CMSC 421: Principles of Operating Systems

Homework 2: Orphans vs. Zombies

This homework is due on Tuesday, February 24, at 11:59:59 PM (Eastern standard time). You must use the submit to turn in your homework like so: submit cs421_jtang hw2 hw2.c

Your program must be named hw2.c, and it will be compiled on Ubuntu 14.04 as follows:

  gcc --std=c99 -Wall -o hw2 hw2.c

There must not be any compilation warnings in your submission; warnings will result in grading penalties. In addition, your code must be properly indented and have a file header comment, as described on the coding conventions page.

In this homework, you will write a program that will produce orphan and zombie processes in Linux.

Part 1: Starting Off

The C program will be divided into several parts as follows. Begin your program by calling getpid() and pipe(), the former to get the process ID (PID) of your running program and the latter to create an unnamed pipe. Display the PID to the screen via printf(); use the %zu specifier when printing the value.

Next, call fork() to create another process. In the parent process, close the reading end of the pipe, while in the child process, close the writing end.

In the parent process, display the PID of its child process. Next, read in a string from the user via scanf().

Meanwhile, in the child process, obtain the PID of its parent process via getppid(). (This value should be the same as the return value from getpid() from before.) Then have the child process read from the unnamed pipe; this will block it.

Part 2: Creating an Orphan

If the user enters an 'o', your program will then create an orphan. Do this by having the parent process write an 'o' to the pipe, and then quit.

This write will unblock the child process. The child should sleep() for a second, and then call getppid() again. Display both the previous parent PID and the new parent PID.

You can confirm that init adopted the orphan by running the command ps x | grep init.

Part 3: Creating a Zombie

If instead the user had entered a 'z', your program will then create a zombie. First, in the parent process open for reading the file /proc/child_pid/stat, where child_pid is the PID of the child process. Next, have the parent write a 'z' to the pipe, and then sleep() for a second.

This write will unblock the child process. The child simply should quit.

The parent resumes from its sleep. For the still open file handle to /proc/child_pid/stat, call rewind() and re-read the third field. Display both the previous state and new state to standard output.

Sample Output

Here are two sample outputs from running the program:

My pid is 6269.
Child pid is 6270.
Create an 'o'rphan or a 'z'ombie? (user enters o)
(after a pause) Old parent pid was 6269, now it is 1.

My pid is 6282.
Child pid is 6283.
Create an 'o'rphan or a 'z'ombie? (user enters z)
(after a pause) State of child process was 'S', now it is 'Z'.

Other Hints and Notes

• Assume the user will enter only an 'o' or 'z' at the scanf().
• You can use strtok() or strsep() to parse the stat file.
• The /proc directory is a special directory in Linux. It is an example of a pseudo-filesystem, where the kernel dynamically generates the directory contents; there is no /proc directory stored on the hard drive. There are lots of other interesting virutal files in this directory.
• This program uses the unnamed pipe so that the parent process can communicate to its child process. Linux has the eventfd() function as an optimization of this software pattern.

Extra Credit

Sorry, there is no extra credit available for this assignment.