Print to Log using Go Language Simple HTTP Server

I am trying to log the IP address of the requestor, what METHOD they are using and what file they are requesting. But for some reason it only outputs on the terminal and doesn't save it to logfile.txt...

package main

import (
  "fmt"
  "net/http"
  "log"
  "encoding/json"
  "io/ioutil"
)

type Options struct {
  Path string
  Port string
}

func Log(handler http.Handler) http.Handler {
  return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
    fmt.Printf("%s %s %s\n", r.RemoteAddr, r.Method, r.URL)
    handler.ServeHTTP(w, r)
  })
}

func main() {

  op := &Options{Path: "./", Port: "8001"}

  data, _ := ioutil.ReadFile("./config.json")

  json.Unmarshal(data, op)

  http.Handle("/", http.FileServer(http.Dir(op.Path)))
  err := http.ListenAndServe(":" + op.Port, Log(http.DefaultServeMux))
  if err != nil {
    log.Fatal("ListenAndServe: ", err)
  }
}

In your Log function, you are using fmt.Printf not fmt.Fprintf.

For example,

package main

import (
	"fmt"
	"log"
	"net/http"
	"os"
)

var logFile *os.File

func Log(handler http.Handler) http.Handler {
	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		fmt.Fprintf(logFile, "%s %s %s\n", r.RemoteAddr, r.Method, r.URL)
		handler.ServeHTTP(w, r)
	})
}

func main() {
	var err error
	logFile, err = os.Create("logfile.txt")
	if err != nil {
		log.Fatal("Log file create:", err)
		return
	}
	defer logFile.Close()
}