Extension modules: marshalling void * to bytearray (and/or vice versa)

Doing some experiments using python (means python3) for preparing data (also as sending them to wire - SPI) shows it is slow (having limited system). So I was thinking of creation of extension module written in C to defer the critical stuff to. I would like either:

the python script would have access to memory block created by the malloc() in the extension module hopefully transparently convertible to bytearray
the extension module would get the pointer to bytearray object created in python hopefully transparently convertible to void *

The goal is to have zero copy also as zero conversion memory block accessible by both python (as bytearray) and extension module (as void *).

Is there any method, how to achieve this?

Solution

OK, it seems to be simpler than expected ;-)

bytearray provides direct support for accessing underlying memory block, which is exactly what is needed
there is a format specifier for extracting bytearray object from function call argument list

C extension module [test.c]:

#include <Python.h>
#include <stdint.h>

/* Forward prototype declaration */
static PyObject *transform(PyObject *self, PyObject *args);

/* Methods exported by this extension module */
static PyMethodDef test_methods[] =
{
     {"transform", transform, METH_VARARGS, "testing buffer transformation"},
     {NULL, NULL, 0, NULL}
};


/* Extension module definition */
static struct PyModuleDef test_module =
{
   PyModuleDef_HEAD_INIT,
   "BytearrayTest",
   NULL,
   -1,
   test_methods,
};


/* 
 * The test function 
 */
static PyObject *transform(PyObject *self, PyObject *args)
{
    PyByteArrayObject *byte_array;
    uint8_t           *buff;
    int                buff_len = 0;
    int                i;


    /* Get the bytearray object */
    if (!PyArg_ParseTuple(args, "Y", &byte_array))
        return NULL;

    buff     = (uint8_t *)(byte_array->ob_bytes); /* data   */
    buff_len = byte_array->ob_alloc;              /* length */

    /* Perform desired transformation */
    for (i = 0; i < buff_len; ++i)
        buff[i] += 65;

    /* Return void */
    Py_INCREF(Py_None);
    return Py_None;
}


/* Mandatory extension module init function */
PyMODINIT_FUNC PyInit_BytearrayTest(void)
{
    return PyModule_Create(&test_module);
}

C extension module build/deployment script [setup.py]:

#!/usr/bin/python3
from distutils.core import setup, Extension

module = Extension('BytearrayTest', sources = ['test.c'])

setup (name = 'BytearrayTest',
       version = '1.0',
       description = 'This is a bytearray test package',
       ext_modules = [module])

Build/install the extension module:

# ./setup.py build
# ./setup.py install

Test it:

>>> import BytearrayTest
>>> a = bytearray(16); a
bytearray(b'\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00')
>>> BytearrayTest.transform(a); a
bytearray(b'AAAAAAAAAAAAAAAA')
>>>