Process creation (fork and exec) in Operating Systems - Mini Project: Build & Apply

Understanding Process Creation with fork() and exec()

📖 Scenario: You are learning how operating systems create new processes. This is important for running programs and managing tasks on your computer.In this project, you will simulate the steps an operating system takes to create a new process using fork() and exec() system calls.

🎯 Goal: Build a simple step-by-step representation of process creation using fork() to duplicate a process and exec() to replace the process image with a new program.

📋 What You'll Learn

Create a variable to represent the original process ID

Create a variable to represent the child process ID after fork

Simulate the fork operation by assigning the child process ID

Simulate the exec operation by changing the program name in the child process

💡 Why This Matters

🌍 Real World

Operating systems use fork() and exec() to run new programs and manage multiple tasks at the same time.

💼 Career

Understanding process creation is essential for system administrators, software developers, and anyone working with operating systems or server management.

1

Create the original process ID

Create a variable called original_pid and set it to the integer 1000 to represent the original process ID.

Operating Systems

# Create a variable called original_pid and set it to 1000
# Your code here

Hint

The process ID is usually a number. Just assign 1000 to original_pid.

2

Simulate the fork operation

Create a variable called child_pid and set it to original_pid + 1 to simulate the new child process ID created by fork().

Operating Systems

original_pid = 1000
# Create a variable called child_pid and set it to original_pid + 1
# Your code here

Hint

fork() creates a new process with a new ID. Add 1 to original_pid to get child_pid.

3

Set the original program name

Create a variable called program_name and set it to the string "original_program" to represent the program running in the original process.

Operating Systems

original_pid = 1000
child_pid = original_pid + 1
# Create a variable called program_name and set it to "original_program"
# Your code here

Hint

The original process runs a program named "original_program". Assign this string to program_name.

4

Simulate exec to run a new program in the child process

Create a variable called child_program_name and set it to the string "new_program" to simulate the exec() call replacing the child process's program.

Operating Systems

original_pid = 1000
child_pid = original_pid + 1
program_name = "original_program"
# Create a variable called child_program_name and set it to "new_program"
# Your code here

Hint

exec() replaces the program in the child process. Assign "new_program" to child_program_name.

Practice

1. What does the fork() system call do in an operating system?

easy

A. Replaces the current process with a new program

B. Creates a new process by copying the current process

C. Terminates the current process immediately

D. Pauses the current process temporarily

Solution

1. Step 1: Understand the role of fork()

   The fork() call creates a new process by duplicating the current one, including its code and data.

2. Step 2: Differentiate from exec()

   Unlike exec(), which replaces the process, fork() makes a copy, so both processes continue running.

3. Final Answer:

   Creates a new process by copying the current process -> Option B

4. Quick Check:

   fork() = process copy [OK]

Hint: Remember: fork copies, exec replaces [OK]

Common Mistakes:

• Confusing fork() with exec()
• Thinking fork() replaces the process
• Believing fork() pauses the process

2. Which of the following is the correct way to use exec() in a program?

easy

A. exec("ls -l")

B. exec("/bin/ls", "-l")

C. exec("/bin/ls", ["ls", "-l"])

D. exec("ls", "-l")

Solution

1. Step 1: Recall exec() syntax

   The exec() family requires the program path and an argument list, where the first argument is the program name.

2. Step 2: Identify correct argument format

   exec("/bin/ls", ["ls", "-l"]) correctly passes the path and an array with the program name and its argument.

3. Final Answer:

   exec("/bin/ls", ["ls", "-l"]) -> Option C

4. Quick Check:

   exec() needs path + argument list [OK]

Hint: exec() needs program path and argument array [OK]

Common Mistakes:

• Passing arguments as separate strings instead of array
• Using command line string instead of path
• Omitting the program name in argument list

3. Consider this code snippet in a Unix-like system:

pid = fork()
if pid == 0:
    exec("/bin/echo", ["echo", "Hello"])
else:
    print("Parent process")

What will be printed when this code runs?

medium

A. Parent process
Hello

B. Parent process

C. Hello

D. Hello
Parent process

Solution

1. Step 1: Understand fork() behavior

   The fork() creates a child process. The child runs the exec() replacing itself with the echo program.

2. Step 2: Analyze output from parent and child

   The parent (pid > 0) prints "Parent process". The child (pid == 0) calls exec(), loading /bin/echo which prints "Hello". Both outputs appear on stdout, typically with parent first.

3. Final Answer:

   Parent process Hello -> Option A

4. Quick Check:

   fork() + child exec() = both print [OK]

Hint: fork() child exec(): both print, parent first [OK]

Common Mistakes:

• Thinking only parent prints
• Reversing the output order
• Believing exec() prevents child output

4. What is wrong with this code snippet?

pid = fork()
if pid == 0:
    exec("ls", ["ls", "-l"])
else:
    print("Parent")

medium

A. The exec call is missing the full path to the program

B. fork() should be replaced with exec()

C. The parent process should call exec() instead

D. The argument list to exec() should be a string, not a list

Solution

1. Step 1: Check exec() usage

   The exec() call requires the full path to the executable, not just the command name.

2. Step 2: Identify the missing full path

   Using "ls" without "/bin/ls" will cause exec to fail because it won't find the program.

3. Final Answer:

   The exec call is missing the full path to the program -> Option A

4. Quick Check:

   exec() needs full path [OK]

Hint: Always use full path in exec() [OK]

Common Mistakes:

• Using command name without path in exec()
• Confusing fork() and exec() roles
• Passing wrong argument types to exec()

5. A program wants to run another program safely without stopping itself. Which sequence of system calls should it use?

hard

A. Call exec() first, then fork()

B. Call exec() directly to run the new program

C. Call fork() twice, then the parent calls exec()

D. Call fork() to create a child, then the child calls exec()

Solution

1. Step 1: Understand process creation and replacement

   Calling exec() replaces the current process, so calling it directly stops the original program.

2. Step 2: Use fork() then exec() in child

   By calling fork(), the program creates a child process. The child can then call exec() to run the new program, leaving the parent running.

3. Final Answer:

   Call fork() to create a child, then the child calls exec() -> Option D

4. Quick Check:

   fork() then exec() = safe new program run [OK]

Hint: fork first, then exec in child to keep parent alive [OK]

Common Mistakes:

• Calling exec() directly and losing original process
• Calling fork() twice unnecessarily
• Calling exec() before fork()