midterm

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CS 471
Operating Systems

Spring 2012
Midterm Exam

Time 2 & 1/2 hours

Open Book & Notes

Name:

Unix Login:

Each Question is 20 points.

Question 1

What is the print out of the following programs:

int main()

{

fork();

printf("Q1a cs471w\n");

}

int main()

{

if (fork())

if (fork())printf("Q1b cs471w \n");

}

int main()

{

if (fork()) execl("Q1cexec", "Q1cexec", 0);

sleep(1);

printf("Q1c cs471w\n");

}

Q1cexec:

int main()

{

printf("Q1cexec cs471w\n");

}

Question 2

Consider the following three processes submitted in the order:

P1, P2 and P3 with estimated CPU times: 8, 6 and 2.

Compute the average waiting time if these 3 processes are scheduled according to:

1) FCFS:

2) SJF:

3) RR with Quantum time = 2. How many context switches are needed?

Question 3

What is the output of the following programs?

#define SIZE 1000

int main(void)

{

int fd[2];

char buf[SIZE];

char *ParentMsg="How are you my child";

char *ChildMsg="I am fine my parent";

int i = SIZE;

pipe(fd);

if (fork()) {

write(fd[1], ParentMsg, strlen(ParentMsg));

wait(NULL);

read(fd[0], buf, 100);

printf("Parent got: %s \nand Parent i = %d\n",buf, i);

}

else {

sleep(1);

read(fd[0], buf, 100);

printf("Child got: %s\n",buf);

sprintf(buf,"%s Child i = %d", ChildMsg, ++i);

write(fd[1], buf, strlen(buf));

}

#define SIZE 3

int

main()

{

int segment_id = shmget(IPC_PRIVATE, SIZE * sizeof(int), S_IRUSR | S_IWUSR);

int *svar = (int *) shmat(segment_id, NULL, 0);

int *nvar = (int *) malloc(SIZE * sizeof(int));

int i;

for (i = 0; i < SIZE; i++) {

svar[i] = SIZE;

nvar[i] = SIZE;

}

pid_t pid = fork();

if (pid == 0) {

for (i = 0; i < SIZE; i++) {

(svar[i])++;

(nvar[i])--;

printf("I am the child: %d, %d\n", svar[i], nvar[i]);

}

exit(0);

}

wait(NULL);

for (i = 0; i < SIZE; i++) {

(svar[i])--;

(nvar[i])++;

printf("I am the parent: %d, %d\n", svar[i], nvar[i]);

}

int sfd;

void One(), Two(), Three();

int main()

{

sfd = open("tmpfile", O_RDWR | O_TRUNC | O_CREAT, 0600);

if (fork() == 0) {

One();

exit(0);

} else {

if (fork() == 0) {

Two();

exit(0);

} else {

wait(NULL);

Three();

}

void One()

{

write(sfd, "123 ", 4);

sleep(2);

exit(0);

}

void Two()

{

sleep(1);

write(sfd, "\n321", 4);

sleep(1);

exit(0);

}

void Three()

{

char buf[10];

int one, two;

lseek(sfd, 0, SEEK_SET);

read(sfd, buf, sizeof(buf));

printf("%s\n", buf);

sscanf(buf, "%d %d", &one, &two);

printf("%d\n", one+two);

exit(0);

}

Question 4

The following programs can serve either as Echo Server or Chat Server

depending on how the function HandleClient is implemented.

#define SA struct sockaddr

#define SIZE 10

void HandleClient(int);

int *connfd ;

int main(int argc, char *argv[])

{

int len, listenfd;

struct sockaddr_in servaddr, cliaddr;

int i;

int segment_id1 = shmget(IPC_PRIVATE, SIZE * sizeof(int), S_IRUSR | S_IWUSR);

connfd = (int *) shmat(segment_id1, NULL, 0);

for (i = 0; i <= SIZE; i++) connfd[i] = -1;

listenfd = socket(AF_INET, SOCK_STREAM, 0);

bzero(&servaddr, sizeof(servaddr));

servaddr.sin_family = AF_INET;

servaddr.sin_addr.s_addr = htonl(INADDR_ANY);

servaddr.sin_port = htons(atoi(argv[1]));

if (bind(listenfd, (SA *) & servaddr, sizeof(servaddr)) < 0) {

perror("bind");

exit(-1);

}

listen(listenfd, 0);

for (;;) {

for (i = 0; i <= SIZE; i++) {

if (connfd[i] == -1) {

len = sizeof(cliaddr);

connfd[i] = accept(listenfd, (SA *) & cliaddr, &len);

if (fork() == 0) HandleClient (i);

}

A. Implement the HandleClient to make the server act as an Echo Server:

void HandleClient(int myIndex)

{

}

B. Implement the HandleClient to make the server act as a Chat Server:

void HandleClient(int myIndex)

{

}

Question 5

What is the output of the following program?

#define MAX 1

void *function1();

void *function2();

pthread_mutex_t mutexA = PTHREAD_MUTEX_INITIALIZER;

pthread_mutex_t mutexB = PTHREAD_MUTEX_INITIALIZER;

pthread_t thread1, thread2;

int A = MAX;

int B = 0;

main(int argc, char *argv[])

{

pthread_create(&thread1, NULL, &function1, NULL);

pthread_create(&thread2, NULL, &function2, NULL);

pthread_join(thread1, NULL);

pthread_join(thread2, NULL);

}

void *function1()

{

int CONTINUE = 1;

while (CONTINUE) {

pthread_mutex_lock(&mutexA);

sleep(1);

pthread_mutex_lock(&mutexB);

A--;

B++;

if ((A <= 0) && (B >= MAX) ) CONTINUE = 0;

printf("Thread 1: A=%d, B=%d\n", A, B);

fflush (stdout);

pthread_mutex_unlock(&mutexB);

pthread_mutex_unlock(&mutexA);

}

printf("Thread 1 is done\n");

pthread_exit(0);

}

void *function2()

{

int CONTINUE = 1;

while (CONTINUE) {

pthread_mutex_lock(&mutexB);

sleep(1);

pthread_mutex_lock(&mutexA);

A--;

B++;

if ((A <= 0) && (B >= MAX) ) CONTINUE = 0;

printf("Thread 2: A=%d, B=%d\n", A, B);

fflush (stdout);

pthread_mutex_unlock(&mutexA);

pthread_mutex_unlock(&mutexB);

}

printf("Thread 2 is done\n");

pthread_exit(0);

}

What is the output of the above program if

sleep(1);

is moved up before the first lock statement in both function1 and function2.