[Baypiggies] A python newbie questoin

[Alex Martelli]

struct.pack returns *a string* and struct.unpack accepts *a string*.  buf
is *a list*, not a string.  struct.unpack[buf[-1]] (or buf[0] as buf only
has one item -- the string in question).  No idea of how it can work in the
case you say it works:-).

Alex

On Fri, Feb 20, 2015 at 3:49 PM, Ashok Chippa <a.n.chippa at gmail.com> wrote:

> A newbie python question:
>
> THIS WORKS:
>
> ethernet.py:
>
> ...
> def test():
>     buf = []
>     #eth_gen(buf)
>     buf = struct.pack('!6s6sH', '\x01\x02\x03\x04\x05\x06', \
>                                 '\x07\x08\x09\x0a\x0b\x0c', 0x0800)
>     eth = Eth()
>     eth.unpack(buf)
>     eth.show()
>
> root at ashok-vb:/home/achippa/ppl# python ethernet.py
> smac = 07:08:09:0a:0b:0c dmac = 01:02:03:04:05:06 type = 0800
>
>
> THIS DOES NOT WORK:
>
> # Build an ethernet header into the specified buffer.
> #
> def eth_gen(buf):
>     n = len(buf)
>     buf[n:] = struct.pack('!6s6sH', '\x01\x02\x03\x04\x05\x06', \
>                                     '\x07\x08\x09\x0a\x0b\x0c', 0x0800)
> def test():
>     buf = []
>     eth_gen(buf)
>     eth = Eth()
>     eth.unpack(buf)
>     eth.show()
>     #
>
> root at ashok-vb:/home/achippa/ppl# !py
> python ethernet.py
> Traceback (most recent call last):
>   File "ethernet.py", line 181, in <module>
>     test()
>   File "ethernet.py", line 173, in test
>     eth.unpack(buf)
>   File "ethernet.py", line 89, in unpack
>     dpkt.Packet.unpack(self, buf)
>   File "/usr/local/lib/python2.7/dist-packages/dpkt/dpkt.py", line 127, in
> unpack
>     struct.unpack(self.__hdr_fmt__, buf[:self.__hdr_len__])):
> struct.error: unpack requires a string argument of length 14
>
> I have verified (by printing) that calcsize(self.__hdr_fmt__) and
> self.__hdr_len__ are both 14. The buf has only 14 bytes of ethernet header.
> Putting the struct.pack(..) in a function (eth_gen()) causes the error...
>
> Any idea what may be happening... Thanks a lot
>
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