type hchan struct {
   qcount   uint           // total data in the queue
   dataqsiz uint           // size of the circular queue
   buf      unsafe.Pointer // points to an array of dataqsiz elements
   elemsize uint16
   closed   uint32
   elemtype *_type // element type
   sendx    uint   // send index
   recvx    uint   // receive index
   recvq    waitq  // list of recv waiters
   sendq    waitq  // list of send waiters
   // lock protects all fields in hchan, as well as several
   // fields in sudogs blocked on this channel.
   //
   // Do not change another G's status while holding this lock
   // (in particular, do not ready a G), as this can deadlock
   // with stack shrinking.
   lock mutex
}

// sudog represents a g in a wait list, such as for sending/receiving
// on a channel.
type sudog struct {
   // The following fields are protected by the hchan.lock of the
   // channel this sudog is blocking on. shrinkstack depends on
   // this for sudogs involved in channel ops.
   g *g
   next *sudog
   prev *sudog
   elem unsafe.Pointer // data element (may point to stack)
   // The following fields are never accessed concurrently.
   // For channels, waitlink is only accessed by g.
   // For semaphores, all fields (including the ones above)
   // are only accessed when holding a semaRoot lock.
   acquiretime int64
   releasetime int64
   ticket      uint32
   // isSelect indicates g is participating in a select, so
   // g.selectDone must be CAS'd to win the wake-up race.
   isSelect bool
   // success indicates whether communication over channel c
   // succeeded. It is true if the goroutine was awoken because a
   // value was delivered over channel c, and false if awoken
   // because c was closed.
   success bool
   parent   *sudog // semaRoot binary tree
   waitlink *sudog // g.waiting list or semaRoot
   waittail *sudog // semaRoot
   c        *hchan // channel
}

func makechan(t *chantype, size int) *hchan {
   elem := t.elem
   // compiler checks this but be safe.
   if elem.size >= 1<<16 {
      throw("makechan: invalid channel element type")
   }
   if hchanSize%maxAlign != 0 || elem.align > maxAlign {
      throw("makechan: bad alignment")
   }
   mem, overflow := math.MulUintptr(elem.size, uintptr(size))
   if overflow || mem > maxAlloc-hchanSize || size < 0 {
      panic(plainError("makechan: size out of range"))
   }
   // Hchan does not contain pointers interesting for GC when elements stored in buf do not contain pointers.
   // buf points into the same allocation, elemtype is persistent.
   // SudoG's are referenced from their owning thread so they can't be collected.
   // TODO(dvyukov,rlh): Rethink when collector can move allocated objects.
   var c *hchan
   switch {
   case mem == 0:
      // Queue or element size is zero.
      c = (*hchan)(mallocgc(hchanSize, nil, true))
      // Race detector uses this location for synchronization.
      c.buf = c.raceaddr()
   case elem.ptrdata == 0:
      // Elements do not contain pointers.
      // Allocate hchan and buf in one call.
      c = (*hchan)(mallocgc(hchanSize+mem, nil, true))
      c.buf = add(unsafe.Pointer(c), hchanSize)
   default:
      // Elements contain pointers.
      c = new(hchan)
      c.buf = mallocgc(mem, elem, true)
   }
   c.elemsize = uint16(elem.size)
   c.elemtype = elem
   c.dataqsiz = uint(size)
   lockInit(&c.lock, lockRankHchan)
   if debugChan {
      print("makechan: chan=", c, "; elemsize=", elem.size, "; dataqsiz=", size, "\n")
   }
   return c
}

// 1
func makechan(t *chantype, size int) *hchan
// 2
mem, overflow := math.MulUintptr(elem.size, uintptr(size))
// 3
var c *hchan
   switch {
   case mem == 0:
      // Queue or element size is zero.
      c = (*hchan)(mallocgc(hchanSize, nil, true))
      // Race detector uses this location for synchronization.
      c.buf = c.raceaddr()
   case elem.ptrdata == 0:
      // Elements do not contain pointers.
      // Allocate hchan and buf in one call.
      c = (*hchan)(mallocgc(hchanSize+mem, nil, true))
      c.buf = add(unsafe.Pointer(c), hchanSize)
   default:
      // Elements contain pointers.
      c = new(hchan)
      c.buf = mallocgc(mem, elem, true)
   }

package main
import (
   "log"
   "time"
)
func main() {
   // 简单设置log参数
   log.SetFlags(log.Lshortfile | log.LstdFlags)
   // 创建 2 个通道，元素数据类型为 int，缓冲区大小为 5
   var ch1 = make(chan int, 5)
   var ch2 = make(chan int, 5)
   // 分别向通道中各自写入数据，咱默认写1吧
   // 直接写一个匿名函数 向通道中添加数据
   go func (){
      var num = 1
      for {
         ch1 <- num
         num += 1
         time.Sleep(1 * time.Second)
      }
   }()
   go func (){
      var num = 1
      for {
         ch2 <- num
         num += 1
         time.Sleep(1 * time.Second)
      }
   }()
   for {
      select {// 读取数据
      case num := <-ch1:
         log.Printf("read ch1 data is  %d\n", num)
      case num := <-ch2:
         log.Printf("read ch2 data is: %d\n", num)
      default:
         log.Printf("ch1 and ch2 is empty\n")
          // 休息 1s 再读
         time.Sleep(1 * time.Second)
      }
   }
}

2021/06/18 17:43:06 main.go:54: ch1 and ch2 is empty
2021/06/18 17:43:07 main.go:48: read ch1 data is  1
2021/06/18 17:43:07 main.go:48: read ch1 data is  2
2021/06/18 17:43:07 main.go:51: read ch2 data is: 1
2021/06/18 17:43:07 main.go:51: read ch2 data is: 2
2021/06/18 17:43:07 main.go:54: ch1 and ch2 is empty
2021/06/18 17:43:08 main.go:48: read ch1 data is  3
2021/06/18 17:43:08 main.go:51: read ch2 data is: 3
2021/06/18 17:43:08 main.go:54: ch1 and ch2 is empty
2021/06/18 17:43:09 main.go:48: read ch1 data is  4
2021/06/18 17:43:09 main.go:51: read ch2 data is: 4
2021/06/18 17:43:09 main.go:54: ch1 and ch2 is empty
2021/06/18 17:43:10 main.go:51: read ch2 data is: 5
2021/06/18 17:43:10 main.go:48: read ch1 data is  5