How to convert pointer to c array to python array

Question

I have a C++ callback function that calls into Python using ctypes. This function\'s parameters are a pointer to an array of double and the number of elements.

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Answer 1

If Data were (c_double*DataLength.value) array then you could:

a = np.frombuffer(Data) # no copy. Changes in `a` are reflected in `Data`

If Data is a POINTER(c_double) you could get numpy array using numpy.fromiter(). It is the same loop as in your question but faster:

a = np.fromiter(Data, dtype=np.float, count=DataLength.value) # copy

To create a numpy array from POINTER(c_double) instance without copying you could use .from_address() method:

ArrayType = ctypes.c_double*DataLength.value
addr = ctypes.addressof(Data.contents)
a = np.frombuffer(ArrayType.from_address(addr))

Or

array_pointer = ctypes.cast(Data, ctypes.POINTER(ArrayType))
a = np.frombuffer(array_pointer.contents)

Both methods convert POINTER(c_double) instance to (c_double*DataLength) before passing it to numpy.frombuffer().

Cython-based solution

Is there anyway to load the data from the C++ array and then convert it to an array fit for scipy?

Here's C extension module for Python (written in Cython) that provide as C API the conversion function:

cimport numpy as np
np.import_array() # initialize C API to call PyArray_SimpleNewFromData

cdef public api tonumpyarray(double* data, long long size) with gil:
    if not (data and size >= 0): raise ValueError
    cdef np.npy_intp dims = size
    #NOTE: it doesn't take ownership of `data`. You must free `data` yourself
    return np.PyArray_SimpleNewFromData(1, &dims, np.NPY_DOUBLE, <void*>data)

It could be used with ctypes as follows:

from ctypes import (PYFUNCTYPE, py_object, POINTER, c_double, c_longlong,
                    pydll, CFUNCTYPE, c_bool, cdll)

import pointer2ndarray
tonumpyarray = PYFUNCTYPE(py_object, POINTER(c_double), c_longlong)(
    ("tonumpyarray", pydll.LoadLibrary(pointer2ndarray.__file__)))

@CFUNCTYPE(c_bool, POINTER(c_double), c_longlong)
def callback(data, size):
    a = tonumpyarray(data, size)
    # call scipy functions on the `a` array here
    return True

cpplib = cdll.LoadLibrary("call_callback.so") # your C++ lib goes here
cpplib.call_callback(callback)

Where call_callback is: void call_callback(bool (*)(double *, long long)).