Python Ctypes passing pointer for data

Question

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.

Answer 1

Dealing with pointers and arrays is explained in [Python.Docs]: ctypes - Type conversions.

I prepared a dummy example for you.

main00.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;
}

code00.py:

#!/usr/bin/env python

import sys
import ctypes as ct


c_float_p = ct.POINTER(ct.c_float)


def main(*argv):
    dll = ct.CDLL("./dll00.so")

    size = dll.size
    size.argtypes = []
    size.restype = ct.c_int


    function = dll.function
    function.argtypes = [ct.c_int, c_float_p, c_float_p]
    function.restype = ct.c_int

    sz = size()
    print(sz)

    data1 = (ct.c_float * sz)()
    data2 = (ct.c_float * sz)()

    res = function(1, ct.cast(data1, c_float_p), ct.cast(data2, c_float_p))
    for i in range(sz):
        print(data1[i], data2[i])


if __name__ == "__main__":
    print("Python {0:s} {1:d}bit on {2:s}\n".format(" ".join(item.strip() for item in sys.version.split("\n")), 64 if sys.maxsize > 0x100000000 else 32, sys.platform))
    rc = main(*sys.argv[1:])
    print("\nDone.")
    sys.exit(rc)

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 dll00.so main00.c
[cfati@cfati-ubtu16x64-0:~/Work/Dev/StackOverflow/q050043861]> python3 code00.py
Python 3.8.5 (default, Jan 27 2021, 15:41:15) [GCC 9.3.0] 64bit on linux

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

Answer 2

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("%f\n%, 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.