Passing numpy arrays in Cython to a C function that requires dynamically allocated arrays

Question

I have some C code that has the following declaration:

int myfunc(int m, int n, const double **a, double **b, double *c);

So a

Answer 1

Reply 1: You can pass NumPy array via Cython to C using the location of the start of the array (see code below).

Reply 2: Your declarations seem correct but I don't use this approach of explicit memory management. You can use NumPy to declare cdef-ed arrays.

Use

cdef double[:,::1] a = np.random.random([p, q])
cdef double[:,::1] b = np.empty([p, q])
cdef double[::1] b = np.empty(q)

Then pass &a[0], the location of the start of the array, to your C function. The ::1 is to ensure contiguousness.

A good reference for this is Jake Vanderplas' blog: https://jakevdp.github.io/blog/2012/08/08/memoryview-benchmarks/

Finally, typically one creates functions in Cython and calls them in Python, so your Python code would be:

import pyximport; pyximport.install()
import mytest
mytest.mywrappedfunc()

where mywrappedfunc is a Python (def and not cdef) function defined in the module that can do the array declaration show above.