[ruby-talk:9669] Re: Non-blocking I/O?

Mathieu Bouchard <[e-mail]> wrote:

> > I'm trying to watch some (log)files for changes. For this purpose,
> > non-blocking I/O and the select function look promising, unfortunately,
> > I don't get them to work.
> > Here's a snippet of my effort
> 
> Do not use Non-blocking I/O. This is not a concept that is useful here.
> NBIO is to be used on pure streams, like pipes, sockets, ... many OS don't
> support NBIO on regular files (e.g. afaik Linux). Anyway this would only
> allow
> you to send several requests to the kernel at the same time.

Apparently it works on Linux. Whatever "it" is exactly. That is, I can't
claim that I'm sure it actually does non-blocking I/O. Here's the script
as it has evolved, so far.


#! /usr/bin/ruby

require 'fcntl'
require 'singleton'


trap("SIGHUP") {
  Logwatch.instance.reload()
}


module Daemon
  def Daemon.init
    puts "Daemon.init"
    
    pid = fork()
    puts "PID #{pid}"
    
    if (pid != nil)
      puts "Parent exit"
      exit(0)
    end
    
    Process.setsid()
    Dir.chdir("/")
    File.umask(0)
  end
end


class Logwatch
  include Daemon
  include Singleton
  
  SLEEP_TIME = 0.2
  @watchfiles = []
  
  def setupWatchfiles()
    # needs to read file paths from config file
    testfile = open("/tmp/messages", "r")
    testfile.fcntl(Fcntl::F_SETFL, Fcntl::O_NONBLOCK)
    testfile.seek(0, IO::SEEK_END)
    @@watchfiles = [testfile]
  end
  
  def watch()
    while (true)
      inputAvail = select(@@watchfiles)[0]
      inputSucc = false
      
      inputAvail.each() do |f|
        begin
          s = f.sysread(100);
          inputSucc = true
          puts s
        rescue EOFError
        end
      end
      
      sleep(SLEEP_TIME) unless inputSucc
      
    end
    puts "Done watching."
  end
  
  def reload()
    puts "Reloading list of watched files..."
    @@instance.setupWatchfiles()
    puts "done."
  end
  
  def run()
    Daemon.init()
    setupWatchfiles()
    watch()
  end
  
end

Logwatch.instance.run()


> I don't know whether select() works at all on regular files.

At least it looks as if it does.

Michael

-- 
Michael Schuerig
mailto:[e-mail]
http://www.schuerig.de/michael/