Passing a packed C structure to a c module

[Philippe Martin]

Hi,

Is it possible to define a packed C structure in python and pass it to the c
module, or should the wrapper do that ?

Regards,

Philippe

[Martin v. Löwis]

Re: Passing a packed C structure to a c module

Philippe Martin wrote:[color=blue]
> Is it possible to define a packed C structure in python and pass it to the c
> module, or should the wrapper do that ?[/color]

You can create a packed structure using string concatenation, and with
the help of the struct module. However, this gives you a string object
in the end, which you still need to pass into your C library.

It is better to fill the struct in the wrapper.

Regards,
Martin

[Philippe Martin]

Re: Passing a packed C structure to a c module

Thanks,

It's a pretty big structure: painfull to pass each item as a param.

Regards,

Philippe


"Martin v. Löwis" wrote:
[color=blue]
> Philippe Martin wrote:[color=green]
>> Is it possible to define a packed C structure in python and pass it to
>> the c module, or should the wrapper do that ?[/color]
>
> You can create a packed structure using string concatenation, and with
> the help of the struct module. However, this gives you a string object
> in the end, which you still need to pass into your C library.
>
> It is better to fill the struct in the wrapper.
>
> Regards,
> Martin[/color]

[Martin v. Löwis]

Re: Passing a packed C structure to a c module

Philippe Martin wrote:[color=blue]
> It's a pretty big structure: painfull to pass each item as a param.[/color]

So how else would you like to pass them? Define the API you want,
and then just implement it. It still shouldn't require to define
the layout in Python.

Regards,
Martin

[Philippe Martin]

Re: Passing a packed C structure to a c module

Well,

The call actually is an IOCtl: depending on the control code, the structure
has a different format.

Although the number of control codes/structures is finite, it would make the
wrapper function fairly large.

You seem to think that building the structure from python would be a
mistake: why is that ?

PS: the wrapper also has to work under multiple OSs

Regards,

Philippe





"Martin v. Löwis" wrote:
[color=blue]
> Philippe Martin wrote:[color=green]
>> It's a pretty big structure: painfull to pass each item as a param.[/color]
>
> So how else would you like to pass them? Define the API you want,
> and then just implement it. It still shouldn't require to define
> the layout in Python.
>
> Regards,
> Martin[/color]

[Jay Parlar]

Re: Passing a packed C structure to a c module


On Apr 14, 2006, at 9:44 AM, Philippe Martin wrote:
[color=blue]
> Thanks,
>
> It's a pretty big structure: painfull to pass each item as a param.
>
> Regards,
>
> Philippe
>[/color]

Maybe this can do something for you?

http://pyconstruct.sourceforge.net/

Jay P.

[Martin v. Löwis]

Re: Passing a packed C structure to a c module

Philippe Martin wrote:[color=blue]
> The call actually is an IOCtl: depending on the control code, the structure
> has a different format.[/color]

Ah. In that case, I recommend to use the ioctl module; you won't need a
C wrapper, then.
[color=blue]
> Although the number of control codes/structures is finite, it would make the
> wrapper function fairly large.[/color]

*Some* code to handle this will be very large, regardless of how you
write it.
[color=blue]
> You seem to think that building the structure from python would be a
> mistake: why is that ?[/color]

It's very error-prone. You have to match the C structure layout of the
compiler precisely, and you even have to get the alignment right,
something that can't be done reliably in Python.
[color=blue]
> PS: the wrapper also has to work under multiple OSs[/color]

That makes it worse: now you not only have to match the C compiler's
expectation on a single processor, but on different ones.

Perhaps you should be using ctypes.

Regards,
Martin

[Philippe Martin]

Re: Passing a packed C structure to a c module

I misslead you: it is a smart card-style ioctl ...

Philippe



"Martin v. Löwis" wrote:
[color=blue]
> Philippe Martin wrote:[color=green]
>> The call actually is an IOCtl: depending on the control code, the
>> structure has a different format.[/color]
>
> Ah. In that case, I recommend to use the ioctl module; you won't need a
> C wrapper, then.
>[color=green]
>> Although the number of control codes/structures is finite, it would make
>> the wrapper function fairly large.[/color]
>
> *Some* code to handle this will be very large, regardless of how you
> write it.
>[color=green]
>> You seem to think that building the structure from python would be a
>> mistake: why is that ?[/color]
>
> It's very error-prone. You have to match the C structure layout of the
> compiler precisely, and you even have to get the alignment right,
> something that can't be done reliably in Python.
>[color=green]
>> PS: the wrapper also has to work under multiple OSs[/color]
>
> That makes it worse: now you not only have to match the C compiler's
> expectation on a single processor, but on different ones.
>
> Perhaps you should be using ctypes.
>
> Regards,
> Martin[/color]

[Philippe Martin]

Re: Passing a packed C structure to a c module

I'll take a look,

thanks.



Jay Parlar wrote:
[color=blue]
>
> On Apr 14, 2006, at 9:44 AM, Philippe Martin wrote:
>[color=green]
>> Thanks,
>>
>> It's a pretty big structure: painfull to pass each item as a param.
>>
>> Regards,
>>
>> Philippe
>>[/color]
>
> Maybe this can do something for you?
>
> http://pyconstruct.sourceforge.net/
>
>
> Jay P.[/color]