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CSCE 451/851 Programming Assignment 3
Implementation of a Monitor
1 Overview of Project
This programming assignment is to familiarize yourself with the synchronization constructs we discuss in
class – mutexes, semaphores, and monitors. The goal is to ensure no deadlocks, no race conditions, and
that data is shared properly between threads.
Part 1 In Part 1 you will solve the producer/consumer problem by utilizing POSIX semaphores to solve
concurrency problems.
Part 2 In Part 2 you will solve the producer/consumer problem by developing a custom monitor.
As usual we have provided you with precompiled binaries (part1 and part2 in the handout folder) that
run on the CSE servers to observe the correct input/output and compare against your solution.
2 Program Specification
Implement switches To make the program easier to evaluate, you will need to implement additional
switches to specify the buffer length, number of producers and the number of consumers as indicated
below.
Switch Specifies
-b Buffer length in bytes
-p Number of producer threads
-c Number of consumer threads
-i Number of items to insert
As an example: cse> ./part1 -b 1000 -p 10 -c 10 -i 100000
should create a buffer of length 1,000 bytes, 10 producer threads, 10 consumer threads, and insert 10,000
items.
Output Convention Whenever an item is produced (i.e., put in the buffer), a message needs to be printed
to the screen with the following (printf()) convention:
“p:<%u>, item: %c, at %d”, threadid, item, index
This indicates a producer with threadid inserted item at index in the buffer. A similar message is
required whenever an item is produced:
“c:<%u>, item: %c, at %d”, threadid, item, index
indicating a consumer with threadid has consumed item at index in the buffer.
Exit The producer thread should exit when it has no items left to insert into the buffer. The consumer
thread should exit when the buffer is empty and it has consumed all the items. Manage this by using a
counter for the number of items inserted and removed.
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3 Submission
Use CSE web handin to hand in your assignment. Submit a single zip file, <UNL username>_<pa#>.zip
(e.g., jdoe2_pa1.zip) containing the following directory structure and files:
<UNL username>_pa3
|—–prob1
| |—-Makefile
| |—-part1.c (or part1.cpp)
|—–prob2
|—-Makefile
|—-part2.c (or part2.cpp)
Be sure to:
1. Replace <UNL username> with your UNL username.
2. Add a separate Makefile for both problems. The Makefiles should:
a. Have an all target as the first target.
b. Produce respective binaries part1 for Part 1, and part2 for Part 2.
NOTE: You can complete this assignment using strict C, but if you want to use C++ you can as long as you
follow the limitations listed in this document regarding the use of pthread_ functions.
4 Evaluation
Your program will be graded according to the following rubric:
Command ND? NRC? ICM? Total
./part1 -b 1 -p 5 -c 5 -i 10 5 5 5 15
./part1 -b 1000 -p 20 -c 20 -i 10000 5 5 5 15
Part 1 Total = 30 points
./part2 -b 1 -p 5 -c 5 -i 10 5 5 5 15
./part2 -b 4 -p 30 -c 30 -i 10 5 5 5 15
./part2 -b 10 -p 1 -c 10 -i 20 5 5 5 15
./part2 -b 1000 -p 20 -c 20 -i 10000 5 5 5 15
Part 2 Total = 60 points
Makefile for Part 1 5
Makefile for Part 2 5
Total 100
where
• “ND?” = No Deadlocks?
– Deadlocks would cause your program to hang, so we will look for that.
• “NRC?” = No Race Conditions?
– Race conditions would manifest most typically by producing a character in the buffer at a certain
index and then consuming from that same index a different character. So be sure if a producer
puts a ‘D’ at index 5 in the buffer, a consumer gets a ‘D’ out at index 5.
• “ICM?” = Item Count Met?
– You should consume and produce the same number of items, which should match the command
line argument for -i.
Additionally if the naming convention described in the Submission section above is not followed you lose
10 points. Remember, if your code doesn’t compile on the CSE servers you will get 0 for all tests. Please
check that your code both compiles and runs on the CSE servers!!! Don’t wait until it works completely on
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your own machine before testing on the CSE servers. Test on the CSE servers at regular milestones in your
development.
Protip You can combine the grep command with the wc command to get the count of producers or
consumers. For example, for producers:
./part2 -b 1000 -p 20 -c 20 -i 10000 | grep p: | wc -l
And for consumers:
./part2 -b 1000 -p 20 -c 20 -i 10000 | grep c: | wc -l
5 Part 1: Producer/Consumer with Semaphores
The goal of Part 1 is to solve the producer/consumer problem using semaphores. You will use the pthread
library to create producer threads and consumer threads. Each producer thread inserts a single ‘X’ character
into a buffer. Each consumer thread removes the most recently inserted ‘X’ from the buffer. Each thread
then repeats the process. Use POSIX semaphore (sem_init, sem_wait, sem_post, sem_destroy). The
pseudocode is given in Pseudocode 1 below. Please stick to the main structure in the pseudocode, but feel free to
add more parameters, variables, and functions as they become necessary.
