Question

I’m trying to parse out some data packed into this binary file and Python’s struct module is causing me all sorts of problems. It won’t seem to give me the correct float variable when it’s trying to do more than one type at a time:

import struct

# a fragment of the binary file
a = '\x39\x00\xFF\x00\x00\x0A\x00\x1F\x05\xDC\x42\x31\x30\x00\xFF\x00\x00\x0A\x00\xB5\x01\xE6\x42'

struct.unpack_from('1sxHxbxf', a)
# returns ('9', 255, 10, 2.8355782166755716e-09), but 
struct.unpack_from('f',a[7:])
# gives the expected (110.01000213623047,)

Answer 1

By default, C types are represented in the machine’s native format and byte order, and properly aligned by skipping pad bytes if necessary (according to the rules used by the C compiler).

The unpacking expects the float to be aligned on an 8-byte boundary and skips over 1 padding byte to get there. You can confirm this by skipping 1 byte yourself:

>>> struct.unpack_from('1sxHxbxf', a)
('9', 255, 10, 2.8355782166755716e-09)
>>> struct.unpack_from('f',a[8:])
(2.8355782166755716e-09,)

To disable alignment, add =, <, >, or ! to the front of the format string.

>>> struct.unpack_from('=1sxHxbxf', a)
('9', 255, 10, 110.01000213623047)