Manipulating pointers in C using ctypes

Daniel Platz at
Fri Jan 8 16:37:49 CET 2010


I have to ask a newbie question about manipulating pointers in C using
ctypes. I have a C dll with two functions. The first one creates a
pointer and returns it to python. The second one takes a pointer as an
argument shows its address and the value at which it is pointing. This
I have implemented using the following code

----------- -------- pointers.c ----------------------------
#include <stdio.h>
#ifdef __cplusplus
extern "C" {  // only need to export C interface if
              // used by C++ source code
using namespace std;

__declspec(dllexport) void* create()
	double number = 2.2;
	double* ptr = &number;
	printf("Pointer address \t %p \n", ptr);
	printf("Value at pointer \t %f \n", ptr[0]);
	return (void*) ptr;

__declspec(dllexport) int show(double* ptr)
	printf("Pointer address \t %p \n", ptr);
	printf("Pointer value \t %f\n", *ptr);
	*ptr = 2.4;
	printf("New pointer value \t %f\n", *ptr);
	return 0;

#ifdef __cplusplus

Please note that in the second function, the show function, I want to
manipulate the value at which the pointer points.
Now, I call this function from python with the following script.

--------------------------- --------------------------
import ctypes as ct

# Load dll
pointers = ct.cdll.LoadLibrary('pointers.dll')
getattr(pointers, 'create')
getattr(pointers, 'show')
pointers.create.restype = ct.c_void_p

# Create pointer in C
ptr = pointers.create()

# Show pointer address and value
print 'Adress returned to python ' +hex(ptr)

Calling this script gives me the following output:

Pointer address          0021E508
Value at pointer         2.200000
Adress returned to python 0x21e508
Pointer address          0021E508
Pointer value    0.000000    (2.20000 expected)
New pointer value        2.400000 (2.40000 expected)

But the script returns also an error.

WindowsError: exception: access violation reading 0x40033333
WARNING: Failure executing file: <>

Another thing that I find strange is that the return value of
pointers.create is actually an integer instead of an ct.c_void_p

Moreover, I also tried to directly manipulate the address of the
pointer given as an argument to But when it returns to
python the pointer points still at the same address as before the
function call.

Can someone help me with this problem? I would be very glad about an

With kind regards,


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