a similar path is taken for messages from the User process).
To complete the assignment:
1. Start with the provided files user.c and server.c. Complete the documentation
in each file to indicate your name and student number. Take the time to document
well your code. Consider taking the time to design your code before you start coding.
2. Complete the following functions:
a. user.c: setupBot(),initUserServer(),
initBotServer(),setupUser()
(also complete main to have these functions run
properly)
b. server.c: generateThreads()
3. The programs should be compiled using the commands “cc –o user user.c”
and “cc –o server server.c –lpthreads” (note that for server
compilation, the pthreads library must be explicitly specified). The file makefile
has been provided to compile both files – simply type in the command make.
4. To submit the assignment, upload the files user.c and server.c. Do not forget to click
on the submit button after (and only after) you have uploaded the file.
5. A word of caution, for debugging the programs, if you wish to write messages to the
terminal, you should write to the standard error using the following library call:
fprintf(stderr,”message string\n”) as the standard input and
standard output are to be modified.
6. See point 4 in “Background Information” for hints on how to observe
processes/threads and pipes to help debug your programs.
7. When user is run, the following output should appear on your screen (Note that
PIDs shall be specific to your execution).
[[email protected] assign1]$ user
Simulation starting
Server Connected (2147)
Server Connected (2149)
Need to recharge. (2145)
Select one option
1) Mobile (2146)
2) Wireless (29786) (2146)
Selected : 1) Mobile (29785) (2145)
1) Credit (2146)
2) Debit (2146)
Selected : 2) Debit (2145)
1) Confirm (2146)
2) Cancel (2146)
Selected : 1) Confirm (2145)
Successful (2146)
Link severed (2149)
Link severed (2147)
Simulation Complete
Bot Server PID
User Server PID
Bot PID
User PID
Background information:
1. An open file descriptor is an integer, used as a handle to identify an open file. Such
descriptors are used in library functions or system calls such as read() and
write() to perform I/O operations.
2. In Unix/Linux, each process has by default three open file descriptors:
a. Standard input (file descriptor 0, i.e. read(0,buf, 4) reads 4 characters from the
standard input to the buffer buf). Typically, the standard input for a program
launched from the command line is the keyboard input.
b. Standard output (file descriptor 1).
c. Standard error (file descriptor 2).
d. When a command is run from the shell, the standard input, standard output
and standard error are connected to the shell tty (terminal). So reading the
standard input reads from the keyboard and writing to the standard output or
standard error writes to the display.
e. Note that many library functions used these file descriptors by default. For
example printf(“String”) writes “String” to the standard output.
f. From the shell it is possible to connect the standard output from one process to
the standard output to another process using the pipe character “|”. For
example, the command “who | wc” connects the standard output from the who
process to the standard input of the wc process such that any data written to
the standard output by the who process is written (via a pipe) to the standard
input of the wc process.
3. You will need the following C library functions:
a. fork() – should be familiar from lectures
b. pthread_create() – should be familiar from lectures
c. pipe()
 should be familiar from lectures
 note that multiple process can be attached to each end of the pipes,
which means that a pipe is maintained until no processes are
connected at either end of the pipe
d. execvp(const char * program, const char *args[]) (or execlp)
 replaces the current process with the program from the file specified in
the first argument
 the second argument is a NULL terminated array of strings
representing the command line arguments
 by convention, args[0] is the file name of the file to be executed
e. execlp(const char * program, const char *arg1, const char *arg2, … NULL)
 replaces the current process with the program from the file specified in
the first argument
 the second argument subsequent arguments are strings representing the
command line arguments.
 by convention, arg1 is the file name of the file to be executed
f. dup2(int newfd, int oldfd) – duplicates the oldfd by the newfd and closes the
oldfd. See http://mkssoftware.com/docs/man3/dup2.3.asp for more
information. For example, the following program:
int main(int argc, char *argv[]) {
int fd;
printf(“Hello, world!”)
fd = open(“outFile.dat”, “w”);
if (fd != -1) dup2(fd, 1);
printf(“Hello, world!”);
close(fd);
}
will redirect the standard output of the program into the file outFile.dat,
i.e. the first “Hello, world!” will go into the console, the second into the
file “outFile.dat”.
g. read(int fd, char *buff, int bufSize) – read from the file (or pipe) identified by
the file descriptor fd bufSize characters into the buffer buff. Returns the
number of bytes read, or -1 if error or 0 if the end of file has been reached (or
the write end of the pipe has been closed and all data read).
h. write(int fd, char *buff, int bufSize) – write into the file/pipe bufSize
characters from the buffer buff
i. close(int fd) – closes an open file descriptor
j. printf(): You may want to use the printf() function to format output. This
function writes to the standard output (fd 1). But be careful since this function
buffers output and does not write immediately to the standard output. To force
and immediate write, used fflush(stdout). Alternatively, you may used
sprintf(), to format the an output into a buffer and use write() to write to the
standard output.
k. fprintf(): this is a version of printf() that provides the means to specify where
output should be send. Use it to write to the standard error with
fprintf(stderr,”a message”, arg, arg,…). This function is useful for debugging
as it will write to the terminal in processes where the standard output has been
redirected to a pipe.
l. getpid(): this function returns the PID of the current process. It is useful in
creating messages printed on the screen to identify the source of the message.
m. Consult the manual pages (by typing ‘man function_name’, i.e. ‘man fork’)
and/or web resources for more information.
