From: "Ara.T.Howard" <Ara.T.Howard@...>
Date: 2005-11-02T02:58:11+09:00
Subject: Re: Best way to free a device

On Wed, 2 Nov 2005, Detlef Reichl wrote:

> Hi,
>
> what is the best way to release an unix device that can't handle
> multiple opens?
>
> I've created a video device class in C, where the device gets opened
> with an instanciation like:
>
>  Wc::Cam.new '/dev/video0'
>
> In this class i also added a destructor that closes the device. But it
> is called to late. From
>
> http://www.rubygarden.org/ruby?ResourceAcquisitionIsInitialization
>
> i've seen, that i cant aspect a direct destruction of the object if it's
> not more in use.
>
> Is there a more elegant way to free the resources automatically, as to
> create a special free or close method for my object?

make 'Wc::Cam::new' return a singleton object and allow new to take a block
that cleans up.

   module Wc
     class Cam
       INSTANCES = {}
       def new path
         cam =
           if INSTANCES.has_key? path
             return INSTANCES[path]
           else
             super
           end
         if block_given?
           begin
             yield cam
           ensure
             cam.close
           end
         else
           cam
         end
       end
     end
   end

that way you can write

   Wc::Cam::new('/dev/video0') do |cam|

   end

and ensure the cam is closed.  when blocks dont' suffice you can write

   a = Wc::Cam::new '/dev/video0'
   b = Wc::Cam::new '/dev/video0'

and both a and b will be the same object.

regards.

-a
-- 
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