代码版本:1.12.7

channel是Go语言里面用来传递数据的一个接口

主要代码在src/runtime/chan.go

常量定义

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const (
	maxAlign  = 8 //偏移量
	hchanSize = unsafe.Sizeof(hchan{}) + uintptr(-int(unsafe.Sizeof(hchan{}))&(maxAlign-1)) //这里就是对齐一下maxAlign
	debugChan = false //debug模式，开启后会输出debug信息
)

类型定义

主要就两个类型，一个是chan的底层hchan，另外一个是waitq用来记录收发队列的goroutine的信息

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type hchan struct {
   qcount   uint               // 缓冲区中已有元素个数
   dataqsiz uint               // 循环队列容量大小
   buf      unsafe.Pointer     // 缓冲区指针
   elemsize uint16             // 元素大小
   closed   uint32             // 关闭标记，0没关闭，1关闭
   elemtype *_type             // 数据项类型
   sendx    uint               // 发送索引
   recvx    uint               // 接收索引
   recvq    waitq              // 等待接收排队链表
   sendq    waitq              // 等待发送排队链表
   lock mutex                  // 锁
}

type waitq struct {
   first *sudog
   last  *sudog
}

功能实现

新建channel ch := make(chan int)

创建一个channel的时候Go的语法是ch := make(chan datatype),这个在底层的实现实际是makechan

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func makechan(t *chantype, size int) *hchan {
	elem := t.elem

	//1、检查待存的数据类型大小，大于1<<16时异常
    // compiler checks this but be safe.
	if elem.size >= 1<<16 {
		throw("makechan: invalid channel element type")
	}	
    //2、检查内存对齐（降低寻址次数，提高内存读取速度），大于最大的内存对齐字节数时，panic
	if hchanSize%maxAlign != 0 || elem.align > maxAlign {
		throw("makechan: bad alignment")
	}
	//3、检查传入的size大小，大于堆可分配的最大内存时，panic，可以看出chan是在堆里面分配内存的
	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.
        // mem为0的时候将c.buf指向自己，避免竞争,具体可看raceaddr注释
		c.buf = c.raceaddr()
	case elem.kind&kindNoPointers != 0:
		// Elements do not contain pointers.
		// Allocate hchan and buf in one call.
        // 数据类型不包含指针的时候，直接分配hchanSize+size个数据的大小，然后将c.buf指向数据起始地址
		c = (*hchan)(mallocgc(hchanSize+mem, nil, true))
		c.buf = add(unsafe.Pointer(c), hchanSize)
	default:
		// Elements contain pointers.
        //包含指针的时候分开申请内存
		c = new(hchan)
		c.buf = mallocgc(mem, elem, true)
	}

    //初始化数据
	c.elemsize = uint16(elem.size)
	c.elemtype = elem
	c.dataqsiz = uint(size)

	if debugChan {
		print("makechan: chan=", c, "; elemsize=", elem.size, "; elemalg=", elem.alg, "; dataqsiz=", size, "\n")
	}
	return c
}

向channel发送数据 ch <- 1

向一个chan发送数据的语法是chan <- data,在底层会被转为chansend1

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// entry point for c <- x from compiled code
//go:nosplit
func chansend1(c *hchan, elem unsafe.Pointer) {
	chansend(c, elem, true, getcallerpc())
}

/*
 * generic single channel send/recv
 * If block is not nil,
 * then the protocol will not
 * sleep but return if it could
 * not complete.
 *
 * sleep can wake up with g.param == nil
 * when a channel involved in the sleep has
 * been closed.  it is easiest to loop and re-run
 * the operation; we'll see that it's now closed.
 */
//c是当前channel，ep是数据的指针，block代表是否阻塞，默认都是阻塞，只有在select中是非阻塞，callerpc是取的PC寄存器
func chansend(c *hchan, ep unsafe.Pointer, block bool, callerpc uintptr) bool {
    //如果当前channel是nil，在非阻塞的情况下会返回false，阻塞情况下会报错，按照历史提交来看的话，throw之前是return false，也就是再调度一轮后返回发送失败，现在的版本是直接在调度的时候就报错了
	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.
    // 如果是非阻塞，并且channel未关闭，并且（缓冲区为空也没有接受者或者缓冲区放满了），直接返回false
	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)

    // 如果channel关闭了，报错
	if c.closed != 0 {
		unlock(&c.lock)
		panic(plainError("send on closed channel"))
	}

    //从接收者队列里面去取一个，直接把ep复制到接收者中，具体看send函数分析，然后返回成功
	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
	}

    //如果没有接收者，缓冲区有空位，就放入缓冲区中
	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
	}

    //没发出去，非阻塞直接返回
	if !block {
		unlock(&c.lock)
		return false
	}

	// Block on the channel. Some receiver will complete our operation for us.
    //阻塞的就将自己休眠进入调度
    
