Thanks. Changing the error to a string worked.
I'm trying to see how can I quickly reuse Go code from Python without too
much wrapping. Seems like I can get get away with it :)
On Thursday, March 9, 2017 at 9:45:30 PM UTC+2, Ian Lance Taylor wrote:
>
> On Thu, Mar 9, 2017 at 7:00 AM, Miki Tebeka <miki....@gmail.com
> <javascript:>> wrote:
> > Hi There,
> >
> > (Originally posted on StackOverflow, 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.
>
> I don't know if this is your problem, but a Go float64 corresponds to
> a C double, not a C float.
>
> You might get more information if you set GOTRACEBACK=system in the
> environment when running the program.
>
> In general you can not return an error from an exported function,
> because an error is an interface value, and in general an interface
> contains a Go pointer, and an exported Go function is not permitted to
> return a Go pointer
> (https://golang.org/cmd/cgo/#hdr-Passing_pointers). Your specific
> program may work because the Go pointer may point to a constant string
> that is statically allocated rather than in heap memory. I'm not
> sure. You might try running your program with GODEBUG=cgocheck=2 to
> see if that picks anything up.
>
> You can't expect Python to do anything useful with a Go error value,
> so this seems like an odd thing to try to get working.
>
> Ian
>
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