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

Grigory Petrov grigory.v.p at gmail.com
Thu Nov 18 13:03:57 CET 2010


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?



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