    //获取当前g
	gp := getg()
    //拿一个可用g
	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.
    // 初始化一下
	mysg.elem = ep
	mysg.waitlink = nil
	mysg.g = gp
	mysg.isSelect = false
	mysg.c = c
	gp.waiting = mysg
	gp.param = nil
    // 放入发送队列中
	c.sendq.enqueue(mysg)
    // 开始休眠调度
	goparkunlock(&c.lock, waitReasonChanSend, traceEvGoBlockSend, 3)
	// 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.
    //保活
	KeepAlive(ep)

	// someone woke us up.
    // 唤醒后gp.waiting应该等于mysg
	if mysg != gp.waiting {
		throw("G waiting list is corrupted")
	}
	gp.waiting = nil
    // 唤醒的时候有传gp.param，所以判断下
	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)
	}
    // 释放g
	mysg.c = nil
	releaseSudog(mysg)
	return true
}

// send processes a send operation on an empty channel c.
// The value ep sent by the sender is copied to the receiver sg.
// The receiver is then woken up to go on its merry way.
// Channel c must be empty and locked.  send unlocks c with unlockf.
// sg must already be dequeued from c.
// ep must be non-nil and point to the heap or the caller's stack.
// c是当前channel，sg是从接收队列中取出的g，ep是数据指针（必须是非空并且指向队或者调用放的栈上），unlockf解锁函数，发送后解锁用， skip作用未知
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++
            // 循环队列置位
			if c.recvx == c.dataqsiz {
				c.recvx = 0
			}
			c.sendx = c.recvx // c.sendx = (c.sendx+1) % c.dataqsiz
		}
	}
    // 接收者数据指针不为空就直接发送，具体看sendDirect分析，发送完后置空
	if sg.elem != nil {
		sendDirect(c.elemtype, sg, ep)
		sg.elem = nil
	}
    // 获取接受者的g
	gp := sg.g
	unlockf()
    // param赋值
	gp.param = unsafe.Pointer(sg)
	if sg.releasetime != 0 {
		sg.releasetime = cputicks()
	}
    // 将接受者的g置为ready状态，让其可以调度
	goready(gp, skip+1)
}

// Sends and receives on unbuffered or empty-buffered channels are the
// only operations where one running goroutine writes to the stack of
// another running goroutine. The GC assumes that stack writes only
// happen when the goroutine is running and are only done by that
// goroutine. Using a write barrier is sufficient to make up for
// violating that assumption, but the write barrier has to work.
// typedmemmove will call bulkBarrierPreWrite, but the target bytes
// are not in the heap, so that will not help. We arrange to call
// memmove and typeBitsBulkBarrier instead.
// 因为按照规定，当前g是不能写别的g的栈的，所以需要一些处理
func sendDirect(t *_type, sg *sudog, src unsafe.Pointer) {
	// src is on our stack, dst is a slot on another stack.

	// Once we read sg.elem out of sg, it will no longer
	// be updated if the destination's stack gets copied (shrunk).
	// So make sure that no preemption points can happen between read & use.
	dst := sg.elem
    // 写屏障处理
	typeBitsBulkBarrier(t, uintptr(dst), uintptr(src), t.size)
	// No need for cgo write barrier checks because dst is always
	// Go memory.
    // 直接复制过去
	memmove(dst, src, t.size)
}

从channel接收数据<- ch,a =<- ch,a,ok = <- ch

整体跟发送差不多，底层对应两种情况chanrecv1和chanrecv2

chanrecv1对应着<- ch,a =<- ch,赋值的操作还是在汇编做的

chanrecv2则对应着a,ok =<- ch

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// entry points for <- c from compiled code
//go:nosplit
func chanrecv1(c *hchan, elem unsafe.Pointer) {
	chanrecv(c, elem, true)
}

//go:nosplit
func chanrecv2(c *hchan, elem unsafe.Pointer) (received bool) {
	_, received = chanrecv(c, elem, true)
	return
}

// chanrecv receives on channel c and writes the received data to ep.
// ep may be nil, in which case received data is ignored.
// If block == false and no elements are available, returns (false, false).
// Otherwise, if c is closed, zeros *ep and returns (true, false).
// Otherwise, fills in *ep with an element and returns (true, true).
// A non-nil ep must point to the heap or the caller's stack.
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")
	}

    //当前chan为空，非阻塞直接返回
	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.
    //判断下是否有非阻塞返回的情况
	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()
	}

	lock(&c.lock)

    // 如果关闭了并且缓冲区为空，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
	}

    // 从发送者队列里面去取一个，直接把值复制到ep中，具体看recv函数分析，然后返回成功
	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).
		recv(c, sg, ep, func() { unlock(&c.lock) }, 3)
		return true, true
	}

    // 如果缓冲区有值，从缓冲区取
	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)
		c.recvx++
		if c.recvx == c.dataqsiz {
			c.recvx = 0
		}
		c.qcount--
		unlock(&c.lock)
		return true, true
	}

    // 都没取到，非阻塞直接返回
	if !block {
		unlock(&c.lock)
		return false, false
	}

	// no sender available: block on this channel.
    // 阻塞的就将自己休眠进入调度
	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.
    // 初始化一下g
	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)
	goparkunlock(&c.lock, waitReasonChanReceive, traceEvGoBlockRecv, 3)

