Using C struct in Python

[Sudheer Gupta]

Hi,

Thanks a lot for your responses. It cleared up a lot  for me !!

Its a superglue developed and used in house and cannot be revealed .. 
sorry for that !!

I am only extending to the existing glue. Have contacted the author 
regarding the problem.

Sorry for confusion

-Sudheer


John Machin wrote:
> On 8/06/2006 7:35 AM, Sudheer Gupta wrote:
>> Hi,
>
> Hi.
>
> Your later "correction" doesn't clear up the confusion below. Quick 
> eye-balling revealed no difference. If you have to correct a minor 
> typo in a posting, please consider saying "change X to Y" instead of 
> reposting the whole thing.
>
>>
>> I am having trouble using C struct in python. Hope anyone can help me 
>> out ...
>>
>> Say, I have my C struct as
>>
>> typedef struct call
>> {
>>       struct call *next;
>>       // .....
>>
>> } call_t;
>>
>> I have a global variable, namely call_pool, which is of type call_t *
>
> You really need to explain what sort of glue you have between your 
> Python code and your C code.
>
> 1. Are you the author of the glue? If not, better ask the author.
> 2. Are you extending Python with a module written in C, or are you 
> embedding Python in a C program?
> 3. If extending, what's your glue? SWIG? something else? hand-crafted?
>
>>
>> My python program:
>>
>> cp = call_pool  # no error doing this, means that call_pool is 
>> accessable
>
> Acessible from where? How do you bind the name "call_pool" to an object?
>
>>
>> while cp:
>>       print cp
>>       print cp.next
>>
>>
>> This is giving me error: " There is no member or method name c_next"
>
> Not a Python error message. Must be coming from inside your extension 
> module.
>
>>
>> Now, If I just do:
>>
>> print cp
>> print cp.next
>>
>> there is no problem.
>
> Sorry, I don't understand. Above you said it was "giving me error".
>
>> But I am seeing a difference in the way python is looking at the struct:
>>
>> print cp       ->  (call_t*) 0xb0...
>> print cp.next ->  (struct call *) 0xb0...
>
> If they are actual results from a Python print statement, then I can 
> only assume the extension module defines types which have (in effect) 
> str() and/or repr() methods which produce such output.
>
> Python does not know that the C type of cp is (call_t*) and that of 
> cp.next is (struct call *). In fact it doesn't care, and it shouldn't 
> care. The extension module could be written in assembly language or 
> APL or even INTERCAL if it obeys the conventions, which don't include 
> exposing a C type for each object. Another way of looking at it: 
> methods in extension modules are mostly expected to behave like 
> methods written in Python.
>
> I get the impression that you are using some "superglue" that parses C 
> declarations and writes (parts of) Python extension modules in C. Do 
> you think you could possibly tell us what the name of this superglue is?
>
>> Is python not intelligent enough to diagnose the next pointer ??
>
>
> To the extent to which I can understand what your question means, the 
> answer is: It by design makes no attempt to be what you are calling 
> intelligent.
>
> Python does only limited inspection of the tables of methods that an 
> extension type says it supports. It uses only the very basic 
> information: is the pointer to method X NULL?
>
> I hope some of the above helps you amplify your question.
>
> Cheers,
> John