On 12/04/2011 09:35 AM, Chris Angelico wrote:
On Mon, Dec 5, 2011 at 1:25 AM, Glen Rice<glen.rice.n...@gmail.com> 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?
Same thing happens in CPython, and it looks to be the result of alignment.
struct.calcsize("4sQ")
16
struct.calcsize("Q4s")
12
The eight-byte integer is aligned on an eight-byte boundary, so when
it follows a four-byte string, you get four padding bytes inserted.
Put them in the other order, and the padding disappears.
NOT disappears. In C, the padding to the largest alignment occurs at
the end of a structure as well as between items. Otherwise, an array of
the struct would not be safely aligned. if you have an 8byte item
followed by a 4 byte item, the total size is 16.
(Caveat: I don't use the struct module much, this is based on conjecture.)
ChrisA
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DaveA
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