Python ctypes example

README

Interfacing python + c + opencv via ctypes
==========================================

This is an example showing how to access
image data, that is consisting of non null-terminated,
non-string char * arrays.

More information is available here
http://www.johnstowers.co.nz/blog/index.php/2011/07/15/interfacing-python-c-opencv-via-ctypes/

libtest.c

#include <stdlib.h>
#include <stdio.h>

char *
test_get_data(unsigned int len)
{
    return malloc(len);
}

char *
test_get_data_nulls(int *len)
{
    *len = 5;
    char *d = malloc(5);
    d[0] = 'a';
    d[1] = 'b';
    d[2] = '\0';
    d[3] = 'c';
    d[4] = '\0';
    return d;
}

void
test_data_print(char *data, int len)
{
    int i;
    for (i = 0; i < len; i++)
        printf("%x (%c),",data[i],data[i]);
    printf("\n");
}

void
test_get_data_nulls_out(char **data, int *len)
{
    *data = test_get_data_nulls(len);
}

void
test_get_fixed_array_size_2(double *data)
{
    data[0] = 1.0;
    data[1] = 2.0;
}

libtest.h

char *
test_get_data(unsigned int len);

char *
test_get_data_nulls(int *len);

void
test_data_print(char *data, int len);

void
test_get_data_nulls_out(char **data, int *len);

void
test_get_fixed_array_size_2(double *data);

Makefile

all: test libtest.so testmodule

libtest.so: libtest.c
	$(CC) -Wall -g -fPIC -shared -o $@ $? -lc

test: test_main.c libtest.o
	$(CC) -o $@ $?

testmodule: testmodule.c
	python setup.py build

clean:
	rm -f test libtest.o libtest.so *.pyc
	rm -rf build

setup.py

from distutils.core import setup, Extension
 
module1 = Extension('testmodule', sources = ['testmodule.c', 'libtest.c'])
 
setup (name = 'PackageName',
        version = '1.0',
        description = 'This is a demo package',
        ext_modules = [module1])

test_main.c

#include <stdio.h>

#include "libtest.h"

int main (void)
{
    char *data, *data2;
    int len, len2;

    data = test_get_data_nulls(&len);
    test_data_print(data, len);

    test_get_data_nulls_out(&data2, &len2);
    test_data_print(data2, len2);
}

testmodule.c

#include <Python.h>

#include "libtest.h"

static PyObject* wrap_test_get_data_nulls(PyObject* self)
{
    char *s;
    int l;
 
    s = test_get_data_nulls(&l);
    return PyString_FromStringAndSize(s, l);
}
 
static PyObject* wrap_test_get_data(PyObject* self, PyObject* args)
{
    char *s;
    unsigned int l;
 
    if (!PyArg_ParseTuple(args, "I", &l))
        return NULL;
 
    s = test_get_data(l);
    return PyString_FromStringAndSize(s, l);
}
 
static PyMethodDef ModuleMethods[] =
{
     {"test_get_data", wrap_test_get_data, METH_VARARGS, "Get a string of variable length"},
     {"test_get_data_nulls", wrap_test_get_data_nulls, METH_NOARGS, "Get a string of fixed length with embedded nulls"},
     {NULL, NULL, 0, NULL},
};
 
PyMODINIT_FUNC
 
inittestmodule(void)
{
     (void) Py_InitModule("testmodule", ModuleMethods);
}

viactypes.py

import os.path
from ctypes import *

me = os.path.abspath(os.path.dirname(__file__))
lib = cdll.LoadLibrary(os.path.join(me, "..", "libtest.so"))

func = lib.test_get_data_nulls
func.restype = POINTER(c_char)
func.argtypes = [POINTER(c_int)]

l = c_int()
data = func(byref(l))

print data,l,data.contents

lib.test_data_print(data,l)

func_out = lib.test_get_data_nulls_out
func_out.argtypes = [POINTER(POINTER(c_char)), POINTER(c_int)]
func.restype = None

l2 = c_int()
data2 = POINTER(c_char)()
func_out(byref(data2), byref(l2))

print data2,l2,data2.contents

lib.test_data_print(data2,l2)

print "equal ", data[0]==data2[0], data[1]==data2[1], data[2]==data2[2], data[3]==data2[3], data[4]==data2[4]

func = lib.test_get_fixed_array_size_2
func_out.argtypes = [POINTER(c_double)]
func.restype = None

data = (c_double * 2)()
func(data)
x,y = data
print "array ", x,y

viamodule.py

import testmodule

data = testmodule.test_get_data_nulls()
print len(data), data

print

data = testmodule.test_get_data(12)
print len(data), data

how to alloc bytes buffer from python side?

You can define a function which calls malloc() in your shared object and call it through ctypes.

http://www.johnstowers.co.nz/blog/index.php/2011/07/15/interfacing-python-c-opencv-via-ctypes/ has changed to http://johnstowers.co.nz/2011/07/15/interfacing-python-c-opencv-via-ctypes/