call Cython function from C++

Question

I have a C++ library that has a Python wrapper (written with SWIG). This library allows executing small user-defined code (a callback), such as element-wise operations on a

Answer 1

Embedding Python in Another Application may be useful reading.

Answer 2

The trick with cython is in using the keyword public

cdef public double cython_function( double value, double value2 ):
    return value + value2

Then the command cythonize <your_file.pyx> along with <your_file.c> will create header <your_file.h> that you can include. Alternatively, you can create the header yourself:

#ifdef __cplusplus {
extern "C"
#endif

double cython_function( double value, double value2 );

#ifdef __cplusplus
}
#endif

Update:

Then with a little overlay from Python you can use ctypes's callback mechanism

func_type = CFUNCTYPE(c_double, c_double, c_double)

your_library.set_callback_function ( func_type(user_modules.cython_function) )

Answer 3

You can achieve that by doing pure cdef functions :

# declare the prototype of your function
ctypedef void (*callback_ptr)(int arg)

# declare your function as cdef
cdef void my_callback(int arg):
    print 'doing some python here', arg

# now, you can use the cdef func as a callback
# in pure C !
cdef void run():
    cdef callback_ptr p = my_callback
    p(42)

if __name__ == '__main__':
    run()

Note: you can use "cython -a" to see that they are no python code involved for the content of run. So it will work with your c library.