Re: [ctypes-users] Problem with alignment under Linux
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Re: [ctypes-users] Problem with alignment under Linux
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From: Axel S. <ax...@gl...> - 2012-04-12 21:28:14
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Hi, Andrew!
It’s been two months since we had our discussion. I have tried to follow your suggestion:
- check each member of a union for a type which requires the 8 byte alignment and mark that union appropriately if such a type is found (ie double or long long);
- check each member of a struct for a type which requires the 8 byte alignment, and if such a member is encountered insert appropriate padding before the member
and then you suggested a code snippet.
The executive summary is: This approach seems to not work.
I believe for Structures, it could work.
However, for Unions I found the following issues:
1. in your example below, you suggest a function align_union(union_class), which could be called in _init_. This could be used when my code instantiates such unions; but what about unions that are created by the server side that I’m only trying to read?
2. Therefore, I changed the sample code; as I know which unions need to be aligned differently, I guess I don’t need this function, and could set class._alignment immediately, e.g.:
class BMCUnion(Union):
_alignment = 8
Unfortunately, this also seems to not work, and I guess, the answer lies here:
class N13ARValueStruct3DOLLAR_1E(BMCUnion):
pass
>>> a=N13ARValueStruct3DOLLAR_1E()
>>> a._alignment
8
>>> alignment(a)
4
What am I missing? Are there other variations on this theme which could lead to a solution?
Thanx for your input!
Axel
Am 14.02.2012 um 09:34 schrieb Andrew MacIntyre:
> I think the comment in the documentation has to be a red herring, as it would make that compiler noncompliant with the Linux i386 ABI by default which would probably cause all sorts of problems compiling other software.
>
> It seems to me that you will need to do several things:
> - check each member of a union for a type which requires the 8 byte alignment and mark that union appropriately if such a type is found (ie double or long long);
> - check each member of a struct for a type which requires the 8 byte alignment, and if such a member is encountered insert appropriate padding before the member.
>
> The approach that occurs to me is to use an attribute which can be tested for using hasattr() (or AttributeError if you prefer exceptions rather than look before leap).
>
> Perhaps something along the lines of (untested! and not necessarily all encompassing):
>
> ...
> from ctypes import *
>
> ...
>
> # this foreign DLL is built with –malign_double
> c_double._alignment = 8
> c_long_long._alignment = 8
>
> ...
>
> # check a Union for members requiring non-standard alignment
> def align_union(union_class):
> for n, t in union_class._fields_:
> if hasattr(t, ‘_alignment’):
> union_class._alignment = t._alignment
> break
>
> # patch the ‘fields’ sequence of a ctypes Structure by inserting
> # dummy fields to achieve required alignment
> def pad_struct(fields):
> result = []
> size = 0
> for n, t in fields:
> if hasattr(t, ‘_alignment’):
> padding = size % t._alignment
> if padding:
> padding = t._alignment – padding
> result.append((‘’, c_char * padding))
> size += padding
> size += sizeof(t)
> result.append((n, t))
> return result
>
> ...
>
> class U(Union):
>
> _fields_ = ((‘a’, c_long),
> (‘b’, c_double))
>
> def __init__(self):
> align_union(self)
> Union.__init__(self)
>
> class S(Structure):
> _fields_ = pad_struct(((‘c’, c_long),
> (‘d’, U)))
>
> ...
>
> -------------------------> "These thoughts are mine alone!" <---------
> Andrew MacIntyre Operations Branch
> tel: +61 2 6219 5356 Communications Infrastructure Division
> fax: +61 2 6253 3277 Australian Communications & Media Authority
> email: and...@ac... http://www.acma.gov.au/
>
> From: Axel Seibert [mailto:ax...@gl...]
> Sent: Tuesday, 14 February 2012 6:34 AM
> To: cty...@li...
> Subject: Re: [ctypes-users] Problem with alignment under Linux [SEC=UNCLASSIFIED]
>
> Hi, Andrew!
>
> According to http://gcc.gnu.org/onlinedocs/gcc/Type-Attributes.html, "aligned" is the attribute to look for. There is another possibility that occurs to me: the use of the gcc option "-malign-double" forcing doubles to be 8 byte aligned when building the library.
>
> This is the hint that brought me in the right direction; looking at one of the supplied examples shows that the Makefile under Linux calls the compiler with the following arguments:
>
> -malign-double
>
> And the documentation says:
> Configure the compiler as follows: Structure member alignment: 8 bytes (default)
>
> In other words, the software supplier requires this for whatever reasons. What can we now deduct from this for my problem?
>
> Thanx again for your help,
> Axel
>
>
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