channel实现的源码在官方的runtime/chan.go下面，加注释一共700+行代码，下面来看他的实现 channel 底层结构是
type hchan struct {
qcount uint // channel中元素的个数
dataqsiz uint // channel循环队列的长度 无缓冲队列为0
buf unsafe.Pointer // channel缓冲区数据指针
elemsize uint16 //channel收发的元素的大小
closed uint32 //channel是否关闭的标识
elemtype *_type // channel收发元素的类型
sendx uint // channel发送下一个元素的索引
recvx uint // channel接收下一个元素的索引
recvq waitq // 等待接收数据的goroutinue列表
sendq waitq // 等待发送数据的goroutine列表
// 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//锁
}
Go 语言提供了一种不同的并发模型，即通信顺序进程（Communicating sequential processes，CSP） Goroutine 和 Channel 分别对应 CSP 中的实体和传递信息的媒介，Goroutine 之间会通过 Channel 传递数据
makechan
通常我们会先初始化一个channel，然后往里面塞数据，用另一个协程接收channel中的数据
c := make(chan int,0)
go func(){
c <- 1
}()
fmt.Println(<-c)
make方法会在编译之后转换成 runtime.makechan 或者 runtime.makechan64 的函数 ，根据传入的参数类型和缓冲区大小创建一个新的 Channel 结构，其中后者用于处理缓冲区大小大于 2 的 32 次方的情况，因为这在 Channel 中并不常见，所以我们重点关注 runtime.makechan
func makechan(t *chantype, size int) *hchan {
elem := t.elem
// compiler checks this but be safe.前面都是做一些合法判断
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. 没有缓冲区，只会为 runtime.hchan 分配一段内存空间
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.存储的类型不是指针类型，会为当前的 Channel 和底层的数组分配一块连续的内存空间
c = (*hchan)(mallocgc(hchanSize+mem, nil, true))
c.buf = add(unsafe.Pointer(c), hchanSize)
default:
// Elements contain pointers.单独为 runtime.hchan 和缓冲区分配内存
c = new(hchan)
c.buf = mallocgc(mem, elem, true)
}
c.elemsize = uint16(elem.size)
c.elemtype = elem
c.dataqsiz = uint(size)
if debugChan {
print("makechan: chan=", c, "; elemsize=", elem.size, "; dataqsiz=", size, "\n")
}
return c
}
发送数据
我们在调用时将 block 参数设置成 true，那么表示当前发送操作是阻塞的
func chansend(c *hchan, ep unsafe.Pointer, block bool, callerpc uintptr) bool {
//如果是nil的通道
if c == nil {
//不是阻塞的操作
if !block {
//发送失败
return false
}
//阻塞的操作，返回错误 chan receive (nil chan)
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.
//非阻塞 && 没关闭 && （(非缓冲通道&&没有接收数据协程方）||（有缓冲通道但是已经满员）） 就返回发送失败 关于最后一个条件 有缓冲通道但已经满员，本来可以挂起协程等待空位，但是当前非阻塞不能等的话那就没办法只能返回失败了
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()
}
//数据操作前先加锁
lock(&c.lock)
//通道已经关闭 解锁 抛出panic
if c.closed != 0 {
unlock(&c.lock)
panic(plainError("send on closed channel"))
}
//情况1：如果现在已经有等待数据的协程
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). 直接把数据复制给这个协程，不用费劲写到缓冲队列了
send(c, sg, ep, func() { unlock(&c.lock) }, 3)
return true
}
//情况2：如果没有等待协程，缓冲队列还没满能塞数据
if c.qcount < c.dataqsiz {
// Space is available in the channel buffer. Enqueue the element to send.找到能塞数据的地址
qp := chanbuf(c, c.sendx)
if raceenabled {
raceacquire(qp)
racerelease(qp)
}
//复制数据到缓冲区
typedmemmove(c.elemtype, qp, ep)
//可以发送数据的索引往后挪一位，因为现在的已经有数据了被占用
c.sendx++
//环形列表，满了回0
if c.sendx == c.dataqsiz {
c.sendx = 0
}
//缓冲区元素数量+1
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.情况3：缓冲区已满，这个协程只能放在等待队列
gp := getg()
//把发送数据的协程相关信息封装在sudog结构体中
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)
//把协程挂起
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
}
直接发送给等待的协程
func send(c *hchan, sg *sudog, ep unsafe.Pointer, unlockf func(), skip int) {
if raceenabled {
if c.dataqsiz == 0 {
racesync(c, sg)
} else {
// Pretend we go through the buffer, even though
// we copy directly. Note that we need to increment
// the head/tail locations only when raceenabled.
