C++ - Create Pythonwrapper to existing Library

Question

I need to write a python wrapper for an existing C++ Module. First I tested the procedere with this basic example (which now actually works fine): C++ - Python Binding with ctypes - Return multiple values in function

Now I tried to change the setting: I want to use the existing lib instead of my single cpp file. I tried it with this:

g++ -c -I. -fPIC projectionWrapper.cpp -o projectionWrapper.o
g++ -shared -Wl,-soname,libproj.so 
    -L./build/liborig_interface.a,./build/liborig_base.a 
    -o libproj.so  projectionWrapper.o

I wanted to link against both .a files from the given library with the -L command. I don't get any errors on that, but when I try to import the module via ipython, I get this:

import myprojection    # I load libproj.so in this python file
OSError: ./libproj.so: undefined symbol: _Z29calibration_loadPKcjbP14camera_typetS2_

There is a function "calibration_load", as well a "camera_type" in the original framework. But I have no clue where the cryptic things in between come from.

Sorry for my vague explanation, I tried to explain it as good as possible, but a C++ Wrapper is not one of my topics where I feel "at home".

score 1 · Answer 1 · answered Sep 14 '15 at 15:21

1

The problem is that you're not linking against the external library that you use in your C++ code; add -l<library> to your second g++ call.

answered Sep 14 '15 at 15:21

Marcus Müller

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score 0 · Accepted Answer · edited May 23 '17 at 12:03

g++ -shared projectionWrapper.o 
    -L./build/base -L./build/interface
    -loriginterface -lorigbase 
    -Wl,-soname,libproj.so  
    -o libproj.so

did the job. Thanks for the hint that I actually didn't link the libraries as I only used the -L option.

Moreover the order of the options was wrong. I had to state "projectionWrapper.o" right at the beginning, as well as "-loriginterface" before "-lorigbase". This was answered here: "undefined reference" when linking against a static library

(complete name of the libs are: liboriginterface.a and liborigbase.a)

C++ - Create Pythonwrapper to existing Library

2 Answers2