CODEWITHSUNDEEP
pythonc++cython
AlekseyStack
AlekseyStack

Reputation: 109

I get an error, when using C-library with Cython

In Qt Creator I created a small library (in 'Squaring' subfolder of Python project):

squaring.h:

int squaring(int a);

squaring.c:

#include "squaring.h"
int squaring(int a){
    return a * a;
}

In Eclipse, I created Python project that tries to use this library (according to the instructions from the official website):

cSquaring.pxd:

cdef extern from "Squaring/squaring.h":
    int squaring(int a)

Functions.pix:

cimport cSquaring
cpdef int count(int value):
    return cSquaring.squaring(value)

setup.py:

from distutils.core import setup
from distutils.extension import Extension
from Cython.Build import cythonize
setup(ext_modules=cythonize([Extension("Functions", ["Functions.pyx"])]))

main.py:

from Functions import count
if __name__ == '__main__':
    data = 1
    returned = count(data)
    print(returned)

Completed the compilation of C code using (without any errors):

python3 setup.py build_ext -i

But when I run main.py on execution, I get ImportError:

File “/home/denis/.p2/pool/plugins/org.python.pydev.core_7.3.0.201908161924/pysrc/pydevd.py”, line 2643, in <module>
main()
File “/home/denis/.p2/pool/plugins/org.python.pydev.core_7.3.0.201908161924/pysrc/pydevd.py”, line 2636, in main
globals = debugger.run(setup, None, None, is_module)
File “/home/denis/.p2/pool/plugins/org.python.pydev.core_7.3.0.201908161924/pysrc/pydevd.py”, line 1920, in run
return self._exec(is_module, entry_point_fn, module_name, file, globals, locals)
File “/home/denis/.p2/pool/plugins/org.python.pydev.core_7.3.0.201908161924/pysrc/pydevd.py”, line 1927, in _exec
pydev_imports.execfile(file, globals, locals) # execute the script
File “/home/denis/.p2/pool/plugins/org.python.pydev.core_7.3.0.201908161924/pysrc/_pydev_imps/_pydev_execfile.py”, line 25, in execfile
exec(compile(contents+“\n”, file, ‘exec’), glob, loc)
File “/home/denis/eclipse-workspace/ConnectionWithCPlusPlus/main.py”, line 1, in <module>
from Functions import count
ImportError: /home/denis/eclipse-workspace/ConnectionWithCPlusPlus/Functions.cpython-37m-x86_64-linux-gnu.so: undefined symbol: squaring

And another project where the code is used Cython (there I did not create A C-library, and wrote the code directly on Cython) works fine.

What is the problem?

Upvotes: 1

Views: 229

Answers (1)

FallenWarrior
FallenWarrior

Reputation: 686

You included the header files in Cython, but you never actually told it about the implementation, i.e. the library where the function is defined. You need to link against the library generated by compiling your C source, as described in the Cython docs.

In your Functions.pyx, you have to add a comment like this to the top.

# distutils: sources = path/to/squaring.c

Upvotes: 1

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