Python and ctypes: how to correctly pass “pointer-to-pointer” into DLL?

Question

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 = m         


        

Answer 1

Post actual code. The C/C++ code doesn't compile as either C or C++. The Python code has syntax errors (] ending function call Foo). The code below works. The main issue after fixing syntax and compiler errors was declaring the function __stdcall so windll could be used in the Python code. The other option is to use __cdecl (normally the default) and use cdll instead of windll in the Python code.

mydll.c (cl /W4 /LD mydll.c)

#include <stdlib.h>

__declspec(dllexport) void __stdcall Foo(unsigned char** ppMem, int* pSize)
{
    char i;
    *pSize = 4;
    *ppMem = malloc(*pSize);
    for(i = 0; i < *pSize; i++)
        (*ppMem)[i] = i;
}

demo.py

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.value,mem[0],mem[1],mem[2],mem[3]

Output

4 0 1 2 3