unpack 4 bytes to a signed integer

W

w wg

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 ?

Thank you.
 
M

Mike Stok

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 ?


There are "i" and "I" for signed and unsigned integer respectively,
which deals in the local size of integer.

Hope this helps,

Mike

--

Mike Stok <[email protected]>
http://www.stok.ca/~mike/

The "`Stok' disclaimers" apply.
 
W

w wg

But the "i" and "I" modifer don't care bytes order.
I'm reading data from network using IO#sysread, I need receive integer
, not unsigned integer.

Thank you for your reply.
 
M

Mike Stok

But the "i" and "I" modifer don't care bytes order.
I'm reading data from network using IO#sysread, I need receive integer
, not unsigned integer.

Thank you for your reply.

OK then, what about "V"? "treat 4 characters as an unsigned long in
little-endian byte order"

The N uses network byte order "big-endian"

Mike


--

Mike Stok <[email protected]>
http://www.stok.ca/~mike/

The "`Stok' disclaimers" apply.
 
J

Joel VanderWerf

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
 
W

w wg

I tried your codes, it works exactly as what I expected. I havn't
tried your library, but I think it should get the correct result too.

Thank you.
 

Ask a Question

Want to reply to this thread or ask your own question?

You'll need to choose a username for the site, which only take a couple of moments. After that, you can post your question and our members will help you out.

Ask a Question

Members online

Forum statistics

Threads
473,744
Messages
2,569,484
Members
44,903
Latest member
orderPeak8CBDGummies

Latest Threads

Top