From: Dale Martenson <dmartenson@...>
Date: 2002-11-09T01:35:20+09:00
Subject: How does one use pack / unpack when dealing with pointers ...


I am using Ruby to talk to a device using ioctl calls. This works great, but
I have a case where I need to pass a pointer of the data area in a packet
and then reference the data, which is structure that varies according the
type of event that is returned. 

I got everything working for the cases that do not involve pointers. I am
not sure how to do it.

I am doing something like...

class Control

  # item1: input (int)
  # item2: output (int)
  GetEZPacket = Struct.new("Get Easy Packet", :item1, :item2)

  # eventQueue: input (int)
  # eventNumber: output (int)
  # eventData: input/output (pointer)

  GetEventPacket = Struct.new("Get Event Packet", :eventQueue, :eventNumber,
:eventData)

  # stuff1: output (int)
  # stuff2: output (int)
  EventDataFor1 = Struct.new("Event Data For 1", :stuff1, :stuff2)
  
  # stuff3: output (int)
  # stuff4: output (int)
  # stuff5: output (int)
  EventDataFor2 = Struct.new("Event Data For 2", :stuff3, :stuff4, :stuff5)

  # This works for cases without pointers
  def getEZStuff( device, item )
    ezPacket = GetEZPacket.new( item, 0 )
    arg = ezPacket.to_a.pack("ii")
    device.ioctl( IOCTL_EZ, arg )
    result = arg.unpack("ii")
    result[1]
  end

  # This is where I need help
  def getEvent( device, queue )
    # How do I set the pointer argument to point to a data region big
    # enough to hold either different return structures?
    area = " " * 256
    gep = GetEventPacket.new( queue, 0, area )
    arg = gep.to_a.pack("iiP256")
    device.ioctl( IOCTL_EVENT, arg )
    result = arg,unpack("iiP256")
    
    if result[1] == 1 then
      # Is this how I unpack the pointer and associate it with my Struct?
      ra = result[2].unpack("i,i")
      edf1 = EventDataFor1.new( ra[0], ra[1] )
      
      if edf1.stuff1 == BLAH then
      ... once I get to here everything is a breeze
  end
end      

Any advice appreciated. 
Thanks,
Dale