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I am accessing an API and can't get the data returned. The two float pointers will point to an array of data. I must assume the API is working properly. A different function call provides a the length of the data I am retrieving. This values is length down below when attempted.

C Header for Function

 int function(int, float * data1, float * data2)

ctypes setup

 dll.function.argtypes = (c_int, POINTER(c_float), POINTER(c_float))
 dll.function.restypes = c_int

Failed Attempt 1:

 x = c_float()
 y = c_float()
 status = dll.function(1, byref(x), byref(y))

Program crashes OR Access violation writing.

Failed Attempt 2:

 x = POINTER(c_float)()
 y = POINTER(c_float)()
 status = dll.function(1, x, y)

Null Pointer Error

Failed Attempt 3:

 dll.function.argtypes = (c_int, c_void_p, c_void_p)
 x = c_void_p()
 y = c_void_p()
 status = dll.function(1, x, y)

Null Pointer Error

Failed Attempt 4:

 array = c_float * length
 x = array()
 y = array()
 status = dll.function(1, byref(x), byref(y))

Program crashes

Failed Attempt 5:

 array = c_float * length
 x = POINTER(array)()
 y = POINTER(array)()
 status = dll.function(1, x, y)

Null Pointer Error OR ArgumentError: expected LP_c_float instance instead of LP_c_float_Array_[length]

Failed Attempt 6:

 x = (c_float*length)()
 y = (c_float*length)()
 a = cast(x, POINTER(c_float))
 b = cast(y, POINTER(c_float))
 status = dll.function(1, a, b)

Program crashes

What am I missing and why?

I believe the argtypes are correct. I am attempting to meet them properly, but there continues to be an issues. Do I need to "malloc" the memory somehow? (I'm sure I need to free after I get the data).

This is on Windows 7 with Python 2.7 32-bit.

I have looked through other similar issues and am not finding a solution. I am wondering if, at this point, I can blame the API for this issue.

Dealing with pointers and arrays is explained in [Python 3]: Type conversions.

I prepared a dummy example for you.

main.c:

#if defined(_WIN32)
# define DECLSPEC_DLLEXPORT __declspec(dllexport)
#else
# define DECLSPEC_DLLEXPORT
#endif


static int kSize = 5;


DECLSPEC_DLLEXPORT int size() 
 return kSize;



DECLSPEC_DLLEXPORT int function(int dummy, float *data1, float *data2) 
 for (int i = 0; i < kSize; i++) 
 data1[i] = dummy * i;
 data2[i] = -dummy * (i + 1);
 
 return 0;

code.py:

#!/usr/bin/env python

import sys
import ctypes


c_float_p = ctypes.POINTER(ctypes.c_float)


def main():
 dll_dll = ctypes.CDLL("./dll.so")

 size_func = dll_dll.size
 size_func.argtypes = []
 size_func.restype = ctypes.c_int


 function_func = dll_dll.function
 function_func.argtypes = [ctypes.c_int, c_float_p, c_float_p]
 function_func.restype = ctypes.c_int

 size = size_func()
 print(size)

 data1 = (ctypes.c_float * size)()
 data2 = (ctypes.c_float * size)()

 res = function_func(1, ctypes.cast(data1, c_float_p), ctypes.cast(data2, c_float_p))
 for i in range(size):
 print(data1[i], data2[i])


if __name__ == "__main__":
 print("Python :s on :sn".format(sys.version, sys.platform))
 main()

Notes:

• The C part tries to mimic what your .dll does (or at least what I understood):
  
  
  • 
    size - gets the arrays sizes
  
  
  • 
    function - populates the arrays (till their size - assuming that they were properly allocated by the caller)
  
  


• 
  Python part is straightforward:
  
  
  • Load the .dll
    
  
  
  • Define argtypes and restype (in your code it's restype's) for the 2 functions (for size_func not necessary)
  
  
  • Get the lengths
  
  
  • Initialize the arrays
  
  
  • Pass them to function_func using ctypes.cast
    
  
  

Output (on Lnx, as building the C code is much simpler, but works on Win as well):

[cfati@cfati-ubtu16x64-0:~/Work/Dev/StackOverflow/q050043861]> gcc -shared -o dll.so main.c
[cfati@cfati-ubtu16x64-0:~/Work/Dev/StackOverflow/q050043861]> python3 code.py
Python 3.5.2 (default, Nov 23 2017, 16:37:01)
[GCC 5.4.0 20160609] on linux

5
0.0 -1.0
1.0 -2.0
2.0 -3.0
3.0 -4.0
4.0 -5.0

It really depends on what you are doing with these float pointers.

If you are trying to traverse it, i.e.

for(int i = 0; i < size; i++)
 printf("%fn%, data1[i])

then for sure this is problematic as no array was allocated. You simply passed a pointer pointing to a float. That is all.

You need to first allocate that memory. To this end Attempt 4 looks like the more promising, but I suspect you have a problem inside your C function leading to the crash.

Difficult to say without seeing the implementation of that function.