w wg wrote:
> Hi
> I' m using unpack to convert 4 bytes to local integer, but ruby just
> supply the "N" modifer  which means unsigned long integer.
> 
> My questions is :
> How to unpack 4 bytes to a signed integer ?

The best way I've found is to unpack with N (to get the swapping right) 
and then do some arithmetic to interpret the unsigned value as signed:

x = -123
s = [x].pack("N")
   # Note that for _pack_ there is no need for a
   # special signed version of N

p s    # ==> "\377\377\377\205"

length = 32
mid = 2**(length-1)
max_unsigned = 2**length
to_signed = proc {|n| (n>=mid) ? n - max_unsigned : n}

p to_signed[s.unpack("N").first]  # ==> -123



This is all very hard for me to remember, so I've written a library to 
do it, bit-struct (http://redshift.sourceforge.net/bit-struct). This 
makes life easier:

require 'bit-struct'

class Packet < BitStruct
   signed    :x, 32
end

pkt = Packet.new
pkt.x = -123

p pkt.to_s  # ==> "\377\377\377\205"
p pkt.x     # ==> -123

# given string data from a network:
pkt2 = Packet.new("\377\377\377\205")
p pkt2.x     # ==> -123

-- 
       vjoel : Joel VanderWerf : path berkeley edu : 510 665 3407