Create a buffer and assign it a pointer in Python

To use SWIG to connect your Python script with C project, the first step is to create an interface file that includes the C functions that will be called by Python (i.e., interface functions), such as buffer.i as follows.

/* File: buffer.i */
%module buffer

%{
#define SWIG_FILE_WITH_INIT
#include "buffer.h"
%}

void wr(int idx, unsigned int val);
unsigned int rd(int idx);

It's a good practice to keep the interface simple, and leave the complexity to C running in the background. Accordingly, you can manually create a C header file (buffer.h) and implementation file (buffer.c), as follows.

Here is buffer.h

/* File: buffer.h */

#include <stdint.h>

void wr(int idx, unsigned int val);
unsigned int rd(int idx);

Here is buffer.c

/* File: buffer.c */

#include "buffer.h"

#define BUF_SIZE 32

unsigned int buf[BUF_SIZE];

void wr(int idx, unsigned int val) {
  buf[idx%BUF_SIZE] = val;
}

unsigned int rd(int idx) {
  return buf[idx%BUF_SIZE];
}

And you'll need a setup file (setup.py):

#!/usr/bin/env python

"""
setup.py file for SWIG buffer
"""

from distutils.core import setup, Extension


buffer_module = Extension('_buffer',
                           sources=['buffer_wrap.c', 'buffer.c'],
                           )

setup (name = 'buffer',
       version = '0.1',
       author      = "SWIG Docs",
       description = """Simple swig buffer from docs""",
       ext_modules = [buffer_module],
       py_modules = ["buffer"],
       )

Now you can use SWIG to build the project by issuing the following commands:

$ swig -python buffer.i
$ python setup.py build_ext --inplace

The above commands will automatically generate buffer.py, which give you the caller framework (on Python side). Open this file, you'll see _buffer.wr and _buffer.rd already being created. In order to convert a very long int (Python style) 32 bits at a time and keep it in a C array, you can add a wrapper function in buffer.py, which calls the automatically generated Python function wr multiple times:

def wr1(args):
  for n in range(0, 32):
    val = args & 0xffffffff
    wr(n, val)
    args >>= 32
    if args == 0:
      break

Now you can run Python. Here is what you see when you run it:

>>> import buffer
>>> buffer.wr1(0x12345678deadbeef)
>>> hex(buffer.rd(0))
'0xdeadbeef'
>>> hex(buffer.rd(1))
'0x12345678'