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

[Diez B. Roggisch]

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