Go并发之atomic

1. atomic介绍：

sync/atomic包提供了原子操作的能力，直接有底层CPU硬件支持，因而一般要比基于操作系统API的锁方式效率高些；这些功能需要非常小心才能正确使用。 除特殊的底层应用程序外，同步更适合使用channel或sync包的功能。 通过消息共享内存; 不要通过共享内存进行通信。

英文介绍如下：

(Package atomic provides low-level atomic memory primitives useful for implementing synchronization algorithms.These functions require great care to be used correctly. Except for special, low-level applications, synchronization is better done with channels or the facilities of the sync package. Share memory by communicating; don't communicate by sharing memory.)

链接：https://golang.org/pkg/sync/atomic/

典型使用场景：

sync/once, 实现源码如下所示：

2. atomic的API介绍

2.1 对于自增和自减的操作，对应的API

• func AddInt32(addr *int32, delta int32) (new int32)
• func AddInt64(addr *int64, delta int64) (new int64)
• func AddUint32(addr *uint32, delta uint32) (new uint32)
• func AddUint64(addr *uint64, delta uint64) (new uint64)
• func AddUintptr(addr *uintptr, delta uintptr) (new uintptr)

伪代码实现如下所示：

*addr += delta
return *addr

2.2对于比较和交换的操作，对应的API

• func CompareAndSwapInt32(addr *int32, old, new int32) (swapped bool)
• func CompareAndSwapInt64(addr *int64, old, new int64) (swapped bool)
• func CompareAndSwapPointer(addr *unsafe.Pointer, old, new unsafe.Pointer) (swapped bool)
• func CompareAndSwapUint32(addr *uint32, old, new uint32) (swapped bool)
• func CompareAndSwapUint64(addr *uint64, old, new uint64) (swapped bool)
• func CompareAndSwapUintptr(addr *uintptr, old, new uintptr) (swapped bool)

伪代码实现如下所示：

if *addr == old {
  *addr = new
  return true
}
return false

2.3 载入操作的API

• func LoadInt32(addr *int32) (val int32)
• func LoadInt64(addr *int64) (val int64)
• func LoadPointer(addr *unsafe.Pointer) (val unsafe.Pointer)
• func LoadUint32(addr *uint32) (val uint32)
• func LoadUint64(addr *uint64) (val uint64)
• func LoadUintptr(addr *uintptr) (val uintptr)

伪代码实现：

 return *addr

2.4存储操作的API：

• func StoreInt32(addr *int32, val int32)
• func StoreInt64(addr *int64, val int64)
• func StorePointer(addr *unsafe.Pointer, val unsafe.Pointer)
• func StoreUint32(addr *uint32, val uint32)
• func StoreUint64(addr *uint64, val uint64)
• func StoreUintptr(addr *uintptr, val uintptr)

伪代码实现：

 *addr = val

2.5 对于交换操作，对应的API：

• func SwapInt32(addr *int32, new int32) (old int32)
• func SwapInt64(addr *int64, new int64) (old int64)
• func SwapPointer(addr *unsafe.Pointer, new unsafe.Pointer) (old unsafe.Pointer)
• func SwapUint32(addr *uint32, new uint32) (old uint32)
• func SwapUint64(addr *uint64, new uint64) (old uint64)
• func SwapUintptr(addr *uintptr, new uintptr) (old uintptr)

伪代码实现：

old = *addr
*addr = new
return old

3.例子

使用atomic的例子：

package main

import (
  "fmt"
  "sync/atomic"
  "sync"
)

func main() {
        var count int32
  fmt.Println("main start...")
  var wg sync.WaitGroup
  for i:=0;i<5;i++ {
    wg.Add(1)
    go func( i int){
      defer wg.Done()
      fmt.Println("goroutine:",i,"start...")
      atomic.AddInt32(&count,1)
    
      fmt.Println("goroutine:",i,"count:",count,"end...")
    }(i)
  }
  wg.Wait()
  fmt.Println("main end...")
}

输出结果：

main start...
goroutine: 4 start...
goroutine: 2 start...
goroutine: 2 count: 2 end...
goroutine: 1 start...
goroutine: 1 count: 3 end...
goroutine: 3 start...
goroutine: 3 count: 4 end...
goroutine: 4 count: 1 end...
goroutine: 0 start...
goroutine: 0 count: 5 end...
main end...

结果分析：

通过执行代码的结果，可以看出来，atomic的AddInt32函数操作是原子性的，在一个goroutine没有执行结束的时候，其他的goroutine在执行AddInt32的时候是被lock住的。

参考文档：

https://golang.org/pkg/sync/atomic/