Python is passing 32bit pointer address to C functions

Question

I would like to call my C functions within a shared library from Python scripts. Problem arrises when passing pointers, the 64bit addresses seem to be truncated to 32bit addresses within the called function. Both Python and my library are 64bit.

The example codes below demonstrate the problem. The Python script prints the address of the data being passed to the C function. Then, the address received is printed from within the called C function. Additionally, the C function proves that it is 64bit by printing the size and address of locally creating memory. If the pointer is used in any other way, the result is a segfault.

CMakeLists.txt

cmake_minimum_required (VERSION 2.6) 
add_library(plate MODULE plate.c)

plate.c

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

void plate(float *in, float *out, int cnt)
{
    void *ptr = malloc(1024);
    fprintf(stderr, "passed address: %p\n", in);
    fprintf(stderr, "local pointer size: %lu\n local pointer address: %p\n", sizeof(void *), ptr);
    free(ptr);
}

test_plate.py

import numpy
import scipy
import ctypes

N = 3
x = numpy.ones(N, dtype=numpy.float32)
y = numpy.ones(N, dtype=numpy.float32)
plate = ctypes.cdll.LoadLibrary('libplate.so')

print 'passing address: %0x' % x.ctypes.data
plate.plate(x.ctypes.data, y.ctypes.data, ctypes.c_int(N))

Output from python-2.7

In [1]: run ../test_plate.py

passing address: 7f9a09b02320

passed address: 0x9b02320

local pointer size: 8

local pointer address: 0x7f9a0949a400

Answer 1

The problem is that the ctypes module doesn't check the function signature of the function you're trying to call. Instead, it bases the C types on the Python types, so the line...

plate.plate(x.ctypes.data, y.ctypes.data, ctypes.c_int(N))

...is passing the the first two params as integers. See eryksun's answer for the reason why they're being truncated to 32 bits.

To avoid the truncation, you'll need to tell ctypes that those params are actually pointers with something like...

plate.plate(ctypes.c_void_p(x.ctypes.data),
            ctypes.c_void_p(y.ctypes.data),
            ctypes.c_int(N))

...although what they're actually pointers to is another matter - they may not be pointers to float as your C code assumes.

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Update

eryksun has since posted a much more complete answer for the numpy-specific example in this question, but I'll leave this here, since it might be useful in the general case of pointer truncation for programmers using something other than numpy.