parameters: (cls)
docstring: Initializes the ``SIGCHLD`` handler.
  
          The signal handler is run on an `.IOLoop` to avoid locking issues.
          Note that the `.IOLoop` used for signal handling need not be the
          same one used by individual Subprocess objects (as long as the
          ``IOLoops`` are each running in separate threads).
  
          .. versionchanged:: 5.0
             The ``io_loop`` argument (deprecated since version 4.1) has been
             removed.
  
          Availability: Unix
docstring_summary: Initializes the ``SIGCHLD`` handler.
docstring_tokens: Initializes the SIGCHLD handler .
function: def initialize(cls) -> None:
          """Initializes the ``SIGCHLD`` handler.
  
          The signal handler is run on an `.IOLoop` to avoid locking issues.
          Note that the `.IOLoop` used for signal handling need not be the
          same one used by individual Subprocess objects (as long as the
          ``IOLoops`` are each running in separate threads).
  
          .. versionchanged:: 5.0
             The ``io_loop`` argument (deprecated since version 4.1) has been
             removed.
  
          Availability: Unix
          """
          if cls._initialized:
              return
          io_loop = ioloop.IOLoop.current()
          cls._old_sigchld = signal.signal(
              signal.SIGCHLD,
              lambda sig, frame: io_loop.add_callback_from_signal(cls._cleanup),
          )
          cls._initialized = True
function_tokens: def initialize ( cls ) -> None : if cls . _initialized : return io_loop = ioloop . IOLoop . current ( ) cls . _old_sigchld = signal . signal ( signal . SIGCHLD , lambda sig , frame : io_loop . add_callback_from_signal ( cls . _cleanup ) , ) cls . _initialized = True