ch := make(chan int)

ch := make(chan int)

// write to channel
ch - x

ch := make(chan int)

// read from channel
x - ch

// another way to read
x = - ch

ch := make(chan int)

close(ch)

v, ok := -ch

ch := make(chan int)    //无缓冲的channel
ch := make(chan int, 3) //带缓冲的channel

func chansend(c *hchan, ep unsafe.Pointer, block bool, callerpc uintptr) bool {
 if c == nil {
  if !block {
   return false
  }
  gopark(nil, nil, waitReasonChanSendNilChan, traceEvGoStop, 2)
  throw("unreachable")
 }

 if debugChan {
  print("chansend: chan=", c, "\n")
 }

 if raceenabled {
  racereadpc(c.raceaddr(), callerpc, funcPC(chansend))
 }

 // Fast path: check for failed non-blocking operation without acquiring the lock.
 //
 // After observing that the channel is not closed, we observe that the channel is
 // not ready for sending. Each of these observations is a single word-sized read
 // (first c.closed and second c.recvq.first or c.qcount depending on kind of channel).
 // Because a closed channel cannot transition from 'ready for sending' to
 // 'not ready for sending', even if the channel is closed between the two observations,
 // they imply a moment between the two when the channel was both not yet closed
 // and not ready for sending. We behave as if we observed the channel at that moment,
 // and report that the send cannot proceed.
 //
 // It is okay if the reads are reordered here: if we observe that the channel is not
 // ready for sending and then observe that it is not closed, that implies that the
 // channel wasn't closed during the first observation.
 if !block  c.closed == 0  ((c.dataqsiz == 0  c.recvq.first == nil) ||
  (c.dataqsiz > 0  c.qcount == c.dataqsiz)) {
  return false
 }

 var t0 int64
 if blockprofilerate > 0 {
  t0 = cputicks()
 }

 lock(c.lock)

 if c.closed != 0 {
  unlock(c.lock)
  panic(plainError("send on closed channel"))
 }

 if sg := c.recvq.dequeue(); sg != nil {
  // Found a waiting receiver. We pass the value we want to send
  // directly to the receiver, bypassing the channel buffer (if any).
  send(c, sg, ep, func() { unlock(c.lock) }, 3)
  return true
 }

 if c.qcount  c.dataqsiz {
  // Space is available in the channel buffer. Enqueue the element to send.
  qp := chanbuf(c, c.sendx)
  if raceenabled {
   raceacquire(qp)
   racerelease(qp)
  }
  typedmemmove(c.elemtype, qp, ep)
  c.sendx++
  if c.sendx == c.dataqsiz {
   c.sendx = 0
  }
  c.qcount++
  unlock(c.lock)
  return true
 }

 if !block {
  unlock(c.lock)
  return false
 }

 // Block on the channel. Some receiver will complete our operation for us.
 gp := getg()
 mysg := acquireSudog()
 mysg.releasetime = 0
 if t0 != 0 {
  mysg.releasetime = -1
 }
 // No stack splits between assigning elem and enqueuing mysg
 // on gp.waiting where copystack can find it.
 mysg.elem = ep
 mysg.waitlink = nil
 mysg.g = gp
 mysg.isSelect = false
 mysg.c = c
 gp.waiting = mysg
 gp.param = nil
 c.sendq.enqueue(mysg)
 gopark(chanparkcommit, unsafe.Pointer(c.lock), waitReasonChanSend, traceEvGoBlockSend, 2)
 // Ensure the value being sent is kept alive until the
 // receiver copies it out. The sudog has a pointer to the
 // stack object, but sudogs aren't considered as roots of the
 // stack tracer.
 KeepAlive(ep)

 // someone woke us up.
 if mysg != gp.waiting {
  throw("G waiting list is corrupted")
 }
 gp.waiting = nil
 gp.activeStackChans = false
 if gp.param == nil {
  if c.closed == 0 {
   throw("chansend: spurious wakeup")
  }
  panic(plainError("send on closed channel"))
 }
 gp.param = nil
 if mysg.releasetime > 0 {
  blockevent(mysg.releasetime-t0, 2)
 }
 mysg.c = nil
 releaseSudog(mysg)
 return true
}