Process forking for parallelism in Ruby - Step-by-Step Execution

Concept Flow - Process forking for parallelism

Start main process

↓

Fork new process

↓

Child process

↓

Child runs code

↓

Child finishes

↓

Processes end

The main process creates a child process by forking. Both run code independently. The parent can wait for the child or continue.

Execution Sample

Ruby

pid = fork do
  puts "Child process: PID=#{Process.pid}"
end
puts "Parent process: PID=#{Process.pid}, Child PID=#{pid}"
Process.wait(pid)

This Ruby code forks a child process that prints its PID. The parent prints its PID and the child's PID, then waits for the child to finish.

Execution Table

Step	Process	Action	PID	Output	Parent Waits?
1	Main	Start main process	1000		No
2	Main	Fork new process	1000		No
3	Child	Run child block	1001	Child process: PID=1001	No
4	Parent	Print parent info	1000	Parent process: PID=1000, Child PID=1001	No
5	Parent	Wait for child	1000		Yes
6	Child	Finish execution	1001		No
7	Parent	Child finished, continue	1000		No

💡 Parent waits for child to finish, then both processes end.

Variable Tracker

Variable	Start	After fork	After child runs	After parent prints	After wait	Final
pid	nil	1001 (child PID)	1001	1001	1001	1001
Process.pid	1000 (parent)	1001 (child)	1001	1000 (parent)	1000 (parent)	1000 (parent)

Key Moments - 3 Insights

Why does the child process print a different PID than the parent?

Because after fork, the child is a new process with its own unique PID, shown in execution_table row 3 and variable_tracker Process.pid changes.

What does the 'pid' variable hold in the parent and child after fork?

In the parent, 'pid' holds the child's PID (1001), but in the child, 'pid' is nil or not used. See execution_table rows 2 and 3 and variable_tracker 'pid' values.

Why does the parent call Process.wait(pid)?

To pause and wait for the child process to finish before continuing, ensuring orderly execution. This is shown in execution_table row 5.

Visual Quiz - 3 Questions

Look at the execution table, what is the PID of the child process?

A 1001

B 1000

C nil

D 2000

💡 Hint

Check the 'PID' column in execution_table row 3 where the child runs.

At which step does the parent wait for the child process to finish?

A Step 3

B Step 5

C Step 2

D Step 7

💡 Hint

Look at the 'Parent Waits?' column in execution_table to find when waiting happens.

If the parent did not call Process.wait(pid), what would change in the execution?

A Parent would wait longer

B Child would not run

C Parent might finish before child

D PID values would swap

💡 Hint

Refer to execution_table row 5 and the role of Process.wait in synchronizing processes.

Concept Snapshot

fork do ... end creates a child process running the block.
Parent gets child's PID from fork return.
Both run independently after fork.
Parent can wait for child with Process.wait(pid).
Child has a unique PID different from parent.
Use fork for parallelism in Ruby.

Full Transcript

This example shows how Ruby uses fork to create a child process. The main process starts with PID 1000. When fork is called, a new child process with PID 1001 is created. The child runs the block printing its PID. The parent prints its own PID and the child's PID, then waits for the child to finish using Process.wait. Variables like 'pid' hold the child's PID in the parent. The child and parent run independently after fork, enabling parallelism.

Practice

1. What does the fork method do in Ruby?

easy

A. It pauses the current process until a condition is met.

B. It creates a new child process that runs concurrently with the parent.

C. It merges two processes into one.

D. It terminates the current process immediately.

Solution

1. Step 1: Understand the purpose of fork

   The fork method creates a new process, called the child, which runs alongside the original parent process.

2. Step 2: Identify the behavior of the child process

   The child process runs the code block given to fork independently, allowing parallel execution.

