[TOC] ``` var ( sema = make(chan struct{}, 1) // a binary semaphore guarding balance balance int ) func Deposit(amount int) { sema <- struct{}{} // acquire token balance = balance + amount <-sema // release token } func Balance() int { sema <- struct{}{} // acquire token b := balance <-sema // release token return b } ``` 等价 ``` import "sync" var ( mu sync.Mutex // guards balance balance int ) func Deposit(amount int) { mu.Lock() balance = balance + amount mu.Unlock() } func Balance() int { mu.Lock() b := balance mu.Unlock() return b } ```