Golang 1.15.7: Return of a Shared library using *C.char

Question

I’m trying to create a Shared library in go, but the return (*C.char) always comes back empty:

How to do to return right?

package main
import (
    "C"
    "fmt"
)
//export Upper
func Upper(palavra *C.char, retorno *C.char, size *int) int {
    fmt.Println(C.GoString(palavra))
    retorno = C.CString(strings.ToUpper(str1))
    *size = int(len(palavra))
    return 0
}
func main() {}