CSCI 340 Exam 1

Duration: 75 Minutes

Closed books, notes, and cell phones.

Q1 [15 points]. Short Answer Questions (5 points each)

(a) List 3 major components of an operating system? Briefly explain their responsibilities.

(b) Briefly define the following terms: (1) multi-tasking (2) timesharing. What’s the goal of multi-tasking? What’s the goal of time-sharing?

(c) Why is it necessary to have two modes of operation, e.g., user and kernel mode, in modern OSs? Briefly explain the difference between the user mode and kernel mode.

Q2 [20 points]. Short Answer Questions (5 points each)

(a) Describe 2 methods by which arguments are passed from a user process to the kernel in system calls. How does Linux pass arguments to the kernel during a system call?

(b) Draw the process transition diagram. Mark each transition appropriately.

(c) What’s the name of the “first” process in Linux. What’s its process id? Who creates it?

(d) What’s a “zombie” process? What information does the kernel store for a zombie process? When does the kernel completely clean up any information related to a zombie process?

Q3 [15 points]. System Call

Consider the hypothetical Linux system call int add(int a, int b, int *c), which computes the sum of a and b in the kernel and copies the result into the buffer pointed to by c. Assume that the system call number for “add” is 40. The system call returns 0 if there is no error, -1 in the case of an error. Further assume that Linux runs on a 32-bit x86 CPU.

(a) Describe the set of actions that occur from the time the user invokes this system call until the system call returns. Specifically, describe in detail how the user program traps to the kernel, how the parameters a, b and c are passed to the kernel, and the set of actions that the Linux kernel takes to execute the system call and return to the user code.

(b) Under what circumstance(s) would the system call return an error, i.e., returns -1?

Q4 [15 points]. Program Output

What is the output of the following code segments when run in Linux?

int g = 0;
 
void T(char *str) {
    g++;
    printf("[%s] g: %d\n", str, g);
}
 
main() {
    int pid;
    char *str = "Parent";
    g++;
 
    if ((pid = fork()) == 0) {
        str = "Child1";
        T(str);
        exit(0);
    }
    waitpid(pid);
 
    g++;
    T(str);
 
    if ((pid = fork()) == 0) {
        str = "Child2";
        T(str);
    } else
        waitpid(pid);
 
    T(str);
}

[Child1] g: 2
[Parent] g: 3
[Child2] g: 4
[Child2] g: 5
[Parent] g: 4

int var = 0;
 
main() {
    int pid;
 
    if ((pid = fork()) == 0) {
        var = 5;
        execl("/bin/ls", "ls", NULL);
        printf("var: %d\n", var);
    } else {
        var = 10;
        waitpid(pid);
    }
 
    printf("var: %d\n", var);
}

<List of files & folders in the current folder>
var: 10

Q5 [15 points]. Process Management

How many processes are created when the following piece of code is executed in Linux? Draw the process tree for the processes thus created and show what’s printed by each process on the screen.

main(int argc, char *argv[]) {
    int i = 3;
 
    while (i <= 8) {
        if (fork() == 0)
            i += 2;
        else
            i += 3;
    }
 
    printf("i = %d\n", i);
    return 0;
}

Q6 [20 points]. IPC

Assume that there is a system utility program “/bin/rand_nums” that prints 5 random integers on the screen. Here is a sample run of this program:

bash% /bin/rand_nums
3 7 1 6 2
bash%

You are asked to implement a program called “sum.c” that will run “/bin/rand_nums”, get the 5 numbers generated, compute their sum and print it on the screen. To make this possible, you will need to create a child process to run “/bin/rand_nums”, create a pipe for the child process to send the generated numbers to your program over the pipe. Make sure that you connect descriptor 1 of “/bin/rand_nums” to the “write end” of the pipe so that the numbers are sent to your program over the pipe. Also connect your descriptor 0 to the “read end” of the pipe so that you can use “scanf” to read the numbers as if they are coming from the keyboard. Here is a sample run of your program:

bash% ./sum
The sum of the 5 random numbers is 19
bash%

Recall that you create a new child using fork(), start a new executable using execl(char *execFilename, char *arg1, …), create a pipe using pipe(int fd[2]) and copy the contents of fd1 to the contents of fd2 using dup2(int fd1, int fd2) system calls.

#include <stdio.h>
#include <unistd.h>
 
int main(int argc, char *argv[]) {
    int fd[2];
    pipe(fd); // From child to parent
 
    if (fork() == 0) {
        // Child
        dup2(fd[1], 1);
        close(fd[0]);
        execl("./rand_nums", "rand_nums", NULL);
    }
 
    close(fd[1]);
    dup2(fd[0], 0);
    close(fd[0]);
 
    int sum = 0;
    for (int i = 0; i < 5; i++) {
        int num;
        scanf("%d", &num);
        sum += num;
    }
 
    printf("The sum of the 5 random numbers is %d\n", sum);
    return 0;
}