update book code

eBook/examples/chapter_12/filecopy.go

@@ -0,0 +1,29 @@
+// filecopy.go
+package main
+
+import (
+	"fmt"
+	"io"
+	"os"
+)
+
+func main() {
+	CopyFile("target.txt", "source.txt")
+	fmt.Println("Copy done!")
+}
+
+func CopyFile(dstName, srcName string) (written int64, err error) {
+	src, err := os.Open(srcName)
+	if err != nil {
+		return
+	}
+	defer src.Close()
+
+	dst, err := os.OpenFile(dstName, os.O_WRONLY|os.O_CREATE, 0644)
+	if err != nil {
+		return
+	}
+	defer dst.Close()
+
+	return io.Copy(dst, src)
+}