From: nobu.nokada@...
Date: 2002-03-26T00:56:32+09:00
Subject: Re: Restoring old signal handlers

Hi,

At Mon, 25 Mar 2002 23:37:34 +0900,
Paul Brannan wrote:
> If I do this twice, then when SIGCHLD is received:
> 
>   handler2 -> calls handler1 -> calls original handler
> 
> How can I remove handler1 in such a way that when SIGCHLD is recieved:
> 
>   handler2 -> calls original handler

You may need a signal handler manager?

module Signal
  @handlers = []

  def self.chain_handler(sig)
    catch (:cancel) do
      @handlers[sig].reverse_each do |handler|
	if String === handler
	  eval(handler)
	else
	  handler.call(sig)
	end
      end
    end
  end

  def self.signum(sig)
    Integer === sig or sig = list[sig.to_s.sub(/^SIG/, "")]
    sig
  end

  def self.install(sig, handler = proc)
    sig = signum(sig)
    if handler
      (@handlers[sig] ||= []) << handler
      trap(sig, method(:chain_handler).to_proc)
    end
    handler
  end

  def self.uninstall(sig, handler)
    sig = signum(sig)
    if handlers = @handlers[sig]
      handlers.delete(handler)
      clear_handler(sig) if handlers.empty?
    end
    handler
  end

  def self.clear(sig = nil)
    if sig
      sig = signum(sig)
      @handlers[sig] = nil
      trap(sig, nil)
    else
      @handlers.each_with_index do |handler, sig|
	trap(sig, nil) if handler
      end
      @handlers.clear
    end
    nil
  end
end

if $0 == __FILE__
  handler1 = Signal.install("CLD") do
    puts "handler 1"
  end

  handler2 = Signal.install("CLD") do
    puts "handler 2"
    throw :cancel
  end

  handler3 = Signal.install("CLD") do
    puts "handler 3"
  end

  Process.waitpid fork {}	# => handler 3 & 2
  Signal.uninstall("CLD", handler2)
  Process.waitpid fork {}	# => handler 3 & 1
end

-- 
Nobu Nakada