package main

import (
	"fmt"
	"sync"
)

func main() {
	var wg sync.WaitGroup
	for i := 0; i < 5; i++ {
		wg.Add(1) //添加请求goroutine的数量，add(n)，counter+=n s
		go func() {
      //每执行完一个goroutine，counter -= 1。
      //当计数器counter为0时通过信号量调用runtime_Semrelease唤醒waiter线程
			defer wg.Done() 
			fmt.Println("hello")
		}()
	}
	wg.Wait() //waiter++，通过信号量runtime_Semacquire(semap)阻塞当前goroutine
}

type Mutex struct {
	state int32 //存储互斥锁的状态，state = 0即是未加锁
	sema  uint32 //信号量，主要用作等待队列
}

type RWMutex struct {
	w           Mutex  // held if there are pending writers 复用互斥锁
	writerSem   uint32 // semaphore for writers to wait for completing readers  写操作的阻塞和唤醒的信号量
	readerSem   uint32 // semaphore for readers to wait for completing writers  读操作的阻塞和唤醒的信号量
	readerCount int32  // number of pending readers 正在执行读的goroutine数量
	readerWait  int32  // number of departing readers 被阻塞的准备读的goroutine（groutine获取了写锁，若后面有goroutine要读，那么这些goroutine会被阻塞）
}

package main

import (
	"fmt"
	"sync"
)

type mydb struct {
	name string
}

//需要声明在外面
var (
	once     sync.Once
	instance *mydb
)

func (s *mydb) getinstance() *mydb {
	once.Do(func() {
		instance = &mydb{ // 实例化，返回mydb结构体的引用
			name: "mysql",
		}
	})
	return instance
}

func main() {
	var s *mydb
	ans1 := s.getinstance()
	ans2 := s.getinstance()

	fmt.Printf("s1: %p\n", ans1) //s1: 0x1400008e210
	fmt.Printf("s2: %p\n", ans2) //s2: 0x1400008e210
}