midterm

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CS 471
Operating Systems

Spring 2011
Midterm Exam

Time 2 & 1/2 hours

Open Book & Notes

Name:

Unix Login:

Each Question is 20 points.

Question 1

Consider the following program:

main(int argc, char **argv)

{

int n, in, out;

char buf[1024];

char InputFile[1024];

char OutputFile[1024];

struct stat st;

printf("enter Input File Name: ");

scanf("%s", InputFile);

if (stat(InputFile, &st) != 0) {

perror("InputFile does not exit");

exit(1);

}

printf("enter Outut File Name: ");

scanf("%s", OutputFile);

if (stat(OutputFile, &st) == 0) {

perror("OutputFile exists");

exit(1);

}

if ((in = open(InputFile, O_RDONLY)) < 0) {

perror(InputFile);

exit(1);

}

if ((out = open(OutputFile, O_CREAT | O_WRONLY, 0600)) < 0) {

perror(OutputFile);

exit(1);

}

while ((n = read(in, buf, sizeof(buf))) > 0)

write(out, buf, n);

close(out);

close(in);

printf("copied %s to %s\n", InputFile, OutputFile);

exit(0);

}

For the above program, list the names of all:

1. File system calls:

2. Standard I/O library functions:

Write a program that creates a file named “cs471” and writes the word “wahab” in that file.

If there is a file with the same name, it is truncated.

Question 2

Consider the following four processes submitted in the order:

P1, P2, P3 and P4 with estimated CPU times: 8, 6, 4 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:

Question 3

What is the output of the following programs?

forkexit.c

main()

{

pid_t pid = fork();

if (pid == 0) {

sleep(1);

printf("I am the child\n");

exit(0);

}

printf("I am the parent\n");

wait(NULL);

if (pid == 0)

printf("pid == 0 after wait\n");

else printf("pid != 0 after wait\n");

}

forknoexit.c

main()

{

pid_t pid = fork();

if (pid == 0) {

sleep(1);

printf("I am the child\n");

}

printf("I am the parent\n");

wait(NULL);

if (pid == 0)

printf("pid == 0 after wait\n");

else printf("pid != 0 after wait\n");

}

forknoshm.c

main()

{

int* var = (int *) malloc(sizeof(int));

*var = 5;

pid_t pid = fork();

if (pid == 0) {

while (*var)-- > 0) sleep (1);

printf("I am the child, var=%d\n", *var);

}

while (*var > 0) sleep (1);

usleep(1);

printf("I am the parent, var=%d\n", *var);

}

forkshm.c

main()

{

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

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

*var = 5;

pid_t pid = fork();

if (pid == 0) {

while (*var)-- > 0) usleep (1);

printf("I am the child, var=%d\n", *var);

}

while (*var > 0) usleep (1);

usleep(1);

printf("I am the parent, var=%d\n", *var);

}

forkexec.c

main(){

pid_t pid;

pid = fork();

if (pid == 0) {

execlp("./forkexec", "forkexec", NULL);

}

else {

printf ("forkexec");

exit(0);

}

Question 4

What is the output of the following program?

% clisrv

clisrv.c

main()
{
   pid_t pid;
     pid = fork();
     if (pid == 0) {
           execlp("./server", "server", NULL);
     }
     else {
            sleep(1);
           execlp("./client", "client", NULL);
     }
}

client.c
main(void)
{
     int             sockfd, n;
     char            buf[SIZE];
     struct sockaddr_in servaddr;
     struct hostent *hp, *gethostbyname();

     bzero(&servaddr, sizeof(servaddr));
     hp = gethostbyname("localhost");
     bcopy(hp->h_addr, &(servaddr.sin_addr.s_addr), hp->h_length);
     servaddr.sin_family = AF_INET;
     servaddr.sin_port = htons(2345);

     sockfd = socket(AF_INET, SOCK_STREAM, 0);

     if (connect(sockfd, (SA *) & servaddr, sizeof(servaddr)) < 0) {
           perror("connect error");
           exit(-1);
     }
     sprintf(buf, "%s", "I am your client\n");
     write(sockfd, buf, strlen(buf));
      read(sockfd, buf, SIZE);
      printf("Client got: %s", buf);
}

