此mmap系统调用有什么问题？

Question

This is an attempt to mmap a file and write a single byte:

package main

import (
  "fmt"
    "os"
    "syscall"
)

func main() {
    file, _ := os.Open("/tmp/data")
    mmap, _ := syscall.Mmap(int(file.Fd()), 0, 100, syscall.PROT_READ|syscall.PROT_WRITE, syscall.MAP_SHARED)
    fmt.Printf("cap is %d", cap(mmap))
    mmap[0] = 0
    syscall.Munmap(mmap)
}

Despite length is set to 100, mmap capacity is always 0. What went wrong in the system call?

Answer 1

Always check for error!

os.Open opens a file for reading only, however mmap call asks to map the file read/write, thus giving a permission denied error, and as a result mapped region size is 0.