Bug in struct.pack?

[Alex Stapleton]

from struct import pack

pack("B", 1) '\x01' pack("BB", 0, 1) '\x00\x01' pack("BI", 0, 1) '\x00\x00\x00\x00\x01\x00\x00\x00' calcsize("BI") 8 calcsize("BB") 2

Why does an unsigned char suddenly become 4 bytes long when you
include an unsigned int in the format string? It's consistent
behaviour but it's incorrect.

Also.
calcsize('BL') 8 calcsize('BBL') 8 calcsize('BBBL') 8 calcsize('BBBBL') 8 calcsize('BBBBBL') 12 pack("BBBL", 255,255,255,0) '\xff\xff\xff\x00\x00\x00\x00\x00' ####### That's 3 255's and 5(!?!?)
0's pack("BBBBL", 255,255,255,255,0)

'\xff\xff\xff\xff\x00\x00\x00\x00' ######### 4 255's and 4 0's!

Which is all kinds of wrong.

BL should be 9
BBL should be 10
.....

Python 2.4.1 (#2, May 5 2005, 11:32:06)
[GCC 3.3.5 (Debian 1:3.3.5-12)] on linux2

Same behaviour on my PowerBook using

Python 2.3.5 (#1, Mar 20 2005, 20:38:20)
[GCC 3.3 20030304 (Apple Computer, Inc. build 1809)] on darwin

sizeof(unsigned long) should be 8 on both of these platforms
sizeof(unsigned char) should be 1 on both as well

So am I just being stupid and not specifying something I should be?
Or is struct really that broken?

[Raymond Hettinger]

[Alex Stapleton]

from struct import pack

>>> pack("B", 1) '\x01' >>> pack("BB", 0, 1) '\x00\x01' >>> pack("BI", 0, 1) '\x00\x00\x00\x00\x01\x00\x00\x00' >>> calcsize("BI") 8 >>> calcsize("BB") 2

Why does an unsigned char suddenly become 4 bytes long when you
include an unsigned int in the format string? It's consistent
behaviour but it's incorrect.

You're seeing native alignment come into play.
Specify a standard alignment to avoid padding.

pack("BI", 0, 1) '\x00\x00\x00\x00\x01\x00\x00\x00' pack("=BI", 0, 1)

'\x00\x01\x00\x00\x00'

All is explained in the docs:
http://docs.python.org/lib/module-struct.html
Raymond