Python将void指针作为参数传递给C函数（SWIG，Numpy）

Question

I need to pass a pointer from Python to a C function. I'm currently trying with SWIG but I'm ready to switch to anything that will solve my issue :)

To simplify my problem, I wrote this empty C function in SWIG:

extern void myvoidp(void *p);

I've setup my .i and .c files, all compile fine and work in Python.

I'd like to pass a Numpy array. I can get the memory address of the Numpy data like this

>>> anp = array([1,2,3])
array([1, 2, 3])
>>> anp.ctypes.data
40546992

I can cast it to a void * like this with ctypes and numpy:

>>> anp.ctypes.data_as(ctypes.c_void_p)
c_void_p(40546992)

I've tried endless variants. Whatever I do, when I invoke my C function I get this error.

>>> myswig.myvoidp(anp.ctypes.data_as(ctypes.c_void_p))
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: in method 'myvoidp', argument 1 of type 'void *'

I can't figure out how to write a Python / numpy statement that allows me to (simply) pass a pointer to my C function. :-(

Answer 1

I guess you want to receive a pointer to the data of the NumPy array, so that you can access the array elements from C. NumPy ships with SWIG support which is easy and convenient to use. Just download numpy.i and place it in your project directory.

Just change the signature of the C function from (void *p) to (double *data, int length) and apply the NumPy typemap.

test.i

%module example
%{
#define SWIG_FILE_WITH_INIT
#include <stdio.h>

void myvoidp(double *data, int length) {
    printf("data = [ ");
    for (int i = 0; i < length; ++i) {
        printf("%f ", data[i]);
    }
    printf("]\n");
}
%}

%include "numpy.i"

%init %{
import_array();
%}

%apply (double* IN_ARRAY1, int DIM1) {(double *data, int length)}
void myvoidp(double *data, int length);

test.py

from example import myvoidp
from numpy import array

anp = array([1,2,3])
myvoidp(anp)

Example invocation:

$ swig -python test.i
$ clang -Wall -Wextra -Wpedantic -I /usr/include/python3.6m/ -fPIC -shared -o _example.so test_wrap.c -lpython3.6m
$ python3 test.py 
data = [ 1.000000 2.000000 3.000000 ]