Python struct format character sizes, why is it bigger than it should be?

Question

Simple question if do the following:

import struct
struct.calcsize("6cHcBHIIQ")

returns 32 when I believe it should be 28.

By doing the following (missing the Q):

import struct
struct.calcsize("6cHcBHII")

it returns 20, which is what I would expect.

and doing:

import struct
struct.calcsize("Q")

returns 8, which is correct.

Why does adding the Q onto the top one result in 12 extra bytes being expected instead of 8?

Python 3, windows machine.

Thanks.

Just an update to this question: same behavior can be reproduced on Ubuntu 14.04. — Fejs, Nov 23 '16 at 12:09

score 5 · Accepted Answer · answered Nov 23 '16 at 12:09

5

Try struct.calcsize("=6cHcBHIIQ").

answered Nov 23 '16 at 12:09

John Zwinck

score 1 · Answer 2 · edited May 23 '17 at 12:24

1

You could also minimize the size by realigning these in a better way:

struct.calcsize("QIIHHB6cc")

yields 28,you should generally expect padding to be the culprit in any struct size issues. See Why isn't sizeof for a struct equal to the sum of sizeof of each member? for a good answer on why struct sizes might sometimes be larger than what they seem.

edited May 23 '17 at 12:24

Community

answered Nov 23 '16 at 12:22

Thanks for this. Id I'm unpacking bytes from a file then the format string would need to be in the correct order though would it not? – BloodSexMagik Nov 23 '16 at 12:33

2 Answers2