Big-endian binary data to/from Python ints?

[William McBrine]

Here are a couple of functions that I feel stupid for having written.
They work, and they're pretty straightforward; it's just that I feel like
I must be missing an easier way to do this...

def net_to_int(numstring):
"""Convert a big-endian binary number, in the form of a string of
arbitrary length, to a native int.
"""
num = 0
for i in numstring:
num *= 256
num += ord(i)
return num

def int_to_net(num):
"""Convert a native int to a four-byte big-endian number, in the form
of a string.
"""
numstring = ''
for i in xrange(4):
numstring = chr(num % 256) + numstring
num /= 256
return numstring

The situation: I'm getting a four-byte packet from a socket that consists
of a big-endian 32-bit integer. (It specifies the length of the data that
follows.) I have to send the same thing in reply. send() and recv() work
with strings... I'm familiar with ntohl() and htonl(), but those expect/
return integers.

--
09 F9 11 02 9D 74 E3 5B D8 41 56 C5 63 56 88 C0 -- pass it on

[Matt Nordhoff]

William McBrine wrote:

Here are a couple of functions that I feel stupid for having written.
They work, and they're pretty straightforward; it's just that I feel like
I must be missing an easier way to do this...

def net_to_int(numstring):
"""Convert a big-endian binary number, in the form of a string of
arbitrary length, to a native int.
"""
num = 0
for i in numstring:
num *= 256
num += ord(i)
return num

def int_to_net(num):
"""Convert a native int to a four-byte big-endian number, in the form
of a string.
"""
numstring = ''
for i in xrange(4):
numstring = chr(num % 256) + numstring
num /= 256
return numstring

The situation: I'm getting a four-byte packet from a socket that consists
of a big-endian 32-bit integer. (It specifies the length of the data that
follows.) I have to send the same thing in reply. send() and recv() work
with strings... I'm familiar with ntohl() and htonl(), but those expect/
return integers.

The struct module should do it, but do you prefer your code or format
strings? :-P

<http://docs.python.org/lib/module-struct.html>
--

[William McBrine]

On Wed, 26 Dec 2007 16:50:53 -0800, Dennis Lee Bieber wrote:

your code might (I've not actually checked it) be incorrect if ported
to another machine.

Nope. :-)

If the problem is that you have the four bytes as a character string,
use the struct module to interpret it as a binary integer and then feed
the results to the above functions, followed by using struct to convert
back to a string representation.

Heck, looking at struct, it already has a format modifier for
net-oriented... (assuming unsigned 32-bit integer)

pinteger = struct.unpack("!I", netstring[0:4])[0]
and
netstring = struct.pack("!I", pinteger)

Thanks, that seems to be what I was looking for.

--
09 F9 11 02 9D 74 E3 5B D8 41 56 C5 63 56 88 C0 -- pass it on