channel的实现是在在runtime包下面，路径为：./src/runtime/chan.go 文件中，其中主要的结构体为：

const (
  maxAlign  = 8
  hchanSize = unsafe.Sizeof(hchan{}) + uintptr(-int(unsafe.Sizeof(hchan{}))&(maxAlign-1))
  debugChan = false
)

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
}

type waitq struct {
  first *sudog
  last  *sudog
}

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

  // isSelect indicates g is participating in a select, so
  // g.selectDone must be CAS'd to win the wake-up race.
  isSelect bool
  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
  parent      *sudog // semaRoot binary tree
  waitlink    *sudog // g.waiting list or semaRoot
  waittail    *sudog // semaRoot
  c           *hchan // channel
}

channel其实就是由一个环形数组实现的队列，用于存储消息元素；两个链表实现的 goroutine 等待队列，用于存储阻塞在 recv 和 send 操作上的 goroutine；一个互斥锁，用于各个属性变动的同步，只不过这个锁是一个轻量级锁。其中 recvq 是读操作阻塞在 channel 的 goroutine 列表，sendq 是写操作阻塞在 channel 的 goroutine 列表。列表的实现是 sudog，其实就是一个对 g 的结构的封装。
和select类似，hchan其实只是channel的头部。头部后面的一段内存连续的数组将作为channel的缓冲区，即用于存放channel数据的环形队列。qcount 和 dataqsiz 分别描述了缓冲区当前使用量【len】和容量【cap】。若channel是无缓冲的，则size是0，就没有这个环形队列了。