AttributeError: python: undefined symbol: when accessing C++ function from Python using ctypes

Question

What I am trying to do is call a C++ method from Python to return a 2D array. The Python filename is: BaxterArm1.py and the C++ filename is: baxIK.cpp. Below is how I compiled my c++ program:

g++ -c -fPIC baxIK.cpp -o baxIK.o
g++ -shared -Wl,-soname,baxIK.so -o baxIK.so baxIK.o

Below is the relevant part of the C++ program:

int main(int argc, char** argv)
{

}

extern "C" float** compute(float x, float y, float z, float q1, float q2, float q3, float q4)
{
    unsigned int num_of_solutions = (int)solutions.GetNumSolutions();
    std::vector<IKREAL_TYPE> solvalues(num_of_joints);
    float** sols;
    sols = new float*[(int)num_of_solutions];

    for(std::size_t i = 0; i < num_of_solutions; ++i) {
        sols[i] = new float[7];
        for( std::size_t j = 0; j < solvalues.size(); ++j)
        {
            printf("%.15f, ", solvalues[j]);
            sols[i][j] = solvalues[j];
        }
        printf("\n");
    }   
    return sols;
}

The idea is to return a Nx7 array for Python to receive. The Python code is as follows:

def sendArm(xGoal, yGoal, zGoal, right, lj):
print "Goals %f %f %f" % (xGoal, yGoal, zGoal)
output = ctypes.CDLL(os.path.dirname('baxIK.so'))
output.compute.restype = POINTER(c_float)
output.compute.argtypes = [c_float, c_float, c_float, c_float, c_float, c_float, c_float]   
res = output.compute(xGoal, yGoal, zGoal, 0.707, 0, 0.7, 0)
print(res)

The error I get is on the line

output.compute.restype = POINTER(c_float)

and the traceback is below:

File "/home/eadom/ros_ws/src/baxter_examples/scripts/BaxterArm1.py", line 60, in sendArm
    output.compute.restype = POINTER(c_float)
  File "/usr/lib/python2.7/ctypes/__init__.py", line 378, in __getattr__
    func = self.__getitem__(name)
  File "/usr/lib/python2.7/ctypes/__init__.py", line 383, in __getitem__
    func = self._FuncPtr((name_or_ordinal, self))
AttributeError: python: undefined symbol: compute

I'm confused as to how I'd fix this. Also, could somebody do a check on this and see if I'm sending the 2D array properly? As it is it's only sending a 1D array but I couldn't find anything on sending a 2D array. I'm grateful for any help.

The return type is pointer to pointer. Did you try output.compute.restype = POINTER(POINTER(c_float)) — Paul Rooney
– Paul Rooney, Commented Nov 25, 2015 at 4:30
Ah true, I will change that. Thank you. But that doesn't solve the other problem — Iche
– Iche, Commented Nov 25, 2015 at 4:52
You're using extern "C", so it shouldn't be name mangled. Check whether or not "compute" is listed in objdump -T baxIK.so, i.e. in the dynamic symbol table where dlsym looks for it. — Eryk Sun
– Eryk Sun, Commented Nov 25, 2015 at 10:03
I just did that. Compute shows up once but the overall terminal output is too long for the terminal to hold. Any other suggestions? — Iche
– Iche, Commented Nov 25, 2015 at 14:53

Iche · Accepted Answer · 2015-11-25 18:39:59Z

0

Finally got it working! Problem is that (os.path.dirname('baxIK.so')was returning an empty string, so I put in the full directory. I fixed the other issue I mentioned earlier by compiling g++ with llapack and now I'm fixing up one last detail. I hope this helps somebody else.

answered Nov 25, 2015 at 18:39

Iche

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1 Comment

Eryk Sun Over a year ago

On POSIX systems CDLL('') references the main module. Because of the undefined symbol error, I didn't even notice that you were using dirname incorrectly. If the file is in the same directory as the script/module, you can use scriptdir = os.path.abspath(os.path.dirname(__file__)); libpath = os.path.join(scriptdir, 'baxIK.so').

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