w said:
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