Accessing C array in golang

I have two files, module.go and test.py. My goal is to speed up some calculations that is done in python, but have an issue accessing array of integers in go.

module.go

package main

import "C"

//export Example
func Example(testArray []C.int) C.int {
    return testArray[2]
}

func main() {}

and simple test file in python:

from ctypes import *

# Load compiled go module
lib = cdll.LoadLibrary("./gomodule.so")
# We are passing an array of 256 elements and recieving integer
lib.Example.argtypes = [c_int * 256]
lib.Example.restype = c_int
pyarr = [x for x in range(256)]
# Make C array from py array
arr = (c_int * len(pyarr))(*pyarr)
print lib.Example(arr)

After compiling go module with go build -buildmode=c-shared -o gomodule.so module.go and fire up python file I got:

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x12 pc=0x7fb18b6e688c]

goroutine 17 [running, locked to thread]:
main.Example(...)
        /home/metro/go/src/github.com/golubaca/carinago/module.go:7
main._cgoexpwrap_53c1c00d0ad3_Example(0xa, 0x7fff33a2eac0, 0x7fff33a2ea70, 0x722a921de6cae100)
        _cgo_gotypes.go:47 +0x1c
Aborted (core dumped)

I get that C array is different from Go, but can’t find any tutorial how to access it’s values without panic.

Answer

This is the idiomatic, efficient Go solution (avoid reflection).

module.go:

package main

import "C"

import "unsafe"

//export Example
func Example(cArray *C.int, cSize C.int, i C.int) C.int {
    gSlice := (*[1 << 30]C.int)(unsafe.Pointer(cArray))[:cSize:cSize]
    return gSlice[i]
}

func main() {}

test.py:

from ctypes import *

# Load compiled go module
lib = cdll.LoadLibrary("./gomodule.so")
# We are passing an array of 256 elements and receiving an integer
lib.Example.argtypes = [c_int * 256]
lib.Example.restype = c_int
pyarr = [x for x in range(256)]
# Make C array from py array
arr = (c_int * len(pyarr))(*pyarr)
print(lib.Example(arr, len(arr), 4))

Output:

$ go build -buildmode=c-shared -o gomodule.so module.go
$ python test.py
4
$