4. Here is a hint at how you can observe the connection of processes to pipes
a. Insert long delays using the standard library function sleep (e.g.
sleep(300)) to allow observation of processes, threads and pipes at
different points in the execution of the programs (particularly in user.c).
b. To see the processes and threads created using the command “ps –Hmu
test1” (replace test1 with your user name if you are using one of the SITE
Linux platforms). The option H has ps print out a tree of processes/threads
and the option m includes all threads. In fact, Linux treats all processes and
threads as tasks assigning each a PID. See below for expected output.
[[email protected] assign1]$ ps -Hmu test1
PID TTY TIME CMD
1114 ? 00:00:56 sshd
1115 pts/0 00:00:01 bash
2103 pts/0 00:00:00 vim
2112 pts/1 00:00:00 bash
2145 pts/1 00:00:00 user
2146 pts/1 00:00:00 user
2147 pts/1 00:00:00 server
2148 pts/1 00:00:00 server
2150 pts/1 00:00:00 server
2149 pts/1 00:00:00 server
2151 pts/1 00:00:00 server
2152 pts/1 00:00:00 server
2153 pts/1 00:00:00 ps
1833 tty1 00:00:00 bash
2148 and 2150 are
threads created by 2147
(using pthreads)
Bot process forked
by User process 2145
User process that
forks three processes
Bot server process
User Server process
See the next page for a hint on
observing file descriptors and pipes.
c. To see how pipes and standard input, standard output, and standard error are
set up for the various processes, use the command “ls –l
/proc/xxx/fd” where xxx is replaced with the PID of a process. This
will display how the various file descriptors of the identified process are
connected. See below for expected output from the programs of this
assignment.
[[email protected]]$ ls -l /proc/2145/fd /proc/2149/fd /proc/2147/fd /proc/2146/fd
/proc/2145/fd:
total 0
lrwx—— 1 test1 test1 64 Feb 3 12:03 0 – /dev/pts/1
l-wx—— 1 test1 test1 64 Feb 3 12:03 1 – pipe:[11096]
lrwx—— 1 test1 test1 64 Feb 3 12:03 2 – /dev/pts/1
/proc/2146/fd:
total 0
lrwx—— 1 test1 test1 64 Feb 3 12:03 0 – /dev/pts/1
l-wx—— 1 test1 test1 64 Feb 3 12:03 1 – pipe:[11093]
lrwx—— 1 test1 test1 64 Feb 3 12:03 2 – /dev/pts/1
/proc/2147/fd:
total 0
lr-x—— 1 test1 test1 64 Feb 3 12:03 0 – pipe:[11093]
lrwx—— 1 test1 test1 64 Feb 3 12:03 1 – /dev/pts/1
lrwx—— 1 test1 test1 64 Feb 3 12:03 2 – /dev/pts/1
lr-x—— 1 test1 test1 64 Feb 3 12:03 4 – pipe:[11094]
l-wx—— 1 test1 test1 64 Feb 3 12:03 7 – pipe:[11095]
/proc/2149/fd:
total 0
lr-x—— 1 test1 test1 64 Feb 3 12:03 0 – pipe:[11096]
lrwx—— 1 test1 test1 64 Feb 3 12:03 1 – /dev/pts/1
lrwx—— 1 test1 test1 64 Feb 3 12:03 2 – /dev/pts/1
l-wx—— 1 test1 test1 64 Feb 3 12:03 5 – pipe:[11094]
lr-x—— 1 test1 test1 64 Feb 3 12:03 6 – pipe:[11095]
Notice that the User process messages written to the standard output traverses
pipe 11096 to the User server process where the message is sent to the Bot
server process who then writes the message to its standard output. A similar
path with different pipes is taken by messages leaving the Bot process. Notice
that both servers have their standard output attached to /dev/pts/1 that
corresponds to a terminal (actually a pseudo-terminal connected to an ssh
client). So messages written to the standard output by the Servers processes
shall appear on the terminal.
User process. Stdout (fd 1) attached to write end of pipe 11096.
Bot process. Stdout (fd 1) attached to write end of pipe 11093.
Bot server process. Stdin (fd 0) attached to read end of pipe 11093.
Note the two pipes used as the communications channel
User server process. Stdin (fd 0) attached to read end of pipe 11096.
Note the two pipes used as the communications channel
Grading criteria;
Compiles and runs correctly by fulfilling the conditions below [100pts].
If any of the points below fail to be implemented; 5 points will be deducted for each nonimplemented part.
If does not compile and/or run correctly; the conditions below will be checked
individually:
· Setting up user and bot [15]
· Setting both the severs correctly [15]
· Connection between the user server and user (bot server and bot)[25]
· Generating threads and in order execution of threads[15]
TA office hours and contacts for questions about the assignment:
Raja Gadamsetty [email protected]
Thursday: 12:00 – 13:30, Room: STE5000J
Wednesday: 10:30 – 12:00, Room: STE5000J
Wednesday: 18:00 – 19:30, Room: STE5000J

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