CS 350 Operating Systems Spring 2021 Labs for processes 1 Practice 1: fork() • Download the source code – $ wget http://www.cs.binghamton.edu/~huilu/teaching/lecture3/cpu-api.tar.gz – $ tar xvf cpu-api.tar.gz – $ cd cpu-api – $ make • Run the first program p1 – $ ./p1 • Read “p1.c” and make sure you understand what the code does 2 Practice 2: wait() • Run the second program p2 – $ ./p2 • Read “p2.c” and answer the questions: – Why is there a delay for the parent process to print out its message? – What is the return value of the wait()? • Please use “waitpid()” to replace wait()? – Use “man waitpid” command in terminal to check out the usage of waitpid – What is the return value and of waitpid()? – What is the return status of the child process? – Can the parent process wait on other processes (not the child process)? • Try it out and see the result. 3 Practice 3: exec() • Run the third program p3 – $ ./p3 • Read “p3.c” and answer the questions: – Does “execvp()” return? What is the evidence to back your answer? – In which case, “execvp()” returns? Please make a case if possible. 4 Practice 4: Redirection • Run the fourth program p4 – $ ./p4 • Read “p4.c” and answer the questions: – Where is the result stored? – Why does this happen? 5 Practice 5: Orphan process • An orphan process is the one that its parent dies (i.e., finishes its execution) • Please modify “p1” to make such a case – to generate an orphan process – Note you can use the following command to check the parent process id – $ ps -o ppid= -p $pid_of_child_process – Or – $ ps -l – Note that, the orphan process’s parent is systemd with pid of 1 – Make sure you kill the orphan process: – $ killall p1 pid pid of the parent 6 Practice 6: Zombie process • A zombie process is the one that dies (i.e., finishes its execution) but its parent does not check on it via wait() • Please modify “p1” to make such a case – to generate a zombie process – $ ps -l – Make sure you kill the orphan process: You catch – $ killall p1 the zombie 7 Practice 7: Address space • Run the fourth program p5 – $ ./p5 • Read “p5.c” and answer the questions: – Why do the parent and child processes print out the different values for the “same” variables? 8 Practice 8 • Write a program that opens a file (with the open() system call) and then calls fork() to create a new process. • Can both the child and parent access the file descriptor returned by open()? • What happens when they are writing to the file concurrently, i.e., at the same time? 9 Practice 1-8 ● Try your best to finish practice 1-8 from lab slides (download from MyCourses). ● To check the manual of unknown system call functions, please type “man <func name>” in terminal (or google instead). ○ (E.g. “man fork” to see how fork is used, what is the return value of the function and what arguments you can pass to the fork function.) ● We will discuss answers and solutions in the second half of the lab. 10 Practice 1: fork() #include <stdio.h> #include <stdlib.h> #include <unistd.h> int main(int argc, char *argv[]) { printf("hello world (pid:%d)\n", (int) getpid()); int rc = fork(); if (rc < 0) { // fork failed; exit fprintf(stderr, "fork failed\n"); exit(1); } else if (rc == 0) { // child (new process) If fork returns 0, then it is in the child process printf("hello, I am child (pid:%d)\n", (int) getpid()); } else { // parent goes down this path (original process) printf("hello, I am parent of %d (pid:%d)\n", If fork returns value greater than 0, then it is in the parent process rc, (int) getpid()); AND the returned value is the child process’s pid. } return 0; } 11 Practice 2: wait() • Run the second program p2 – $ ./p2 • Read “p2.c” and answer the questions: – Why is there a delay for the parent process to print out its message? Wait for the child process and child process sleep for 2 seconds – What is the return value of the wait()? Child process pid • Please use “waitpid()” to replace wait()? – Use “man waitpid” command in terminal to check out the usage of waitpid Child process pid – What is the return value and of waitpid()? – What is the return status of the child process? Stored in wstatus with value 0, means normally terminated – Can the parent process wait on other processes (not the child process)? No, can only wait for its children • Try it out and see the result. 12 Practice 3: exec() • Run the third program p3 – $ ./p3 • Read “p3.c” and answer the questions: – Does “execvp()” return? What is the evidence to back your answer? No, execvp will replace address space to another program, normally it won’t return. – In which case, “execvp()” returns? Please make a case if possible. When execvp runs failed, such as the program does not exist. 13 Practice 4: Redirection • Run the fourth program p4 – $ ./p4 • Read “p4.c” and answer the questions: To the file named “p4.output” – Where is the result stored? – Why does this happen? write-only, offset: 0 This will point to some other files, in our case, p4.output 14 Practice 5: Orphan process • An orphan process is the one that its parent dies (i.e., finishes its execution) • Please modify “p1” to make such a case – to generate an orphan process – Note you can use the following command to check the parent process id – $ ps -o ppid= -p $pid_of_child_process – Or – $ ps -l – Note that, the orphan process’s parent is systemd with pid of 1 – Make sure you kill the orphan process: – $ killall p1 pid pid of the parent 15 Practice 6: Zombie process • A zombie process is the one that dies (i.e., finishes its execution) but its parent does not check on it via wait() • Please modify “p1” to make such a case – to generate a zombie process – $ ps -l – Make sure you kill the orphan process: You catch – $ killall p1 the zombie 16 Practice 7: Address space • Run the fourth program p5 – $ ./p5 • Read “p5.c” and answer the questions: – Why do the parent and child processes print out the different values for the “same” variables? 17 Practice 8 • Write a program that opens a file (with the open() system call) and then calls fork() to create a new process. • Can both the child and parent access the file descriptor returned by open()? • What happens when they are writing to the file concurrently, i.e., at the same time? 18 References • Chapter 4&5 of OSTEP book (please read) – http://pages.cs.wisc.edu/~remzi/OSTEP/cpu-intro.pdf – http://pages.cs.wisc.edu/~remzi/OSTEP/cpu-api.pdf • Man pages for different system calls – Try “man 2 <syscall_name>” • E.g. man 2 exec – Syscalls are normally listed in section 2 of the man page 19.