	// someone woke us up
	if mysg != gp.waiting {
		throw("G waiting list is corrupted")
	}
	gp.waiting = nil
	if mysg.releasetime > 0 {
		blockevent(mysg.releasetime-t0, 2)
	}
	closed := gp.param == nil
	gp.param = nil
	mysg.c = nil
	releaseSudog(mysg)
	return true, !closed
}

// recv processes a receive operation on a full channel c.
// There are 2 parts:
// 1) The value sent by the sender sg is put into the channel
//    and the sender is woken up to go on its merry way.
// 2) The value received by the receiver (the current G) is
//    written to ep.
// For synchronous channels, both values are the same.
// For asynchronous channels, the receiver gets its data from
// the channel buffer and the sender's data is put in the
// channel buffer.
// Channel c must be full and locked. recv unlocks c with unlockf.
// sg must already be dequeued from c.
// A non-nil ep must point to the heap or the caller's stack.
func recv(c *hchan, sg *sudog, ep unsafe.Pointer, unlockf func(), skip int) {
    // 如果无缓冲
	if c.dataqsiz == 0 {
		if raceenabled {
			racesync(c, sg)
		}
		if ep != nil {
			// copy data from sender
            // 如果接收者有接收值，直接从发送者复制一份到ep中
			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.
        // 从缓冲区取值
		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 {
            // 如果接收者有接收值，复制一份到ep中
			typedmemmove(c.elemtype, ep, qp)
		}
		// copy data from sender to queue
		typedmemmove(c.elemtype, qp, sg.elem)
		c.recvx++
        // 循环队列置位
		if c.recvx == c.dataqsiz {
			c.recvx = 0
		}
		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()
	}
    // 将发送者的g置为ready状态，让其可以调度
	goready(gp, skip+1)
}

// 与sendDirect 一样
func recvDirect(t *_type, sg *sudog, dst unsafe.Pointer) {
	// dst is on our stack or the heap, src is on another stack.
	// The channel is locked, so src will not move during this
	// operation.
	src := sg.elem
	typeBitsBulkBarrier(t, uintptr(dst), uintptr(src), t.size)
	memmove(dst, src, t.size)
}

关闭channel close(ch)

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//c是当前channel
func closechan(c *hchan) {
    // 当前channel为空直接报错
	if c == nil {
		panic(plainError("close of nil channel"))
	}

	lock(&c.lock)
    // 之前已经关过
	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())
	}

    // 置位关闭状态
	c.closed = 1

	var glist gList

	// release all readers
    // 释放所有的接收方
	for {
        // 从读取队列里取一个g
		sg := c.recvq.dequeue()
		if sg == nil {
			break
		}
        // 接收者接收值不为空清理一下
		if sg.elem != nil {
			typedmemclr(c.elemtype, sg.elem)
			sg.elem = nil
		}
		if sg.releasetime != 0 {
			sg.releasetime = cputicks()
		}
        // 添加到glist等待释放
		gp := sg.g
		gp.param = nil
		if raceenabled {
			raceacquireg(gp, c.raceaddr())
		}
		glist.push(gp)
	}

	// release all writers (they will panic)
    // 释放所有发送者，会panic
	for {
        // 从发送者队列取
		sg := c.sendq.dequeue()
		if sg == nil {
			break
		}
        // 直接置空发送者的值，方便gc回收
		sg.elem = nil
		if sg.releasetime != 0 {
			sg.releasetime = cputicks()
		}
        // 添加到glist等待释放
		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.
    // 将所有g置为ready状态让其可以被调度
	for !glist.empty() {
		gp := glist.pop()
		gp.schedlink = 0
		goready(gp, 3)
	}
}

select相关

在底层select会被转成对应的语法来调用上面的chansend,chanrecv

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// compiler implements
//
//	select {
//	case c <- v:
//		... foo
//	default:
//		... bar
//	}
//
// as
//
//	if selectnbsend(c, v) {
//		... foo
//	} else {
//		... bar
//	}
//
func selectnbsend(c *hchan, elem unsafe.Pointer) (selected bool) {
	return chansend(c, elem, false, getcallerpc())
}

// compiler implements
//
//	select {
//	case v = <-c:
//		... foo
//	default:
//		... bar
//	}
//
// as
//
//	if selectnbrecv(&v, c) {
//		... foo
//	} else {
//		... bar
//	}
//
func selectnbrecv(elem unsafe.Pointer, c *hchan) (selected bool) {
	selected, _ = chanrecv(c, elem, false)
	return
}

// compiler implements
//
//	select {
//	case v, ok = <-c:
//		... foo
//	default:
//		... bar
//	}
//
// as
//
//	if c != nil && selectnbrecv2(&v, &ok, c) {
//		... foo
//	} else {
//		... bar
//	}
//
func selectnbrecv2(elem unsafe.Pointer, received *bool, c *hchan) (selected bool) {
	// TODO(khr): just return 2 values from this function, now that it is in Go.
	selected, *received = chanrecv(c, elem, false)
	return
}