Undefined symbol with Python extention using OpenCV C API

Question

I am expanding a C-extension to use uses OpenCV, and to start, cvtColor, but I am getting an undefined symbol error. I think it may be me mixing up C and C++, or maybe loading the library incorrectly. I have the error boiled down to this C code:

test.c:

#include <cv.h>

void BGR2HLS(const void * srcv, void * dstv, int elems) {
    const uint8_t * src = (uint8_t *) srcv;
    uint8_t * dst = (uint8_t *) dstv;
    cvtColor(src, dst, CV_BGR2HLS);
}

which I compile using:

gcc -fPIC -shared -o test.so test.c -L/usr/lib/ `pkg-config --cflags opencv` -lopencv_imgproc -lopencv_core

I see /usr/lib/libopencv_imgproc.so exists, and everything compiles without error.

I then try to run this Python script:

import numpy as np
import os.path
import ctypes
import cv2

test_module = ctypes.CDLL(
    os.path.join(os.path.dirname(__file__), './test.so'))

test_module.BGR2HLS.argtypes = [
    ctypes.c_void_p, ctypes.c_void_p, ctypes.c_int
]

if __name__ == '__main__':
    img_data = cv2.imread('../test_data/FixedUIDesign.jpg')
    h, w, d = img_data.shape
    img_data_out = np.array(img_data, dtype=np.uint8)
    test_module.BGR2HLS(img_data.ctypes.data, img_data_out.ctypes.data, h*w)

and get the following error (I was experiment with adding the RTLD_GLOBAL flag):

Traceback (most recent call last):
  File "c_tests.py", line 6, in <module>
    os.path.join(os.path.dirname(__file__), './test.so'), ctypes.RTLD_GLOBAL)
  File "/usr/lib/python2.7/ctypes/__init__.py", line 365, in __init__
    self._handle = _dlopen(self._name, mode)
OSError: ./test.so: undefined symbol: cvtColor

I see opencv_imgproc exists, and I can point to it when I compile without errors. I can see there is something similar to the symbol I want as well:

$ nm -D /usr/lib/libopencv_imgproc.so | grep cvtColor
00000000000b65a0 T _ZN2cv8cvtColorERKNS_11_InputArrayERKNS_12_OutputArrayEii

Is the odd name a symptop of the issue? What is wrong/missing with how I'm building/using the extension?

possible duplicate of Ctypes "symbol not found" for dynamic library in OSX — n. m. could be an AI
– n. m. could be an AI, Commented Nov 30, 2014 at 18:28
I never solved this, but I swithced to using C++ and it works. — Charles L.
– Charles L., Commented Apr 12, 2015 at 17:02

Charles L. · Accepted Answer · 2015-04-12 22:09:55Z

1

Switching to C++ solves the problem. Luckily I have the flexibility.

answered Apr 12, 2015 at 22:09

Charles L.

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