Channel 在运行时的内部表示是 runtime.hchan，该结构体中包含了用于保护成员变量的互斥锁，从某种程度上说，Channel 是一个用于同步和通信的有锁队列，使用互斥锁解决程序中可能存在的线程竞争问题是很常见的，我们能很容易地实现有锁队列。

数据结构

Go 语言的 Channel 在运行时使用 runtime.hchan 结构体表示。我们在 Go 语言中创建新的 Channel 时，实际上创建的都是如下所示的结构：

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
}

runtime.hchan 结构体中的五个字段 qcount、dataqsiz、buf、sendx、recv 构建底层的循环队列：

qcount — Channel 中的元素个数；
dataqsiz — Channel 中的循环队列的长度；
buf — Channel 的缓冲区数据指针；
sendx — Channel 的发送操作处理到的位置；
recvx — Channel 的接收操作处理到的位置；

除此之外，elemsize 和 elemtype 分别表示当前 Channel 能够收发的元素类型和大小；sendq 和 recvq 存储了当前 Channel 由于缓冲区空间不足而阻塞的 Goroutine 列表，这些等待队列使用双向链表 runtime.waitq 表示，链表中所有的元素都是 runtime.sudog 结构：

type waitq struct {
	first *sudog
	last  *sudog
}

runtime.sudog 表示一个在等待列表中的 Goroutine，该结构中存储了两个分别指向前后 runtime.sudog 的指针以构成链表。

// sudog represents a g in a wait list, such as for sending/receiving
// on a channel.
//
// sudog is necessary because the g ↔ synchronization object relation
// is many-to-many. A g can be on many wait lists, so there may be
// many sudogs for one g; and many gs may be waiting on the same
// synchronization object, so there may be many sudogs for one object.
//
// sudogs are allocated from a special pool. Use acquireSudog and
// releaseSudog to allocate and free them.
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
}

创建管道

Go 语言中所有 Channel 的创建都会使用 make 关键字。编译器会将 make(chan int, 10) 表达式转换成 OMAKE 类型的节点，并在类型检查阶段将 OMAKE 类型的节点转换成 OMAKECHAN 类型：

func typecheck1(n *Node, top int) (res *Node) {
	switch n.Op {
	case OMAKE:
		...
		switch t.Etype {
		case TCHAN:
			l = nil
			if i < len(args) { // 带缓冲区的异步 Channel
				...
				n.Left = l
			} else { // 不带缓冲区的同步 Channel
				n.Left = nodintconst(0)
			}
			n.Op = OMAKECHAN
		}
	}
}

这一阶段会对传入 make 关键字的缓冲区大小进行检查，如果我们不向 make 传递表示缓冲区大小的参数，那么就会设置一个默认值 0，也就是当前的 Channel 不存在缓冲区。

OMAKECHAN 类型的节点最终都会在 SSA 中间代码生成阶段之前被转换成调用 runtime.makechan 或者 runtime.makechan64 的函数：

func walkexpr(n *Node, init *Nodes) *Node {
	switch n.Op {
	case OMAKECHAN:
		size := n.Left
		fnname := "makechan64"
		argtype := types.Types[TINT64]

		if size.Type.IsKind(TIDEAL) || maxintval[size.Type.Etype].Cmp(maxintval[TUINT]) <= 0 {
			fnname = "makechan"
			argtype = types.Types[TINT]
		}
		n = mkcall1(chanfn(fnname, 1, n.Type), n.Type, init, typename(n.Type), conv(size, argtype))
	}
}

runtime.makechan 和 runtime.makechan64 会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.
	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)

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

上述代码根据 Channel 中收发元素的类型和缓冲区的大小初始化 runtime.hchan 和缓冲区：

如果当前 Channel 中不存在缓冲区，那么就只会为 runtime.hchan 分配一段内存空间；
如果当前 Channel 中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；
在默认情况下会单独为 runtime.hchan 和缓冲区分配内存；

在函数的最后会统一更新 runtime.hchan 的 elemsize、elemtype 和 dataqsiz 几个字段。

【Golang】源码 - Channel 实现

数据结构

创建管道

发送数据

接收数据

关闭 Channel

Reference

FEATURED TAGS

FRIENDS

TOC