如何使用ctypes将数组从Go [lang]返回到Python？

Question

I am trying to write some code that creates an array in GoLang, and returns it to a python script ctypes (and some numpy). What I have got so far doesn't work, and I cannot figure out why... I would appreciate any help!

My Go code goes something like this:

func Function(physics_stuff... float64,  N int ) []float64{
    result := make([]float64, N)
    for i:= 0; i< N; i++{
        result[i] =  blah....
    }
    return result;
}

and I am currently trying to import this functionality to python using:

from ctypes import c_double, cdll, c_int
from numpy.ctypeslib import ndpointer

lib = cdll.LoadLibrary("./go/library.so")
lib.Function.argtypes = [c_double]*6 + [c_int]

def lovely_python_function(stuff..., N):
    lib.Function.restype = ndpointer(dtype = c_double, shape = (N,))
    return lib.Function(stuff..., N)

This python function never returns. Other functions from the same library work just fine, but they all return a single float64 (c_double in python).

Answer 1

In your code restype is expecting _ndtpr type, see:

lib.Function.restype = ndpointer(dtype = c_double, shape = (N,))

See too in numpy document:

def ndpointer(dtype=None, ndim=None, shape=None, flags=None)

[others texts]

Returns

klass : ndpointer type object

A type object, which is an _ndtpr instance containing
dtype, ndim, shape and flags information.

[others texts]

In this way lib.Function.restype is pointer type which appropriated type in Golang must be unsafe.Pointer.

However you want of an slice that need be passed as pointer:

func Function(s0, s1, s2 float64, N int) unsafe.Pointer {
    result := make([]float64, N)
    for i := 0; i < N; i++ {
        result[i] = (s0 + s1 + s2)
    }
    return unsafe.Pointer(&result)//<-- pointer of result
}

This cause a problem in Rules for passing pointers between Go and C.

1. C code may not keep a copy of a Go pointer after the call returns.

Source: https://github.com/golang/proposal/blob/master/design/12416-cgo-pointers.md

So you must convert unsafe.Pointer to uintptr golang type.

func Function(s0, s1, s2 float64, N int) uintptr {
    result := make([]float64, N)
    for i := 0; i < N; i++ {
        result[i] = (s0 + s1 + s2)
    }
    return uintptr(unsafe.Pointer(&result[0]))//<-- note: result[0]
}

In this way you will work fine!

Note: The structure of slice in C is represented by typedef struct { void *data; GoInt len; GoInt cap; } GoSlice;, but C expect only data, for this is need result only void *data (first field, or field[0]).

PoC: https://github.com/ag-studies/stackoverflow-pointers-ref-in-golang