def flock(file, mode)
  success = file.flock(mode)
  if success
    begin
      yield file
    ensure
      file.flock(File::LOCK_UN)
    end  
  end  
  return success
end
open('output', 'w') do |f|
  flock(f, File::LOCK_EX) do |f|
    f << "Kiss me, I've got a write lock on a file!"
  end
end
def open_lock(filename, openmode="r", lockmode=nil)
  if openmode == 'r' || openmode == 'rb'
    lockmode ||= File::LOCK_SH
  else
    lockmode ||= File::LOCK_EX
  end
  value = nil
  open(filename, openmode) do |f|
    flock(f, lockmode) do
      begin
        value = yield f
      ensure
        f.flock(File::LOCK_UN) # Comment this line out on Windows.
      end
    end
    return value
  end
end
t1 = Thread.new do 
  puts 'Thread 1 is requesting a lock.'
  open_lock('output', 'w') do |f| 
    puts 'Thread 1 has acquired a lock.'
    f << "At last we're alone!"
    sleep(5)
  end
  puts 'Thread 1 has released its lock.'
end
t2 = Thread.new do
  puts 'Thread 2 is requesting a lock.'
  open_lock('output', 'r') do |f| 
    puts 'Thread 2 has acquired a lock.'
    puts "File contents: #{f.read}"
  end
  puts 'Thread 2 has released its lock.'
end
t1.join
t2.join

Thread 1 is requesting a lock.
Thread 1 has acquired a lock.
Thread 2 is requesting a lock.
Thread 1 has released its lock.
Thread 2 has acquired a lock.
File contents: At last we're alone!
Thread 2 has released its lock.

Thread 1 is requesting a lock.
Thread 1 has acquired a lock.
Thread 2 is requesting a lock.
Thread 1 has released its lock.
Thread 2 has acquired a lock.
File contents: At last we're alone!
Thread 2 has released its lock.

def try_lock
  puts "I couldn't get a lock." unless 
    open_lock('contested', 'w', File::LOCK_EX | File::LOCK_NB) do
    puts "I've got a lock!" 
    true
  end
end
try_lock

I've got a lock!

I've got a lock!

open('contested', 'w').flock(File::LOCK_EX) # Get a lock, hold it forever.
try_lock

I couldn't get a lock.

I couldn't get a lock.