Python is passing 32bit pointer address to C functions -

i call c functions within shared library python scripts. problem arrises when passing pointers, 64bit addresses seem truncated 32bit addresses within called function. both python , library 64bit.

the example codes below demonstrate problem. python script prints address of data being passed c function. then, address received printed within called c function. additionally, c function proves 64bit printing size , address of locally creating memory. if pointer used in other way, result 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 python-2.7

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

passing address: 7f9a09b02320

passed address: 0x9b02320

local pointer size: 8

local pointer address: 0x7f9a0949a400

the problem ctypes module doesn't check function signature of function you're trying call. instead, bases c types on python types, line...

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

...is passing the first 2 params integers. see eryksun's answer reason why they're being truncated 32 bits.

to avoid truncation, you'll need tell ctypes params pointers like...

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

...although they're pointers to matter - may not pointers float c code assumes.

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update

eryksun has since posted more complete answer numpy-specific example in question, i'll leave here, since might useful in general case of pointer truncation programmers using other numpy.