Re: How to correctly pass “pointer-to-pointer” into DLL via ctypes?

[Grigory Petrov]

Thank you a lot!

I have stripped down my production code to the sample - and it worked.
Bug was in another part of my code where free() was called for the
memory in question.

On Fri, Nov 19, 2010 at 3:39 PM, Diez B. Roggisch <deets at web.de> wrote:
> Grigory Petrov <grigory.v.p at gmail.com> writes:
>
>> Hello.
>>
>> I have a DLL that allocates memory and returns it. Function in DLL is like this:
>>
>> void Foo( unsigned char** ppMem, int* pSize )
>> {
>>   * pSize = 4;
>>   * ppMem = malloc( * pSize );
>>   for( int i = 0; i < * pSize; i ++ ) (* pMem)[ i ] = i;
>> }
>>
>> Also, i have a python code that access this function from my DLL:
>>
>> from ctypes import *
>> Foo = windll.mydll.Foo
>> Foo.argtypes = [ POINTER( POINTER( c_ubyte ) ), POINTER( c_int ) ]
>> mem = POINTER( c_ubyte )()
>> size = c_int( 0 )
>> Foo( byref( mem ), byref( size ) ]
>> print size, mem[ 0 ], mem[ 1 ], mem[ 2 ], mem[ 3 ]
>>
>> I'm expecting that print will show "4 0 1 2 3" but it shows "4 221 221
>> 221 221" O_O. Any hints what i'm doing wrong?
>
> After correcting quite a few obvious errors in your code, it worked just
> fine for me. So I guess what you think you test is not what you test.
>
> --- test.py
> from ctypes import *
>
> foo = cdll.LoadLibrary("/Users/deets/projects/GH28/kinect/kinect/c/foo/libtest.dylib").foo
> foo.argtypes = [ POINTER( POINTER( c_ubyte ) ), POINTER( c_int ) ]
> mem = POINTER( c_ubyte )()
> size = c_int( 0 )
> foo( byref( mem ), byref( size ) )
> print size, mem[ 0 ], mem[ 1 ], mem[ 2 ], mem[ 3 ]
> ---
>
> --- test.c
> void foo( unsigned char** ppMem, int* pSize )
> {
>  int i;
>  * pSize = 4;
>  * ppMem = malloc( * pSize );
>  for( i = 0; i < * pSize; i ++ ) (* ppMem)[ i ] = i;
> }
> ---
>
> --- CMakeLists.txt
> cmake_minimum_required (VERSION 2.6)
> project (Test)
> add_library(test SHARED test.c)
> ---
>
> --
> Diez
> --
> http://mail.python.org/mailman/listinfo/python-list
>