Problem in passing numpy array to a C function

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I have the following C function:

int c_sum(const int *a, int len) {

    int c = 0;
    for (int i=0; i<len; i++){
        c += a[i];
    }
    return c;
}

After creating the shared lib with the following:

cc -fPIC -shared -o c_sum.so c_sum.c

I run the following Python code in Jupyter notebook

import numpy as np
import time
from ctypes import *

lib = cdll.LoadLibrary("c_sum.so")
c_sum = lib.c_sum
c_sum.restype = c_int


a = np.random.randint(1,5,6)
print(a)

c = c_sum(c_void_p(a.ctypes.data), c_int(len(a)))
print(c)

It prints the following:

[1 4 3 1 2 2]
8

What might be the problem here? It prints the sum of the first 3 numbers. I tried random arrays of different lengths, it turns out that the code always sums the first half of the numbers in the array. Thanks.

ctypes c python
1 Answers

It's easier to explain in an answer rather than in comments. It has nothing to do with different int sizes. Yes, the types do differ, but not in the way OP thinks, as the int array is a NumPy (which is written in C and C++) one. Check [SO]: Maximum and minimum value of C types integers from Python for more details on this topic.

Listing [Python.Docs]: ctypes - A foreign function library for Python.
Also listing [NumPy]: C-Types Foreign Function Interface (numpy.ctypeslib) - convenience library for conversions between NumPy and CTypes.

dll00.c:

#if defined(_WIN32)
#  define DLL00_EXPORT_API __declspec(dllexport)
#else
#  define DLL00_EXPORT_API
#endif


#if defined(__cplusplus)
extern "C" {
#endif

DLL00_EXPORT_API int arrSum(const int *arr, int len);

#if defined(__cplusplus)
}
#endif


int arrSum(const int *arr, int len)
{
    int s = 0;
    for (int i = 0; i < len; ++i)
        s += arr[i];
    return s;
}

code00.py:

#!/usr/bin/env python

import ctypes as ct
import sys
import numpy as np


IntPtr = ct.POINTER(ct.c_int)

DLL_NAME = "./dll00.{:s}".format("dll" if sys.platform[:3].lower() == "win" else "so")


def main(*argv):
    dll00 = ct.CDLL(DLL_NAME)
    arrSum = dll00.arrSum
    arrSum.argtypes = (IntPtr, ct.c_int)
    arrSum.restype = ct.c_int

    a = np.random.randint(1, 5, 6)
    print(a, sum(a))

    datas = [
        np.ctypeslib.as_ctypes(a),  # Using ctypeslib
        a.ctypes.data_as(IntPtr),  # Using manual conversion
        #IntPtr(ct.c_long(a.ctypes.data)),  # Original way - wrong
    ]
    for data in datas:
        res = arrSum(data, len(a))
        print("\n{0:s} returned: {1:d}".format(arrSum.__name__, res))


if __name__ == "__main__":
    print("Python {:s} {:03d}bit on {:s}\n".format(" ".join(elem.strip() for elem in sys.version.split("\n")),
                                                   64 if sys.maxsize > 0x100000000 else 32, sys.platform))
    rc = main(*sys.argv[1:])
    print("\nDone.")
    sys.exit(rc)

Output:

[cfati@CFATI-5510-0:e:\Work\Dev\StackOverflow\q070633293]> sopr.bat
### Set shorter prompt to better fit when pasted in StackOverflow (or other) pages ###

[prompt]> "c:\Install\pc032\Microsoft\VisualStudioCommunity\2019\VC\Auxiliary\Build\vcvarsall.bat" x64 >nul

[prompt]> dir /b
code00.py
dll00.c

[prompt]> cl /nologo /MD /DDLL dll00.c  /link /NOLOGO /DLL /OUT:dll00.dll
dll00.c
   Creating library dll00.lib and object dll00.exp

[prompt]> dir /b
code00.py
dll00.c
dll00.dll
dll00.exp
dll00.lib
dll00.obj

[prompt]>
[prompt]> "e:\Work\Dev\VEnvs\py_pc064_03.09_test0\Scripts\python.exe" code00.py
Python 3.9.9 (tags/v3.9.9:ccb0e6a, Nov 15 2021, 18:08:50) [MSC v.1929 64 bit (AMD64)] 064bit on win32

[2 2 2 3 4 4] 17

arrSum returned: 17

arrSum returned: 17

Done.
user_47885460
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