passing pointer from python var to c lib

Question

I have a fixed C libary as DLL file. The DLL file has some security functions which is why i cant look into the libary. I only know the functionname and the type of data is needed.

I need to give the function a Pointer to my var in Python, the adress in the memory. I try using ctypes but it wont work. For the loading of the lib. i use ctypes.

This is the function with all needed variables i need to start.

function  (
 const unsigned char* input_array,
 unsigned int multiply_by,
 unsigned char* output_array
);

So i initialize my vars in Python like this:

input_array = "100"
multiply_by = 5
output_array = "000"

After that i load my lib and start the function.

lib = CDLL('func_lib.dll')
lib.function( hex(id(input_array)),
                 multiply_by,
                 hex(id(output_array))
             )

The function wont change the variable output_array, which is the goal of it. I can see that the function is called because no error is called. Do i handle the pointers wrong?

EDIT: I simplified the function to 3 vars, it would be the same for more. Also i think the input_array is the problem. The called function gets me a feedback-code which says, that the input_array is wrong. I dont know how to send the pointer adress to the function.

Neywat · Accepted Answer · 2020-02-17 16:10:25Z

1

The parameter bigarray is declared const unsigned char * in the function signature. If this is correct, then there are few chances for the bigarray variable to change.

EDIT To summarize comments, here is a hopefully working sample, given the original function signature.

    # Load the lib
    lib = CDLL('func_lib.dll')

    # Defines signature, based on C one
    lib.function.argtypes = [ c_char_p, c_uint, c_char_p ]

    # Call parameters
    data_in = b'Sample input'
    data_out = create_string_buffer(16)

    # Call
    lib.function(data_in, 16, data_out)

One guess here, i supposed that the second parameter is a buffer size, hence in the sample call i gave the same value as to create_string_buffer.
The call to create_string_buffer is required here, has it is an output for your function. Beware that there is an obvious buffer overrun risk here.

edited Feb 17, 2020 at 16:10

answered Feb 13, 2020 at 15:35

Neywat

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11 Comments

tryanderror Over a year ago

So u mean the problem is the "const"? I give the function a pointer to a start of a array from python which should not be protected or am i wrong?

Neywat Over a year ago

What i mean is if the function signature is correct, then the first argument is not supposed to be modified by the function. Did you get the signature from DLL doc ? Or was it guessed ?

tryanderror Over a year ago

thx u are right, i did post it wrong. i did a edit on my post

Neywat Over a year ago

I dont think using hex(...) will give you the expected result. For input and output parameters i guess you need to use ctypes' byref() (see docs.python.org/2/library/…) or using c_char_p type.

tryanderror Over a year ago

i tried using it like 'c_char_p(input_array)' but with this i get an error which says: bytes or integer address expected, so i tried, making a c var: 'c_input_array = c_wchar_p("100")' and use 'c_char_p(addressof(c_input_array))' in my function, but still no change

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