fatal error: concurrent map iteration and map write; fatal error: concurrent map writes

iter_read_write_map.go

package main

import (
	//	"fmt"
	"time"
)

func main() {
	m := map[int]int{}
	go func() {
		for i := 0; i < 1000000; i++ {
			m[i] = i
		}
	}()
	for {
		for i := 0; i < 1000000; i++ {
			//for k, _ := range m {
			//	k = k * 2
			//}
			k := len(m)
			k = k * 2
		}
	}
	for {
		time.Sleep(time.Second / 100)
	}
}