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struct.pack behavior

Can anyone explain to me why
struct.pack('HB',1,2) gives 3 bytes, whereas struct.pack('BH',1,2)
gives 4 bytes?

-Steven
Jun 27 '08 #1
5 2528
On Jun 26, 9:00 am, "Steven Clark" <steven.p.cl...@gmail.comwrote:
Can anyone explain to me why
struct.pack('HB',1,2) gives 3 bytes, whereas struct.pack('BH',1,2)
gives 4 bytes?
Alignment -- read the manual.
Jun 27 '08 #2
On Wed, Jun 25, 2008 at 7:03 PM, John Machin <sj******@lexicon.netwrote:
On Jun 26, 9:00 am, "Steven Clark" <steven.p.cl...@gmail.comwrote:
>Can anyone explain to me why
struct.pack('HB',1,2) gives 3 bytes, whereas struct.pack('BH',1,2)
gives 4 bytes?
Alignment -- read the manual.
--
http://mail.python.org/mailman/listinfo/python-list
If "the manual" is the help files for the struct module, I've read it
several times over. I understand endianness; I don't understand
alignment. Could anyone give a less cryptic / terse answer?
Jun 27 '08 #3
En Wed, 25 Jun 2008 23:38:54 -0300, Steven Clark
<st************@gmail.comescribi�:
On Wed, Jun 25, 2008 at 7:03 PM, John Machin <sj******@lexicon.net>
wrote:
>On Jun 26, 9:00 am, "Steven Clark" <steven.p.cl...@gmail.comwrote:
>>Can anyone explain to me why
struct.pack('HB',1,2) gives 3 bytes, whereas struct.pack('BH',1,2)
gives 4 bytes?
Alignment -- read the manual.

If "the manual" is the help files for the struct module, I've read it
several times over. I understand endianness; I don't understand
alignment. Could anyone give a less cryptic / terse answer?
A good start might be this Wikipedia article:
<http://en.wikipedia.org/wiki/Data_structure_alignment>

--
Gabriel Genellina

Jun 27 '08 #4
On Jun 26, 12:38*pm, "Steven Clark" <steven.p.cl...@gmail.comwrote:
On Wed, Jun 25, 2008 at 7:03 PM, John Machin <sjmac...@lexicon.netwrote:
On Jun 26, 9:00 am, "Steven Clark" <steven.p.cl...@gmail.comwrote:
Can anyone explain to me why
struct.pack('HB',1,2) gives 3 bytes, whereas struct.pack('BH',1,2)
gives 4 bytes?
Alignment -- read the manual.
--
http://mail.python.org/mailman/listinfo/python-list

If "the manual" is the help files for the struct module, I've read it
several times over. I understand endianness; I don't understand
alignment. Could anyone give a less cryptic / terse answer?
google("struct alignment")
Jun 27 '08 #5
For efficiency reasons many CPUs require particular primitive data
types (integers/pointers of various sizes) to be placed in memory at
particular boundaries. For example, shorts ("H" above, usually two bytes
and probably always so in the struct module) are often required to be
on even addresses, and longer objects to be on 4 or 8 byte boundaries.

This allows for much more efficient memory access on many platforms
(of course the rules depend on the platform). Although RAM _appears_ to
the random access to arbitrary bytes, the underlying hardware will often
fetch chunks of bytes in parallel. If a number spanned the boundaries of
such a chunk it would require two fetch cycles instead of one. So
this is avoided for performance reasons.

So, packing "HB" puts a short at offset 0 (even) and then a byte.
Conversely, packing "BH" puts a byte at offset zero but puts the short
at offset 2 (to be even), leaving a gap after the byte to achieve this,
thus the 4 byte size of the result (byte, gap, short).

This layout procedure is called "alignment".

Cheers,
--
Cameron Simpson <cs@zip.com.auDoD#743
http://www.cskk.ezoshosting.com/cs/

Thanks for taking the time to type a detailed, helpful response,
Cameron. Much appreciated!
Jun 28 '08 #6

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