Last Updated: 2021-01-28 Thu 12:04

CSCI 4061 HW02: Fork, Spoon, and Wait


  • Download the code distribution every HW
  • See further setup instructions below


1 Rationale

Processes are created in Unix by fork()'ing an existing process so mastery of these system calls is essential to bringing new running programs into the system. Processes created in this way are the children of the creating parent process which can detect changes in their state using the wait() family of calls. Many programming paradigms rely on the parent tracking its children necessitating an understanding of the wait() family. Children execute the same code code as parents unless a call to a member of the exec() family is made which replaces a running child's instructions with a different program.

1.1 Associated Reading

  • Ch 8 of Stevens and Rago introduces and shows many examples of fork(), wait(), exec()
  • Ch 3 of Robbins & Robbins also discusses these system calls

1.2 Grading Policy

Credit for this HW is earned by taking the associate Quiz which is linked under Gradescope. The quiz will ask similar questions as those that are present in the QUESTIONS.txt file and those that complete all answers in QUESTIONS.txt should have no trouble with the quiz.

See the full policy in the syllabus.

2 Codepack

The codepack for the HW contains the following files:

File Description
QUESTIONS.txt Questions to answer
fork_report.c C file for Problem 1
run_child.c C file for Problem 2
  NOTE: No Makefile is provided; use gcc file.c for these simple programs

3 What to Understand

Make sure you get a good understanding of

  • How fork is used to clone a running process to create a child
  • How each process has a unique pid, process ID, available via library calls
  • How wait() and waitpid() can be used to ensure order in execution by causing a parent to wait for a child to finish
  • How execvp() is used to replace a running program with a different program

All of these things will show up in projects.

4 Questions about fork(), wait(), and exec()


                            HW 02 QUESTIONS

- Name: (FILL THIS in)
- NetID: (THE kauf0095 IN

Write your answers to the questions below directly in this text file.
HW quiz questions will be related to the questions in this file.

PROBLEM 1 `fork_report.c'


  Compile and run the program in `fork_report.c'. Run the program
  several times. Show the output of a typical run below.


  Is the output ordered or disordered with respect to the "I am
  number..." statements? Why?


  Add calls to the wait() or waitpid() function to ensure that the
  output is ordered. Compile and rerun the code to ensure that your code
  will enforce ordered output. Paste your completed code below.


  How many parents and children are there in this program? What is their
  arrangement? How are the parent process(es) numbered?

PROBLEM 2 `run_child.c'


  Compile and run the program in `run_child.c'. Show it's output below.


  Adjust the contents of the `child_argv[]' array. Try removing the "-l"
  element, try adding on ".." prior to the final NULL, or make other
  modifications INCLUDING for the 0th "ls" string.  Recompile and
  describe whether the output has changed.


  Change the child_cmd string from "ls" to be something else like "gcc"
  or "cat". A good combination is "gcc" with the "--version" as the only
  option in the child_argv array. Recompile and explain what happens


  Currently `run_child.c' does not reach the bottom statements.
  |   printf("------------------\n");
  |   printf("Child Finished\n");
  Correct this by doing the following:
  - Fork a child process
  - Have ONLY the child process call execvp()
  - Have the parent process wait() for the child
  Paste your completed code below.

Author: Chris Kauffman (
Date: 2021-01-28 Thu 12:04