Glen Rice wrote:
> In IPython:
>>import struct
>>struct.calcsize('4s')
> 4
>>struct.calcsize('Q')
> 8
>>struct.calcsize('4sQ')
> 16
>
> This doesn't make sense to me. Can anyone explain?
A C compiler can insert padding bytes into a struct:
"""By default, the result of packing a given C struct includes pad bytes in
order to maintain proper alignment for the C types involved; similarly,
alignment is taken into account when unpacking. This behavior is chosen so
that the bytes of a packed struct correspond exactly to the layout in memory
of the corresponding C struct. To handle platform-independent data formats
or omit implicit pad bytes, use standard size and alignment instead of
native size and alignment: see Byte Order, Size, and Alignment for details.
"""
http://docs.python.org/library/struct.html#struct-alignment
You can avoid this by specifying a non-native byte order (little endian, big
endian, or "network"):
>>> struct.calcsize("4sQ")
16
>>> struct.calcsize("!4sQ")
12
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