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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.

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
$ 

An array in C cannot be automatically casted to a slice in Go. Note that in Go, a slice has two parts: a length, and a pointer to the backing data.

So you would have to manually create the slice from the pointer.

package main

import (
 "C"
 "reflect"
 "unsafe"
)

//export Example
func Example(carr *C.int, size int, idx int) C.int 

 // Build the slice manually using unsafe
 var slice C.int
 header := (*reflect.SliceHeader)(unsafe.Pointer(&slice))
 header.Cap = size
 header.Len = size
 header.Data = uintptr(unsafe.Pointer(carr))

 return slice[idx]


func main() 

Then you would call the Go exported function in your python code like:

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, len(arr), 4)

The example above should print the 4th index element of the array