Re: [go-nuts] plugin - How to open and lookup for interface implementation

[Ian Lance Taylor]

[ +crawshaw ]

On Thu, Nov 3, 2016 at 12:51 AM, Mateusz Dymiński <dymin...@gmail.com> wrote:
> Hi,
>
> I am trying to load dynamically implementation of particular interface. I've
> created the example in following repo:
> https://github.com/mateuszdyminski/go-plugin
>
> I have interface - let's call it Printer:
> package printer
>
> type Printer interface {
> Print(text string)
> }
>
>
> And implementation of this interface:
> package main
>
> import (
> "fmt"
>
> "github.com/mateuszdyminski/go-plugin/printer"
> )
>
> func main() {
> Impl.Print("test")
> }
>
> type PrinterImpl struct{}
>
> func (p PrinterImpl) Print(text string) {
> fmt.Printf("[PrinterImpl] %s\n", text)
> }
>
> var Impl printer.Printer = PrinterImpl{}
>
>
> Then I can build that implementation of Printer as following:
> go build -buildmode=plugin printer.go
>
>
> The last step is to load dynamically created library 'printer.so':
> package main
>
> import (
> "fmt"
> "plugin"
> "reflect"
>
> "github.com/mateuszdyminski/go-plugin/printer"
> )
>
> func main() {
> lib, err := plugin.Open("printer.so")
> if err != nil {
> panic(err)
> }
>
> p, err := lib.Lookup("Impl")
> if err != nil {
> panic(err)
> }
>
> printerImpl, ok := p.(printer.Printer)
> if !ok {
> fmt.Printf("wrong type: %+v \n", reflect.TypeOf(p))
> panic("wrong type")
> }
>
> printerImpl.Print("test")
> }
>
>
> But when I run it with 'go run main.go' I got following error:
> wrong type: *printer.Printer
> panic: wrong type
>
> goroutine 1 [running]:
> panic(0x50fda0, 0xc42000c2d0)
> /home/md/.gvm/gos/master/src/runtime/panic.go:531 +0x1cf
> main.main()
> /home/md/workspace/go/src/github.com/mateuszdyminski/go-plugin/main.go:25
> +0x231
> exit status 2
>
>
> The type of the 'Impl' taken from 'reflect' package is '*printer.Printer'.
> Is it possible to cast it to the printer.Printer and not to the pointer to
> the interface?
>
> Thanks for any help!
>
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