# struct.calcsize problem

Larry Bates larry.bates at websafe.com
Tue Nov 8 01:39:35 CET 2005

```Chandu wrote:
> In using the following struct format I get the size as 593. The same C
> struct is 590 if packed on byte boundary and 596 when using pragma
> pack(4). I am using pack(4) and added 3 spares at the end to get by.
>    hdrFormat = '16s 32s 32s B 8s H 8s H 4s H H 20s 64s 64s 64s 32s 32s
> 64s L L B B B B B 64s 64s'
>
> Any ideas on what I am doing wrong?
>

Maybe this will help:

import struct
hdrFormats = ['16s','32s', '32s','B','8s','H','8s','H','4s','H','H',
'20s','64s','64s','64s','32s','32s','64s','L','L','B',
'B','B','B','B','64s','64s']

sumofcalcsize=0
i=0
for fmt in hdrFormats:
l=struct.calcsize(fmt)
sumofcalcsize+=l
print "fmt='%s', calcsize=%i, sumofcalcsize=%i, calcsize=%i" % \
(fmt, l, sumofcalcsize, struct.calcsize(' '.join(hdrFormats[:i+1])))
i+=1

Outputs:

fmt='16s', calcsize=16, sumofcalcsize=16, calcsize=16
fmt='32s', calcsize=32, sumofcalcsize=48, calcsize=48
fmt='32s', calcsize=32, sumofcalcsize=80, calcsize=80
fmt='B', calcsize=1, sumofcalcsize=81, calcsize=81
fmt='8s', calcsize=8, sumofcalcsize=89, calcsize=89
fmt='H', calcsize=2, sumofcalcsize=91, calcsize=92    <====
fmt='8s', calcsize=8, sumofcalcsize=99, calcsize=100
fmt='H', calcsize=2, sumofcalcsize=101, calcsize=102
fmt='4s', calcsize=4, sumofcalcsize=105, calcsize=106
fmt='H', calcsize=2, sumofcalcsize=107, calcsize=108
fmt='H', calcsize=2, sumofcalcsize=109, calcsize=110
fmt='20s', calcsize=20, sumofcalcsize=129, calcsize=130
fmt='64s', calcsize=64, sumofcalcsize=193, calcsize=194
fmt='64s', calcsize=64, sumofcalcsize=257, calcsize=258
fmt='64s', calcsize=64, sumofcalcsize=321, calcsize=322
fmt='32s', calcsize=32, sumofcalcsize=353, calcsize=354
fmt='32s', calcsize=32, sumofcalcsize=385, calcsize=386
fmt='64s', calcsize=64, sumofcalcsize=449, calcsize=450
fmt='L', calcsize=4, sumofcalcsize=453, calcsize=456  <====
fmt='L', calcsize=4, sumofcalcsize=457, calcsize=460
fmt='B', calcsize=1, sumofcalcsize=458, calcsize=461
fmt='B', calcsize=1, sumofcalcsize=459, calcsize=462
fmt='B', calcsize=1, sumofcalcsize=460, calcsize=463
fmt='B', calcsize=1, sumofcalcsize=461, calcsize=464
fmt='B', calcsize=1, sumofcalcsize=462, calcsize=465
fmt='64s', calcsize=64, sumofcalcsize=526, calcsize=529
fmt='64s', calcsize=64, sumofcalcsize=590, calcsize=593

Larry Bates

```