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Manipulating pointers in C using ctypes

Mark Tolonen metolone+gmane at gmail.com
Sat Jan 9 00:09:05 EST 2010 
"Daniel Platz" <mail.to.daniel.platz at googlemail.com> wrote in message 
news:08bda34e-9bee-44b1-b2de-80d647151786 at a15g2000yqm.googlegroups.com...
> Hello!
>
> 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;
> #endif
>
> __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
> }
> #endif
> ------------------------------------------------------------------------
>
> 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.
>
> --------------------------- pointers.py --------------------------
> 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)
> pointers.show(ct.c_void_p(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: <pointers.py>
>
> 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
> object.
>
> Moreover, I also tried to directly manipulate the address of the
> pointer given as an argument to pointers.show. 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
> answer.

As Jason observed, your create() function creates a number on the local 
stack, which becomes invalid once the function returns.  If you don't want 
to worry about freeing the object created in Jason's solution, you can 
create the double in python yourself and let create initialize it:


-------------- pointers.py ----------------------
import ctypes as ct

pointers = ct.CDLL('pointers.dll')
pointers.create.restype = None
pointers.show.restype = None

dbl = ct.c_double()
print ct.byref(dbl),dbl
ptr = pointers.create(ct.byref(dbl))

# Show pointer address and value
print 'Value returned to python: %f' % dbl.value
pointers.show(ct.byref(dbl))
print 'Value returned to python: %f' % dbl.value

-------------- pointers.c ------------------------
#include <stdio.h>

__declspec(dllexport) void create(double* ptr)
{
 *ptr = 2.2;
 printf("Pointer address   %p\n", ptr);
 printf("Value at pointer  %f\n", *ptr);
}

__declspec(dllexport) void show(double* ptr)
{
 printf("Pointer address   %p\n", ptr);
 printf("Pointer value     %f\n", *ptr);
 *ptr = 2.4;
 printf("New pointer value %f\n", *ptr);
}

-------------- OUTPUT -------------------------
<cparam 'P' (00A05DC8)> c_double(0.0)
Pointer address   00A05DC8
Value at pointer  2.200000
Value returned to python: 2.200000
Pointer address   00A05DC8
Pointer value     2.200000
New pointer value 2.400000
Value returned to python: 2.400000
-----------------------------------------------------

HTH,
Mark
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