Problem

If you need to read from and write to a map from concurrently executing goroutines, the accesses must be mediated by some kind of synchronization mechanism. One common way to protect maps is with sync.RWMutex.

package main
func main() {
	m := make(map[int]int)
	go func() {
		for {
			_ = m[1]
		}
	}()
	go func() {
		for {
			m[2] = 2
		}
	}()
	select {}

}

错误信息是: fatal error: concurrent map read and map write。

Solution

Solution 1

This statement declares a counter variable that is an anonymous struct containing a map and an embedded sync.RWMutex.

var counter = struct{
    sync.RWMutex
    m map[string]int
}{m: make(map[string]int)}

To read from the counter, take the read lock:

counter.RLock()
n := counter.m["some_key"]
counter.RUnlock()
fmt.Println("some_key:", n)

To write to the counter, take the write lock:

counter.Lock()
counter.m["some_key"]++
counter.Unlock()

Solution 2

sync.map 对 map 的读写，不需要加锁。取而代之，它通过空间换时间的方式，使用 read 和 dirty 两个 map 来进行读写分离，降低锁时间来提高效率。

package main
import (
    "sync"
    "fmt"
)

func main() {
    //开箱即用
    var sm sync.Map
    //store 方法,添加元素
    sm.Store(1,"a")
    //Load 方法，获得value
    if v,ok:=sm.Load(1);ok{
        fmt.Println(v) // print a
    }
    //LoadOrStore方法，获取或者保存
    //参数是一对key：value，如果该key存在且没有被标记删除则返回原先的value（不更新）和true；不存在则store，返回该value 和false
    if vv,ok:=sm.LoadOrStore(1,"c");ok{
        fmt.Println(vv) // print a
    }
    if vv,ok:=sm.LoadOrStore(2,"c");!ok{
        fmt.Println(vv) // print c
    }
    //遍历该map，参数是个函数，该函数参的两个参数是遍历获得的key和value，返回一个bool值，当返回false时，遍历立刻结束。
    sm.Range(func(k,v interface{})bool{
        fmt.Print(k)
        fmt.Print(":")
        fmt.Print(v)
        fmt.Println()
        return true
    })
}

Reference

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【Golang】map 并发问题