Process creation (fork and exec) in Operating Systems - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the fork() system call do in process creation?

The fork() system call creates a new process by duplicating the calling process. The new process is called the child process, and it is almost an exact copy of the parent process.

beginner

What is the purpose of the exec() family of functions?

The exec() functions replace the current process's memory and code with a new program. This means the process starts running a different program without creating a new process.

intermediate

How do fork() and exec() work together in creating a new process?

First, fork() creates a copy of the current process. Then, the child process uses exec() to replace its code with a new program. This way, a new process runs a different program.

intermediate

What is the difference between the parent and child process after a fork() call?

After fork(), both processes run independently. The parent gets the child's process ID as the return value, while the child gets zero. They have separate memory spaces but start as copies.

advanced

Why is fork() considered a cheap operation in modern operating systems?

Because modern OS use 'copy-on-write', which means the actual copying of memory happens only if either process changes it. This saves time and memory when creating new processes.

What does the fork() system call return to the child process?

A -1

B Child's process ID

C Parent's process ID

D 0

The fork() call returns 0 to the child process and the child's process ID to the parent.

Which system call replaces the current process's program with a new one?

A exec()

B exit()

C wait()

D fork()

exec() replaces the current process's code and memory with a new program.

What happens if fork() fails?

A Returns -1

B Returns child's PID

C Returns 0

D Creates a zombie process

If fork() fails, it returns -1 indicating an error.

Why do operating systems use copy-on-write with fork()?

A To speed up process termination

B To share memory permanently

C To avoid copying memory until necessary

D To prevent process creation

Copy-on-write delays copying memory until either process modifies it, saving resources.

Which of these is true about the child process after fork()?

A It immediately runs a different program

B It runs the same program as the parent initially

C It shares the same memory space as the parent

D It cannot execute <code>exec()</code>

After fork(), the child runs the same program as the parent until it calls exec() or exits.

Explain how fork() and exec() work together to create a new process running a different program.

Think about copying first, then replacing the program.

Describe what happens in the system when fork() is called and why copy-on-write is important.

Focus on memory handling after fork.

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()