add template functions

printer.go

@@ -17,6 +17,7 @@ import (
 
 	"github.com/kenshaw/escpos"
 )
+import "errors"
 
 //export PrintImg
 func PrintImg(printer *C.char, imagePath *C.char) *C.char {
@@ -77,6 +78,8 @@ func PrintImg(printer *C.char, imagePath *C.char) *C.char {
 		// 	reader := bufio.NewReader(rw)
 		// 	writer := bufio.NewWriter(rw)
 		// 	w = bufio.NewReadWriter(reader, writer)
+	} else {
+		return NewErr(errors.New("invalid printer"))
 	}
 	prt := escpos.New(w)