qp := chanbuf(c, c.recvx)
raceacquire(qp)
racerelease(qp)
raceacquireg(sg.g, qp)
racereleaseg(sg.g, qp)
c.recvx++
if c.recvx == c.dataqsiz {
c.recvx = 0
}
c.sendx = c.recvx // c.sendx = (c.sendx+1) % c.dataqsiz
}
}
if sg.elem != nil {
//步骤一 把数据直接赋值给等待的协程
sendDirect(c.elemtype, sg, ep)
sg.elem = nil
}
gp := sg.g
unlockf()
gp.param = unsafe.Pointer(sg)
if sg.releasetime != 0 {
sg.releasetime = cputicks()
}
//步骤二 将等待接收数据的 Goroutine 标记成可运行状态 Grunnable 并把该 Goroutine 放到发送方所在的处理器的 runnext 上等待执行，该处理器在下一次调度时会立刻唤醒数据的接收方 并不是立即执行
goready(gp, skip+1)
}
接收数据
func chanrecv(c *hchan, ep unsafe.Pointer, block bool) (selected, received bool) {
// raceenabled: don't need to check ep, as it is always on the stack
// or is new memory allocated by reflect.
if debugChan {
print("chanrecv: chan=", c, "\n")
}
//nil的通道接收数据会失败
if c == nil {
if !block {
return
}
gopark(nil, nil, waitReasonChanReceiveNilChan, traceEvGoStop, 2)
throw("unreachable")
}
// Fast path: check for failed non-blocking operation without acquiring the lock.
//
// After observing that the channel is not ready for receiving, we observe that the
// channel is not closed. Each of these observations is a single word-sized read
// (first c.sendq.first or c.qcount, and second c.closed).
// Because a channel cannot be reopened, the later observation of the channel
// being not closed implies that it was also not closed at the moment of the
// first observation. We behave as if we observed the channel at that moment
// and report that the receive cannot proceed.
//
// The order of operations is important here: reversing the operations can lead to
// incorrect behavior when racing with a close.
if !block && (c.dataqsiz == 0 && c.sendq.first == nil ||
c.dataqsiz > 0 && atomic.Loaduint(&c.qcount) == 0) &&
atomic.Load(&c.closed) == 0 {
return
}
var t0 int64
if blockprofilerate > 0 {
t0 = cputicks()
}
//数据操作前先加锁
lock(&c.lock)
//已经关闭并且缓冲区没有数据 直接返回
if c.closed != 0 && c.qcount == 0 {
if raceenabled {
raceacquire(c.raceaddr())
}
unlock(&c.lock)
if ep != nil {
typedmemclr(c.elemtype, ep)
}
return true, false
}
//情况1：发送协程有等待的
if sg := c.sendq.dequeue(); sg != nil {
// Found a waiting sender. If buffer is size 0, receive value
// directly from sender. Otherwise, receive from head of queue
// and add sender's value to the tail of the queue (both map to
// the same buffer slot because the queue is full).直接复制给发送方 不用费劲放缓冲区
recv(c, sg, ep, func() { unlock(&c.lock) }, 3)
return true, true
}
//情况2：缓冲区有数据待接收
if c.qcount > 0 {
// Receive directly from queue
qp := chanbuf(c, c.recvx)
if raceenabled {
raceacquire(qp)
racerelease(qp)
}
if ep != nil {
typedmemmove(c.elemtype, ep, qp)
}
typedmemclr(c.elemtype, qp)
//接收索引+1 因为现在的已经被拿走了
c.recvx++
if c.recvx == c.dataqsiz {
c.recvx = 0
}
c.qcount--
unlock(&c.lock)
return true, true
}
if !block {
unlock(&c.lock)
return false, false
}
//情况3：缓冲区没有数据，把接收数据协程挂起等待有协程发送数据
// no sender available: block on this channel.
