Here are some concurrent and distributed system problems

 

 

CS 475 Spring 2017
Assignment 2 - PART A
DUE 11:59 PM Thursday, March 9
Instructions:
• PART A of Assignment 2 is to be completed individually by each student. No collaboration among
students is permitted.
• Please upload your solutions to PART A to Blackboard in PDF or MS Word format.
• Late submissions will not be graded. There are no slip days for Assignment 2 - PART A. Problem 1. (15 points):
This problem tests your understanding of exceptional control flow in C programs. Assume we are running
code on a Unix machine. The following problems all concern the value of the variable counter. Part I
int counter = 0;
int
{ main()
int i;
for (i = 0; i < 2; i ++){
fork();
counter ++;
printf("counter = %d\n", counter);
}
printf("counter = %d\n", counter);
return 0; } A. How many times would the value of counter be printed: ____________
B. What is the value of counter printed in the first line? ____________
C. What is the value of counter printed in the last line? ____________
Page 1 of 6 Part II
pid_t pid;
int counter = 0;
void handler1(int sig)
{
counter ++;
printf("counter = %d\n", counter);
fflush(stdout);
/* Flushes the printed string to stdout */
kill(pid, SIGUSR1);
}
void handler2(int sig)
{
counter += 3;
printf("counter = %d\n", counter);
exit(0);
}
main() {
signal(SIGUSR1, handler1);
if ((pid = fork()) == 0) {
signal(SIGUSR1, handler2);
kill(getppid(), SIGUSR1);
while(1) {};
}
else {
pid_t p; int status;
if ((p = wait(&status)) > 0) {
counter += 2;
printf("counter = %d\n", counter);
}
}
} What is the output of this program? Page 2 of 6 Part III
int counter = 0;
void handler(int sig)
{
counter ++;
} int
{ main()
int i;
signal(SIGCHLD, handler);
for (i = 0; i < 5; i ++){
if (fork() == 0){
exit(0);
}
}
/* wait for all children to die */
while (wait(NULL) != -1);
printf("counter = %d\n", counter);
return 0; } A. Does the program output the same value of counter every time we run it? Yes No B. If the answer to A is Yes, indicate the value of the counter variable. Otherwise, list all possible values
of the counter variable.
Answer: counter = __________________ Page 3 of 6 Problem 2. (25 points):
You have been hired by a company to work on a program that does climate modelling. The inner loop of the
program matches atoms of different types as they form molecules. In an excessive reliance on threads, each
atom is represented by a thread.
A Your task is to write code to form carbon dioxide out of one carbon thread and two oxygen threads (C02).
You are to write the two procedures: CArrives() and OArrives(). A carbon dioxide molecule
forms when two O threads are present and one C thread; otherwise the atoms must wait. Once all
three are present, one of the threads calls MakeCO2(), and only then, all three depart.
B Extending the product line into beer production, your task is to write code to form alcohol (C2H60) out
of two carbon atoms, six hydrogens, and one oxygen.
You must use locks and Mesa-style condition variables to implement your solutions. Obviously, an atom
that arrives after the molecule is made must wait for a different group of atoms to be present. There should
be no busy-waiting. There should also be no useless waiting: atoms should not wait if there is a sufficient
number of each type to make a particular molecule. Page 4 of 6 Problem 3. (10 points):
Consider the code written by Harry Q. Bovik for the following problem. Please assume that all necessary
header files are included in the code and all system calls and accessory functions always succeed. You may
assume that the locks have been initialized correctly in main(), which we do not show.
1. Bovik comes to you and complains that func1() seems to misbehave. Is he lying or is there something wrong with his code? Defend your answer in 1-3 sentences.
2. Bovik is complaining about func2() as well. He insists to your boss that this program sometimes
hangs and your boss would like your opinion. Is Bovik lying again or is there a problem? Defend
your answer in 1-3 sentences. pthread_mutex_t *count_l, *l_count;
int ref_count, tid_1, tid_2, tid_3, tid_4;
void *thread1(void *vargp) {
tid_2 = pthread_self();
pthread_mutex_lock(count_l);
ref_count++;
pthread_mutex_unlock(count_l);
return(0);
}
void *thread2(void *vargp) {
tid_1 = pthread_self();
pthread_mutex_lock(count_l);
pthread_kill(pthread_self(), SIGKILL);
ref_count++; pthread_mutex_unlock(count_l);
return(0);
}
void *thread3(void *vargp) {
tid_3 = pthread_self();
pthread_mutex_lock(count_l);
pthread_mutex_lock(l_count);
ref_count++;
pthread_mutex_unlock(l_count);
pthread_mutex_unlock(count_l);
return(0);
}
void *thread4(void *vargp) {
tid_4 = pthread_self();
pthread_mutex_lock(l_count);
pthread_mutex_lock(count_l);
ref_count--;
pthread_mutex_unlock(l_count);
pthread_mutex_unlock(count_l);
return(0);
} Page 5 of 6 void func1(void) {
pthread_t t1,t2;
pthread_create(&t1,NULL, thread1, NULL );
pthread_create(&t2, NULL, thread2, NULL);
pthread_join(t2, NULL); pthread_join(t1, NULL);
exit(0);
}
void func2(void) {
pthread_t t3, t4;
pthread_create(&t4,NULL, thread4, NULL );
pthread_create(&t3, NULL, thread3, NULL);
pthread_join(t3, NULL);
pthread_join(t4, NULL);
exit(0);
} Problem 4. (10 points):
Consider the following three threads and four semaphores: /*
x
/*
s1
s2
s3
s4 Initialize x */
= 1;
Initialize semaphores */
=
=
=
= void thread1()
{
while (x != 360)
{ void thread2()
{
while (x != 360)
{ x = x * 2; x = x * 3; }
exit(0);
} void thread3()
{
while (x != 360)
{ x = x * 5; }
exit(0);
} }
exit(0);
} Provide initial values for the four semaphores and add P(), V() semaphore operations (using the four
semaphores) in the code for thread 1, 2 and 3 such that the process is guaranteed to terminate. Page 6 of 6

Attachments:

• 1493696512-assignment2a.pdf