how to pass numpy array to Cython function correctly?

This is described in many places but i simply cannot get it to work. I am calling a C++ function from Cython:

cimport numpy as np
cdef extern from "test.h" namespace "mytest":
   void test(double *A, int m)

cdef int foo():
  cdef np.ndarray[double,mode="c"] a = np.array([1,2,3,4,5],dtype=float)
  # pass ptr to first element of 'a'
  test(&a[0], len(a))
  return 0

foo()

test.cpp is just:

#include <stdio.h>
namespace mytest {
    void test(double *A, int m)
    {
    for (int i = 0; i < m; i++)
    {
        printf("%d is %f\n", i, A[i]);
    }
    }
}

test.h just has:

namespace mytest {
  void test(double *A, int m);
}

This seems to work but when is np.ascontiguousarray needed? Is it sufficient to do:

cdef np.ndarray[double,mode="c"] a = np.array([1,2,3,4,5],dtype=float)

or do you need:

cdef np.ndarray[double,mode="c"] a = np.ascontiguousarray(np.array([1,2,3,4,5],dtype=float))

second and more importantly, how can this generalize to 2d arrays?

Handling 2d arrays

Here is my attempt at passing 2d numpy arrays to C++ which does not work:

cdef np.ndarray[double,mode="c",ndim=2] a = np.array([[1,2],[3,4]],dtype=float)

which is called as:

test(&a[0,0], a.shape[0], a.shape[1])

in the cpp code:

void test(double *A, int m, int n) 
{ 
  printf("reference 0,0 element\n");
  printf("%f\n", A[0][0]);
}

UPDATE: The correct answer

The correct answer is to use linear indexing for the array and not the [][] syntax. The correct way to print the 2d array is:

for (int i = 0; i < m; i++)
{
    for (int j = 0; j < n; j++)
    {
    printf("%d, %d is %f\n", i, j, A[i*m + j]);
    }
}