Packing data in Python

Question

I'm trying to use Struct in python to pack some data, but experiencing some weird behavior:

My format is:

struct.Struct('B B I 15s I')

The output is (Pipes added for clarity):

ff|01|000022000000|4650766e65564369797a4531416f41|0001000000

The first two entries are perfect, and so is the string in the middle. But the integer values I am passing are 34 and 1, respectively, and would expect 00000022, 00000001 instead of the weird 6 byte data that I am getting...

NPE · Accepted Answer · 2014-11-02 13:08:56Z

1

There are two issues at play: endianness and padding. For example, the 00002200000000 is two bytes of zero-padding followed by 0x22 in little-endian encoding:

00 00 22 00 00 00
^^^^^             padding
      ^^^^^^^^^^^ 0x22

To fix both, specify the desired endianness explicitly:

struct.Struct('> B B I 15s I')

(The reason this gets rid of the padding is that struct only pads structures when you use the default native encoding.)

answered Nov 2, 2014 at 13:08

NPE

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