Process forking for parallelism in Ruby - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of simple forked process

What will this Ruby code print?

Ruby

pid = fork do
  puts "Child process"
end
Process.wait(pid)
puts "Parent process"

A Child process

B Child process\nParent process

C Parent process\nChild process

D Parent process

💡 Hint

Remember the parent waits for the child to finish before printing.

The fork creates a child process that prints first. The parent waits for the child with Process.wait before printing.

❓ Predict Output

intermediate

Variable value after fork

What is the value of x printed by this Ruby code?

Ruby

x = 10
pid = fork do
  x = 20
  puts x
end
Process.wait(pid)
puts x

A 10\n20

B 20\n20

C 20\n10

D 10\n10

💡 Hint

Child and parent have separate memory after fork.

The child changes x to 20 and prints it. The parent keeps x as 10 and prints it after waiting.

🔧 Debug

advanced

Why does this forked code hang?

What is wrong with this Ruby code?

Ruby

pid = fork do
  puts "Child"
end
puts "Parent"

A Parent does not wait for child, so child may still run after parent exits

B Fork block syntax is incorrect, causing infinite loop

C Missing <code>Process.wait(pid)</code> causes parent to hang

D Child process never prints because of missing <code>puts</code>

💡 Hint

Think about what happens if the parent exits before the child finishes.

The parent does not wait for the child, so the child runs independently. The code does not hang, but the child may print after the parent exits. The code does not hang indefinitely.

❓ Predict Output

advanced

Number of processes created

How many processes will be created and print "Hello" in this Ruby code?

Ruby

3.times do
  fork do
    puts "Hello"
  end
end
Process.waitall

A 4

B 1

C 0

D 3

💡 Hint

Each fork creates one child process.

The loop runs 3 times, each time creating one child process that prints "Hello". So 3 child processes print "Hello".

❓ Predict Output

expert

Output of nested forks with variable increments

What is the output of this Ruby code?

Ruby

x = 0
pid1 = fork do
  x += 1
  pid2 = fork do
    x += 1
    puts x
  end
  Process.wait(pid2)
  puts x
end
Process.wait(pid1)
puts x

A 2\n1\n0

B 1\n2\n0

C 0\n1\n2

D 2\n2\n2

💡 Hint

Remember each fork creates a new process with its own copy of variables.

The innermost child increments x to 2 and prints it. Its parent increments x to 1 and prints it after waiting. The original parent prints 0 because its x is unchanged.