log printf and Write connection - go

Question

I cannot for the life of me from below golang code, why when you call write function at the bottom,
func write(message string) {
log.Printf("%v\n", message)
}
why log.Printf calls below method
func (fl fileLog) Write(data []byte) (int, error ) {
fmt.Println("does this ever get called?")
f, err := os.OpenFile(string(fl), os.O_CREATE|os.O_WRONLY|os.O_APPEND, 0600)
if err != nil {
return 0, err
}
defer f.Close()
return f.Write(data)
}
Logger's printf definition is below
func (*Logger) Printf ¶
func (l *Logger) Printf(format string, v ...interface{})
-- Full Code below, someone please explain to me why this is so please--
package main
import (
"fmt"
stlog "log"
"os"
)
var log *stlog.Logger
type fileLog string
func (fl fileLog) Write(data []byte) (int, error ) {
fmt.Println("does this ever get called?")
f, err := os.OpenFile(string(fl), os.O_CREATE|os.O_WRONLY|os.O_APPEND, 0600)
if err != nil {
return 0, err
}
defer f.Close()
return f.Write(data)
}
func registerHandlers() {
var msgRaw = "how are you"
write(msgRaw)
}
func run(destination string) {
log = stlog.New(fileLog(destination), "", stlog.LstdFlags)
}
func main() {
fmt.Println("here we go")
run("../test.log")
registerHandlers()
}
func write(message string) {
log.Printf("%v\n", message)
}

Answer 1

The logger writes the log messages to an io.Writer, which is an interface defined as:
type Writer interface {
Write([]byte) (int,error)
}
The fileLog type implements the io.Writer interface, and you set it as the output of the new logger. So whenever the logger tries to write a log, it calls the Write function of its writer, which is fileLog.Write.