Kernel#fork

fork -> integer or nil

Returns: Integer or nil · Updated March 13, 2026 · Kernel Methods

processes parallel multiprocessing child

The fork method creates a new process by duplicating the current process. The child process gets a copy of all memory, while the parent receives the child’s process ID.

Basic Usage

pid = fork do
  # This runs in child process
  puts "Child process: #{Process.pid}"
end

# This runs in parent
puts "Parent: #{Process.pid}, Child PID: #{pid}"

# Wait for child
Process.wait(pid)

Practical Examples

Parallel Processing

tasks = [1, 2, 3, 4]

tasks.each do |task|
  fork do
    # Process task in parallel
    result = process(task)
    File.write("result_#{task}.txt", result)
  end
end

# Wait for all children
Process.waitall

Creating Worker Pool

def spawn_workers(count)
  count.times do |i|
    fork do
      loop do
        job = get_next_job
        process_job(job) if job
      end
    end
  end
end

Daemon Process

def daemonize
  fork do
    Process.setsid  # New session
    fork do
      Dir.chdir("/")
      File.umask(0000)
      STDIN.reopen("/dev/null")
      STDOUT.reopen("/dev/null", "a")
      STDERR.reopen("/dev/null", "a")
      run_daemon
    end
    exit!
  end
  Process.waitall
end

With Block

# Block version - runs block in child
fork do
  puts "Child: #{Process.pid}"
end
puts "Parent: #{Process.pid}"

Handling Child Exit

# Parent can wait for specific child
pid = fork { sleep 1 }
Process.wait(pid)
puts "Child finished"

# Non-blocking check
pid = fork { }
if Process.waitpid(pid, Process::WNOHANG)
  puts "Child done"
end

Exit Status

pid = fork { exit(42) }
_, status = Process.wait2(pid)
puts status.exitstatus  # => 42

Important Notes

Resource Management

# Children inherit file descriptors
# Close what you don't need
child = fork do
  STDIN.close
  # ...
end

Copy-on-Write

# Modern OS uses copy-on-write
# Parent and child share memory until modified
# Efficient for read-heavy tasks

On Platforms Without Fork

# On Windows, fork not available
# Use spawn instead
spawn("ruby", "script.rb")

Comparison with Threads

# fork - separate processes, no GIL
# threads - same process, shared memory