Re: feature request for setting _pack_ to a higher value

Hi, Thomas!

Thanx for your suggestion. I took your code and changed my union definition. I believe your code has an issue if size happens to be the same as value (8 in our example), because then modulo operation in the line delta = 8 - size % 8 yields 0, and then delta is set to 8, while it actually should be 0. Right?

So, the original fill_union raises the following exception:
Assert sizeof(N13ARValueStruct3DOT_1E) == 8, sizeof(N13ARValueStruct3DOT_1E
AssertionError: 16

After changing the logic to return the fields unchanged in case size == value, I get another exception:
assert alignment(N13ARValueStruct3DOT_1E) == 8, alignment(N13ARValueStruct3DOT_1E)
AssertionError: 4

So, while the size is now calculated ok, the alignment still is not ok. 
Any thoughts?

Thanx,
Axel



Am 13.04.2010 um 22:01 schrieb Thomas Heller:

> Axel Seibert schrieb:
>> Hi, Thomas!
>> 
>> Thanx for asking -- no, I have not found any other solution yet.
> 
> Ok, so the remaining question was:  How can I change a (generated) ctypes Union
> definition so that the packing is a multiple of an arbitrary value, 8 in this case.
> 
> Background:  A Union contains all the _fields_ in the same memory block (they overlay
> each other; only one is normally valid), so the Union size is the size of the largest
> field.  We have now the goal to bring the union size up to a multiple of 8 by adding
> a dummy field.  Here is the ode that I wrote:
> 
> <snip>
> from ctypes import *
> 
> def fill_union(value, fields):
>    size = 0
>    for name, typ in fields:
>        size = max(size, sizeof(typ))
>    delta = 8 - size % 8
>    return fields + [("", c_char * (size + delta))]
> 
> class X(Union):
>    _fields_ = fill_union(8, [("a", c_char),
>                              ("b", c_short),
>                              ("c", c_char * 31)])
> 
> print sizeof(X)
> <snip/>
> 
>> So long,
>> Axel
>> 
>> Am 09.04.2010 um 10:02 schrieb Thomas Heller:
>> 
>>> Axel Seibert schrieb:
>>>> Hi, Thomas!
>>>> 
>>>> I tried to apply your solution to my problem. I found two issues with that:
>>>> a) a couple of my structs that require padding are Unions. Do you have a solution for that?
>>>> b) I believe that line "missing = a - size % a" needs to be replaced by missing = a - sizeof(t) % a
>>> 
>>> Axel - sorry for this delayed reply; I'm very short on time.  Is the question
>>> still open or did you find a solution yourself in the meantime?
>>> 
>>> -- 
>>> Thanks,
>>> Thomas
>>> 
>>> 
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>> 
>> 
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> 
> 
> -- 
> Thanks,
> Thomas
> 
> 
> ------------------------------------------------------------------------------
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> Try the new software tools for yourself. Speed compiling, find bugs
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------------------------------------------------------------------------------
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proactively, and fine-tune applications for parallel performance.
See why Intel Parallel Studio got high marks during beta.
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