gp := getg()
//把协程封装成sudog的结构体
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
gp.waiting = mysg
mysg.g = gp
mysg.isSelect = false
mysg.c = c
gp.param = nil
c.recvq.enqueue(mysg)
gopark(chanparkcommit, unsafe.Pointer(&c.lock), waitReasonChanReceive, traceEvGoBlockRecv, 2)
// someone woke us up
if mysg != gp.waiting {
throw("G waiting list is corrupted")
}
gp.waiting = nil
gp.activeStackChans = false
if mysg.releasetime > 0 {
blockevent(mysg.releasetime-t0, 2)
}
closed := gp.param == nil
gp.param = nil
mysg.c = nil
releaseSudog(mysg)
return true, !closed
}
情况1由协程等待的时候，调用了recv方法
func recv(c *hchan, sg *sudog, ep unsafe.Pointer, unlockf func(), skip int) {
//无缓冲区
if c.dataqsiz == 0 {
if raceenabled {
racesync(c, sg)
}
if ep != nil {
// copy data from sender
//直接把协程数据复制给接收方
recvDirect(c.elemtype, sg, ep)
}
} else {
// Queue is full. Take the item at the
// head of the queue. Make the sender enqueue
// its item at the tail of the queue. Since the
// queue is full, those are both the same slot.
qp := chanbuf(c, c.recvx)
if raceenabled {
raceacquire(qp)
racerelease(qp)
raceacquireg(sg.g, qp)
racereleaseg(sg.g, qp)
}
// copy data from queue to receiver
if ep != nil {
//把缓冲队列里的数据复制给接收方
typedmemmove(c.elemtype, ep, qp)
}
// copy data from sender to queue
//现在缓冲区空了一个位置，把当前等待发送数据协程数据复制到这个空位
typedmemmove(c.elemtype, qp, sg.elem)
//当前位置应该是队列末尾，下一个要接收的缓冲数据索引应该是下一个所以+1
c.recvx++
//环形，满了回0
if c.recvx == c.dataqsiz {
c.recvx = 0
}
//发送数据的索引应该和接收是一致的，因为现在缓冲区是满了的状态 当最后一个发送协程取完了，缓冲区空一格位置的时候 sendx应该也是指向这一个位置，这里就是这个作用
c.sendx = c.recvx // c.sendx = (c.sendx+1) % c.dataqsiz
}
sg.elem = nil
gp := sg.g
unlockf()
gp.param = unsafe.Pointer(sg)
if sg.releasetime != 0 {
sg.releasetime = cputicks()
}
//唤起挂起的协程
goready(gp, skip+1)
}
关闭通道
func closechan(c *hchan) {
//关闭nil通道 会panic
if c == nil {
panic(plainError("close of nil channel"))
}
lock(&c.lock)
//关闭已经关闭的通道 会panic
if c.closed != 0 {
unlock(&c.lock)
panic(plainError("close of closed channel"))
}
if raceenabled {
callerpc := getcallerpc()
racewritepc(c.raceaddr(), callerpc, funcPC(closechan))
racerelease(c.raceaddr())
}
c.closed = 1
var glist gList
// release all readers
for {
sg := c.recvq.dequeue()
if sg == nil {
break
}
if sg.elem != nil {
typedmemclr(c.elemtype, sg.elem)
sg.elem = nil
}
if sg.releasetime != 0 {
sg.releasetime = cputicks()
}
gp := sg.g
gp.param = nil
if raceenabled {
raceacquireg(gp, c.raceaddr())
}
glist.push(gp)
}
// release all writers (they will panic)
for {
sg := c.sendq.dequeue()
if sg == nil {
break
}
sg.elem = nil
if sg.releasetime != 0 {
sg.releasetime = cputicks()
}
gp := sg.g
gp.param = nil
if raceenabled {
raceacquireg(gp, c.raceaddr())
}
glist.push(gp)
}
unlock(&c.lock)
// Ready all Gs now that we've dropped the channel lock.
for !glist.empty() {
gp := glist.pop()
gp.schedlink = 0
goready(gp, 3)
}
}
往一个nil的channel里面发送接收数据都会失败
问题 为啥不会panic ，注释去掉就panic
一年后追加： 我认为不panic是因为main主进程已经结束，所有里面的报错还没来得及走到就被跟着主进程一起结束了，去掉注释，又执行了一行代码拖延一点时间，所以协程里面的发送数据执行到了，所以panic，跟具体最后一行代码里面是什么没有多大关系，具体证据如下，如果最后在hang住几秒就会给协程足够的时间执行到报panic，所以是go func里面的panic，不是最后一行代码导致的，从一个关闭的通道读取数据不会panic，发送数据会
另外在网上看到的，close通道的时候发送数据协程队列底层源码解析说了这句话
验证了一下，两个协程给一个只有缓冲1个元素的通道赛数据，其中一个会进入发送数据协程等待队列里面，关闭通道之后结果是两个协程随机的panic，j jo随机出现
