原文链接： https://mp.weixin.qq.com/s/RvFWCMX8P3VxjJrZRJZzMQ
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
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func AddInt32(addr *int32, delta int32) (new int32)
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func AddInt64(addr *int64, delta int64) (new int64)
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func AddUint32(addr *uint32, delta uint32) (new uint32)
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func AddUint64(addr *uint64, delta uint64) (new uint64)
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func AddUintptr(addr *uintptr, delta uintptr) (new uintptr)
伪代码实现如下所示：
*addr += deltareturn *addr
2.2对于比较和交换的操作，对应的API
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func CompareAndSwapInt32(addr *int32, old, new int32) (swapped bool)
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func CompareAndSwapInt64(addr *int64, old, new int64) (swapped bool)
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func CompareAndSwapPointer(addr *unsafe.Pointer, old, new unsafe.Pointer) (swapped bool)
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func CompareAndSwapUint32(addr *uint32, old, new uint32) (swapped bool)
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func CompareAndSwapUint64(addr *uint64, old, new uint64) (swapped bool)
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func CompareAndSwapUintptr(addr *uintptr, old, new uintptr) (swapped bool)
伪代码实现如下所示：
if *addr == old {*addr = newreturn true}return false
2.3 载入操作的API
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func LoadInt32(addr *int32) (val int32)
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func LoadInt64(addr *int64) (val int64)
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func LoadPointer(addr *unsafe.Pointer) (val unsafe.Pointer)
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func LoadUint32(addr *uint32) (val uint32)
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func LoadUint64(addr *uint64) (val uint64)
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func LoadUintptr(addr *uintptr) (val uintptr)
伪代码实现：
return *addr
2.4存储操作的API：
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func StoreInt32(addr *int32, val int32)
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func StoreInt64(addr *int64, val int64)
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func StorePointer(addr *unsafe.Pointer, val unsafe.Pointer)
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func StoreUint32(addr *uint32, val uint32)
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func StoreUint64(addr *uint64, val uint64)
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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)
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func SwapInt64(addr *int64, new int64) (old int64)
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func SwapPointer(addr *unsafe.Pointer, new unsafe.Pointer) (old unsafe.Pointer)
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func SwapUint32(addr *uint32, new uint32) (old uint32)
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func SwapUint64(addr *uint64, new uint64) (old uint64)
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func SwapUintptr(addr *uintptr, new uintptr) (old uintptr)
伪代码实现：
old = *addr*addr = newreturn old
3.例子
使用atomic的例子：
package mainimport ("fmt""sync/atomic""sync")func main() {var count int32fmt.Println("main start...")var wg sync.WaitGroupfor 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/
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