os: Mkdir can't add write permission to group/other user

[math345]

1. go version go1.5.1 linux/amd64
2. centos 7
3. go run xxx.go : success
4. ls -ll
   drw-r--r-- 2 root root 4096 Apr 7 10:03 testdir

question: Mkdir failed to add write permission to group/other user. Why?

code:

package main

import (
    "fmt"
    "os"
)

func main() {
    savepath := "testdir"
    if err := os.Mkdir(savepath, 0666); err != nil {
        fmt.Println("mkdir(", savepath, ") error:", err)
    } else {
        fmt.Println("mkdir success:", savepath)
    }
}

[bradfitz]

See https://en.wikipedia.org/wiki/Umask