3. Final Answer:

   It creates a new child process that runs concurrently with the parent. -> Option B

4. Quick Check:

   fork creates child process = A [OK]

Hint: Fork means start a new process running alongside parent [OK]

Common Mistakes:

• Thinking fork pauses the process
• Confusing fork with process termination
• Assuming fork merges processes

2. Which of the following is the correct syntax to create a child process using fork in Ruby?

easy

A. child_pid = fork puts 'Hello from child'

B. fork = child_pid { puts 'Hello from child' }

C. fork child_pid { puts 'Hello from child' }

D. child_pid = fork { puts 'Hello from child' }

Solution

1. Step 1: Recall the correct fork syntax

   The fork method takes a block with curly braces or do-end to run code in the child process.

2. Step 2: Check each option's syntax

   child_pid = fork { puts 'Hello from child' } correctly assigns the child process ID and uses a block with curly braces. Options A, C, and D have syntax errors or incorrect usage.

3. Final Answer:

   child_pid = fork { puts 'Hello from child' } -> Option D

4. Quick Check:

   Use block with fork = B [OK]

Hint: Fork needs a block with braces or do-end [OK]

Common Mistakes:

• Omitting braces for the block
• Assigning fork incorrectly
• Using wrong order of arguments

3. What will be the output of this Ruby code?

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

medium

A. Child process
Parent process

B. Child process

C. Parent process
Child process

D. Parent process

Solution

1. Step 1: Understand the fork and wait behavior

   The fork creates a child process that prints 'Child process'. The parent waits for the child to finish using Process.wait(pid).

2. Step 2: Determine output order

   The child prints first, then the parent prints 'Parent process' after the child ends.

3. Final Answer:

   Child process Parent process -> Option A

4. Quick Check:

   Child prints before parent after wait = C [OK]

Hint: Parent waits, so child prints first [OK]

Common Mistakes:

• Assuming parent prints first
• Ignoring Process.wait effect
• Confusing output order

4. Identify the error in this Ruby code using fork:

pid = fork do
  puts 'Child running'
end
puts 'Parent running'
Process.wait

medium

A. Process.wait called without PID argument

B. Missing block for fork

C. fork does not return a PID

D. puts statements are inside the fork block

Solution

1. Step 1: Check usage of Process.wait

   The Process.wait method should be called with the child's PID to wait for that specific process.

2. Step 2: Identify the error

   In the code, Process.wait is called without any argument, which waits for any child process but can cause unexpected behavior or errors if multiple children exist.

3. Final Answer:

   Process.wait called without PID argument -> Option A

4. Quick Check:

   Always pass PID to Process.wait = A [OK]

Hint: Pass child PID to Process.wait to avoid errors [OK]

Common Mistakes:

• Calling Process.wait without PID
• Confusing fork return value
• Misplacing puts statements

5. You want to run two tasks in parallel using fork and wait for both to finish before continuing. Which code correctly achieves this?

hard

A. pids = fork { task1; task2 }
Process.wait(pids)
puts 'Both done'

B. fork { task1 }
fork { task2 }
puts 'Both done'

C. pids = []
pids << fork { task1 }
pids << fork { task2 }
pids.each { |pid| Process.wait(pid) }
puts 'Both done'

D. fork { task1; Process.wait }
fork { task2; Process.wait }
puts 'Both done'

Solution

1. Step 1: Understand how to fork multiple processes

   Each fork creates a child process. To run two tasks in parallel, fork twice and store both child PIDs.

2. Step 2: Wait for both child processes

   Use Process.wait(pid) for each PID to ensure both finish before continuing.

3. Step 3: Analyze options

   pids = [] pids << fork { task1 } pids << fork { task2 } pids.each { |pid| Process.wait(pid) } puts 'Both done' correctly forks twice, stores PIDs, waits for both, then prints. Others either don't wait or misuse fork.

4. Final Answer:

   pids = [] pids << fork { task1 } pids << fork { task2 } pids.each { |pid| Process.wait(pid) } puts 'Both done' -> Option C

5. Quick Check:

   Fork twice, wait both PIDs = D [OK]

Hint: Store PIDs, wait each to sync parallel tasks [OK]

Common Mistakes:

• Not waiting for child processes
• Calling Process.wait inside child
• Forking once for multiple tasks