Pseudocode 1
Pseudocode for producer/consumer problem using semaphores:
# define N 10000000
semaphore mutex = 1;
semaphore empty = N;
semaphore full = 0;
void main( void )
{
// create -p # of producer threads
// create -c # of consumer threads
}
void * producer( void )
{
while(1)
{
sem_wait(&empty);
sem_wait(&mutex);
// insert X into the first available slot in the buffer
insert(‘X’);
sem_post(&mutex);
sem_post(&full);
}
}
void * consumer( void )
{
while(1)
{
sem_wait(&full);
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sem_wait(&mutex);
// remove X from the last used slot in the buffer
remove();
sem_post(&mutex);
sem_post(&empty);
}
}
6 Part 2: Producer/Consumer with Monitor
The goal of Part 2 is to create your own monitor to provide synchronization support for the producer/consumer problem. You will use the pthread library again to create producer threads and consumer
threads. Each producer thread inserts a randomly generated character from the alphabet (upper and lower
cases) into the first available slot in a buffer. Each consumer thread removes a character from the most
recently used slot of the buffer. Each thread then repeats the process. Pseudocode 2 provides the basic outline
your code should take, but feel free to add more parameters, variables, and functions as they become necessary.
Pseudocode 2
Pseudocode for producer/consumer problem using monitors:
// ===== pro_con.c ===== //
void main( void )
{
// any functions you think necessary
// create producer threads
// create consumer threads
// join all threads
}
void * producer( ) // add more parameters as needed
{
char alpha;
while(1) {
alpha = generate_random_alphabet();
mon_insert(alpha);
}
}
void * consumer( ) //add more parameters as needed
{
char result;
while(1) {
result = mon_remove();
}
}
// ===== monitor.c ===== //
#define N 10000000
typedef struct {
// condition variable fields
} cond;
int count(cond cv)
{
// return # of threads blocked on cv
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}
void wait(cond cv)
{
// give up exclusive access to monitor
// and suspend appropriate thread
// implement either Hoare or Mesa paradigm
}
void signal(cond cv)
{
// unblock suspended thread at head of queue
// implement either Hoare or Mesa paradigm
}
cond empty, full;
int count = 0;
char buf[N];
// add more variables as necessary
// define condition variable struct
// define monitor struct
void mon_insert(char alpha)
{
// implement either Hoare or Mesa paradigm
// synchronization and bookkeeping
while(count == N)
wait(full);
insert_item(alpha);
count = count+1;
signal(empty);
}
char mon_remove()
{
// implement either Hoare or Mesa paradigm
// synchronization and bookkeeping
char result;
while(count == 0)
wait(empty);
result = remove_item();
count = count-1;
signal(full);
return result;
}
Helpful Tips and Instructions
1. Create a new variable type for condition variables (CV). CVs are used to delay processes or threads
that cannot continue executing due to a specific monitor state (e.g., full buffer). They are also used to
awaken delayed processes or threads when the conditions are satisfiable. The variable type (probably
a struct) consists of an integer variable that indicates the number of threads blocked on a condition
variable and a semaphore that is used to suspend threads. There are three operations that can be
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performed on the CV. They are:
• count(cv)—returns the number of threads blocked on the cv.
• wait(cv)— relinquishes exclusive access to the monitor and then suspends the executing
threads.
• signal(cv)—unblocks one thread suspended at the head of the cv blocking queue. The
signaled thread resumes execution where it was last suspended.
You are not allowed to use existing condition variables or synchronization mechanisms such as
the one from pthread library (pthread_cond_init, or any of the other pthread_ functions) You
should be able to complete the assignment with:
• sem_wait() – decrement (lock) a semaphore
• sem_post() – increment (unlock) a sempahore
• sem_init() – initialize a semaphore
• sem_destroy() – destroy the semaphore
• pthread_create() – create a thread
• pthread_exit() – exit a thread
• pthread_join() – join a thread
If you use other pthread_ function calls you will lose points. Think about the following questions
as you design your solution:
a. How would you guarantee that only one thread is inside the monitor at one time?
b. Will your monitor follow the signal and wait or signal and continue discipline?
c. How would you make sure that a suspended thread (due to wait) resumes where it left off?
d. How would you initialize the necessary data structures to support your monitor and make them
visible to all threads?
2. Create a function mon_insert that inserts a character into the buffer. If the buffer is full, it invokes
wait on the condition variable full. It also invokes signal on condition variable empty.
3. Create a function mon_remove that removes a character from the buffer. If the buffer is empty, it
invokes wait on the condition variable empty. It also invokes signal on condition variable full. The
function returns the removed character.
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