官方的faq里有说明，考虑到有性能损失，map没有设计成原子操作，在并发读写时会有问题。
Map access is unsafe only when updates are occurring. As long as all goroutines are only reading—looking up elements in the map, including iterating through it using a for range loop—and not changing the map by assigning to elements or doing deletions, it is safe for them to access the map concurrently without synchronization.
查看源码，进一步立即实现机制
const (
...
hashWriting = 4 // a goroutine is writing to the map
...
)
type hmap struct
...
flags uint8
...
map是检查是否有另外线程修改h.flag来判断，是否有并发问题。
// 在更新map的函数里检查并发写
if h.flags&hashWriting == 0
throw("concurrent map writes")
// 在读map的函数里检查是否有并发写
if h.flags&hashWriting != 0
throw("concurrent map read and map write")
测试并发问题的例子：一个goroutine不停地写，另一个goroutine不停地读
package main
import (
"fmt"
"time"
)
func main()
c := make(map[string]int)
go func() //开一个goroutine写map
for j := 0; j < 1000000; j++
c[fmt.Sprintf("%d", j)] = j
()
go func() //开一个goroutine读map
for j := 0; j < 1000000; j++
fmt.Println(c[fmt.Sprintf("%d", j)])
()
time.Sleep(time.Second * 20)
立马产生错误
0
fatal error: concurrent map read and map write
goroutine 19 [running]:
runtime.throw(0x10d6ea4, 0x21)
/usr/local/go/src/runtime/panic.go:774 +0x72 fp=0xc00009aef0 sp=0xc00009aec0 pc=0x10299c2
runtime.mapaccess1_faststr(0x10b41e0, 0xc000066180, 0x116ae71, 0x1, 0x1)
/usr/local/go/src/runtime/map_faststr.go:21 +0x44f fp=0xc00009af60 sp=0xc00009aef0 pc=0x100ffff
main.main.func2(0xc000066180)
加sync.RWMutex来保护map
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()
针对上面有并发问题的测试例子，可以修改成以下代码：
package main
import (
"fmt"
"sync"
"time"
)
func main()
var c = struct
sync.RWMutex
m map[string]int
m: make(map[string]int)
go func() //开一个goroutine写map
for j := 0; j < 1000000; j++
c.Lock()
c.m[fmt.Sprintf("%d", j)] = j
c.Unlock()
()
go func() //开一个goroutine读map
for j := 0; j < 1000000; j++
c.RLock()
fmt.Println(c.m[fmt.Sprintf("%d", j)])
c.RUnlock()
()
time.Sleep(time.Second * 20)
第三方包的实现都大同小异，基本上都是使用分离锁来实现并发安全的，具体分离锁来实现并发安全的原理可参考下面的延伸阅读
concurrent-map
m:= cmap.New()
//写
m.Set("foo", "hello world")
m.Set("slice", []int1, 2, 3, 4, 5, 6, 7)
m.Set("int", 1)
//读
m.Get("foo")
m.Get("slice")
m.Get("int")
go-concurrentMap
m := concurrent.NewConcurrentMap()
m.Put("foo", "hello world")
m.Put("slice", []int1, 2, 3, 4, 5, 6, 7)
m.Put("int", 1)
//读
m.Get("foo")
m.Get("slice")
m.Get("int")
sync.Map 是官方出品的并发安全的 map，他在内部使用了大量的原子操作来存取键和值，并使用了 read 和 dirty 二个原生 map 作为存储介质，具体实现流程可阅读相关源码。
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