[go-nuts] “invalid memory address” when calling Go from Python/C

[Miki Tebeka]

Hi There,

(Originally posted on StackOverflow 
<http://stackoverflow.com/questions/42689133/invalid-memory-address-when-calling-go-from-python>,
 
trying my luck here as well).

I'm trying to extend the code in Calling Go Functions from Other Languages. 
I'd like see how a function returning an error can be handled. See the code 
below, I get "panic: runtime error: invalid memory address or nil pointer 
dereference" when running the Python code. Any ideas on how to fix this?

gmath.go

package main


import (
    "fmt"
)


import "C"


//export Div
func Div(x, y float64) (float64, error) {
    fmt.Printf("Div called: x=%f, y=%f\n", x, y)
    if y == 0 {
        return 0, fmt.Errorf("ZeroDivisionError")
    }
    return x / y, nil
}


func main() {}


gmath.py

import ctypes




class GoInterface(ctypes.Structure):
    _fields_ = [
        ('t', ctypes.c_void_p),
        ('v', ctypes.c_void_p),
    ]




class DivRet(ctypes.Structure):
    _fields_ = [
        ('result', ctypes.c_float),
        ('error', GoInterface),
    ]




lib = ctypes.cdll.LoadLibrary('./gmath.so')
lib.Div.argtypes = [ctypes.c_double, ctypes.c_double]
lib.Div.restype = DivRet


r = lib.Div(1.2, 2.3)

Example run

$ make
go build -o gmath.so -buildmode=c-shared gmath.go
$ python gmath.py
Div called: x=1.200000, y=2.300000
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x17 pc=0x7fbe5fda928d
]


goroutine 17 [running, locked to thread]:
main._cgoexpwrap_448ea9090bef_Div.func1(0xc42003cea8)
    command-line-arguments/_obj/_cgo_gotypes.go:46 +0x42
main._cgoexpwrap_448ea9090bef_Div(0x3ff3333333333333, 0x4002666666666666, 
0x3fe0b21642c8590b, 0x0, 0x0)


    command-line-arguments/_obj/_cgo_gotypes.go:48 +0xaa
[1]    29009 abort (core dumped)  python gmath.py

I've tried calling the Go code directly from C - with same results.

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