Need help with C extension module

Robert Kern rkern at ucsd.edu
Thu Sep 8 00:38:51 CEST 2005


chris wrote:
> This is my first attempt at undertaking a C extension module.  I want
> to wrap an existing C library so I can call the functions from Python.
> There are only two functions I'm interested in calling.  I did mess
> with Pyrex a bit and Swig, to no avail, so I turned to doing it by
> hand.  Using the example in Programming Python, I did get the easier of
> the two functions working--only takes a string parameter.  I'm stuck
> now on the other function and not sure how to wrap it, because it
> involves some structs.  Here's a simplified version of the C:
> 
> struct In
> {
>   int x;
>   char* s;
>   ... (only primitive data types)
> };
> 
> struct Out
> {
>   int y;
>   char* s;
>   ... (only primitive data types)
> };
> 
> Out* func(In* x, Out* y);
> 
> So the function takes pointers to the two structs, and fills out the
> output struct and also returns the pointer to it.  I would envision the
> Python looking like
> 
> in = In()
> in.y = 1
> in.s = "abc"
> ...
> 
> out = func(in)
> 
> maybe?  Just no idea how to deal with the structs in the C extension
> module code.

It's *not* a straightforward task. You have to define an extension type. See

  http://docs.python.org/ext/defining-new-types.html

However, Pyrex really does make defining extension types much, much
easier. I highly suggest trying to debug your Pyrex code before learning
all of the boilerplate necessary to create extension types in pure C.

-- 
Robert Kern
rkern at ucsd.edu

"In the fields of hell where the grass grows high
 Are the graves of dreams allowed to die."
  -- Richard Harter




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