Using Python Ctypes for ssdeep's fuzzy.dll but receive Error [SOLVED]

Question

I am trying to use python and ctypes to use the fuzzy.dll from ssdeep. So far all I have tried fails with and access violation error. Here is what I do after changing to the proper directory which contains the fuzzy.dll and fuzzy.def files.

>>> import os,sys
>>> from ctypes import *
>>> fn = create_string_buffer(os.path.abspath("fuzzy.def"))
>>> fuzz = windll.fuzzy
>>> chash = c_char_p(512)
>>> hstat = fuzz.fuzzy_hash_filename(fn,chash)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
WindowsError: exception: access violation writing 0x00000200
>>>

From what I understand, I have passed the proper c_types. From fuzzy.h

extern int fuzzy_hash_filename(char * filename, char * result)

I just cannot get past that access violation. Thank you in advance.

Answer 1

There are two problems with your code:

1. You should not use windll.fuzzy, but cdll.fuzzy -- from ctypes documentation:

   cdll loads libraries which export functions using the standard cdecl calling convention, while windll libraries call functions using the stdcall calling convention.

2. For return value (chash), you should declare a buffer rather than creating a pointer to 0x0000200 (=512) -- this is where the access violation comes from. Use create_string_buffer('\000' * 512) instead.

So your example should look like this:

>>> import os, sys
>>> from ctypes import *
>>> fn = create_string_buffer(os.path.abspath("fuzzy.def"))
>>> fuzz = cdll.fuzzy
>>> chash = create_string_buffer('\000' * 512)
>>> hstat = fuzz.fuzzy_hash_filename(fn,chash)
>>> print hstat
0 # == success

Answer 2

@DzinX

Perfect. Thank you.

Marked as SOLVED