server.c

main(void)
{
     int             len, listenfd, connfd;
     struct sockaddr_in servaddr, cliaddr;
     char            buf[SIZE];

     bzero(&servaddr, sizeof(servaddr));
     servaddr.sin_family = AF_INET;
     servaddr.sin_addr.s_addr = htonl(INADDR_ANY);
     servaddr.sin_port = htons(2345);

     listenfd = socket(AF_INET, SOCK_STREAM, 0);
     bind(listenfd, (SA *) & servaddr, sizeof(servaddr));
     listen(listenfd, 0);
     len = sizeof(cliaddr);
     connfd = accept(listenfd, (SA *) & cliaddr, &len);
     read(connfd, buf, SIZE);
     printf("Server got: %s", buf);
      write(connfd, buf, strlen(buf));
}

What is the output of the following program?

pipe.c:

#define SIZE 10
char *msg1="child1\n";
char *msg2="child2\n";

int main(void)
{
     int fd[2];
      char buf[SIZE];

     pipe(fd);

     if (fork() == 0) {
           write(fd[1], msg1, strlen(msg1));
           sleep(1);
           read(fd[0], buf, 10);
           printf("%s",buf);
     }
     else if (fork() == 0) {
           read(fd[0], buf, 10);
           printf("%s",buf);
            sleep(1);
           write(fd[1], msg2, strlen(msg2));
     }
      else sleep(2);
}

Question 5

Modify the following program to create at most N concurrent echo threads.

For example:

% EchoServerThreadMax 2 10123

Will create at most 2 concurrent echo threads.

EchoServerThreadMax.c

#define SA      struct sockaddr
sem_t           available;

main(int argc, char *argv[])
{
     int             len, listenfd, connfd;
     struct sockaddr_in servaddr, cliaddr;
     char            buff[512];
     int             nread;
     pthread_t       tid;
     pthread_attr_t attr;

      int             MaxThreads = atoi(argv[1]);
     sem_init(&available, 0, MaxThreads);

      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[2]));
     bind(listenfd, (SA *) & servaddr, sizeof(servaddr));
     listen(listenfd, 0);
     pthread_attr_init(&attr);
     for (;;) {
          len = sizeof(cliaddr);

           connfd = accept(listenfd, (SA *) & cliaddr, &len);
           pthread_create(&tid, &attr, &EchoToClient, (void *) connfd);
     }
}

void     *EchoToClient(void *sockfd)
{
     int             nread;
     char            buffer[512];
     for (;;) {
           nread = read((int) sockfd, buffer, 512);
           if (nread == 0) {

                pthread_exit(0);

} else
write((int) sockfd, buffer, nread);

}
}

Modify the following program so that each echo thread uses mutual exclusion to save the received messages into the LogFile.

EchoServerThreadLog.c
#define SA      struct sockaddr
int             logfd;
pthread_mutex_t filemutex = PTHREAD_MUTEX_INITIALIZER;

main(int argc, char *argv[])
{
     int             len, listenfd, connfd;
     struct sockaddr_in servaddr, cliaddr;
     char           buff[512];
     int             nread;
     pthread_t       tid;
     pthread_attr_t attr;

     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]));

     logfd = open("LogFile", O_RDWR | O_TRUNC | O_CREAT, 0600);
     bind(listenfd, (SA *) & servaddr, sizeof(servaddr));
     listen(listenfd, 0);
     pthread_attr_init(&attr);
     for (;;) {
           len = sizeof(cliaddr);
           connfd = accept(listenfd, (SA *) & cliaddr, &len);
           pthread_create(&tid, &attr, &EchoToClient, (void *) connfd);
     }
}

void *EchoToClient(void *sockfd)
{
     int             nread;
     char            buffer[512];
     for (;;) {
           nread = read((int) sockfd, buffer, 512);
           if (nread == 0)
                pthread_exit(0);
           else {

write((int) sockfd, buffer, nread);

}
}
}