Issue 3481: Length of struct.pack('HL', 1,1) incorrect (8 instead of 6 bytes) (original) (raw)

Python Versions tested: Windows 2.5.2 (r252:60911) Debian Linux Python 2.4.4

Example:

import struct struct.pack('HL', 1, 1)

results in '\x01\x00\x00\x00\x01\x00\x00\x00' although '\x01\x00\x01\x00\x00\x00' was expected.

if concatenated or done separately

struct.pack('H', 1) + struct.pack('L', 1)

result is as expected '\x01\x00\x01\x00\x00\x00' or '\x01\x00' and '\x01\x00\x00\x00' respectively

Error: Length is 8 where it should be 6 This is as well true for "hl", "hL" and "Hl".

Free description: I could not find another error regarding that, nor any information using popular search. Further no explanation found why that might be valid behaviour.

Regards, Dominique

The struct module is correct. You see here the result of "alignment": the address of a long is always a multiple of the size of a long.

The struct module mimics the C compiler: a "struct { short x; long y; }", will actually occupy 8 bytes in memory (on a 32bit processor). There are two padding bytes, that are not used.