本文作者：何海涛
一、什么是channel
我们来看《Go语言编程》中的一段话
channel是Go语言在语言级别提供的goroutine间的通信方式，是一种进程内的通信方式。
通俗点儿解释就是channel可以在两个或者多个goroutine之间传递消息。在Go中，goroutine和channel是并发编程的两大基石，goroutine用来执行并发任务，channel用来在goroutine之间来传递消息。
Do not communicate by sharing memory; instead, share memory by communicating. 不要通过共享内存来通信，而要通过通信来实现共享内存。
二、channel的实现
1、引入概念
首先我们来看两个例子来简单看下golang中channel是如何使用的：
package mainimport ( "fmt")func goroutineA(a chan int) { for { select { case val := fmt.Println(val) } }}func main() { ch := make(chan int) go goroutineA(ch) ch 3 ch 5}很简单的一段程序，初始化了一个非缓冲的channel，然后并发一个协程去接收channel中的数据，然后往channel中连续发送两个值，首先大家先理解一组概念，什么是非缓冲型channel和缓冲型channel？对，其实很简单，make时如果channel空间不为0，就是缓冲型的channel。
ch := make(chan int)//非缓冲型ch := make(chan int， 1024)//缓冲型如果我们将go goroutineA(ch)这行代码往下移，会发生什么？对，会报
fatal error: all goroutines are asleep - deadlock!
因为channel没有缓冲，也没有正在等待接收的goroutine，这个概念接下来我会讲到。
另外，我们会看到goroutineA的入参是a
var a chan var a chanint
大家一定要清楚接收和发送的概念
接收代表从channel读取数据 发送代表往channel写入数据
再看一个复杂点儿的例子
package mainimport ( "fmt" "os" "os/signal" "syscall" "time")var exit = make(chan string, 1)func main() { go dealSignal() exited := make(chan struct{}, 1) go channel1(exited) count := 0 t := time.Tick(time.Second)Loop: for { select { case count++ fmt.Printf("main run %d\n", count) case fmt.Println("main exit begin") break Loop } } fmt.Println("main exit end")}func dealSignal() { c := make(chan os.Signal, 1) signal.Notify(c, os.Interrupt, syscall.SIGTERM) go func() { exit "shutdown" }()}func channel1(exited chanstruct{}) { t := time.Tick(time.Second) count := 0 for { select { case count++ fmt.Printf("channel1 run %d\n", count) case fmt.Println("channel1 exit") close(exited) return } }}
这个例子首先并发出一个dealsign方法，用来接收关闭信号，如果接收到关闭信号后往exit channel发送一条消息，然后并发运行channel1，channel1中定了一个ticker，正常情况下channel1每秒打印第一个case语句，如果接收到exit的信号，进入第二个case，然后关闭传入的exited channel，那么main中的Loop，接收到exited关闭的信号后，打印“main exit begin”， 然后退出循环，进程成功退出。这个例子演示了channel在goroutine中起到的传递消息的作用。这个例子是为了向大家展示channel在多个goroutine之间进行通信。
2、数据结构
channel为什么会天生具备这种传递消息的特性呢，我们不禁对其底层的数据结构产生兴趣，我们来看下runtime/chan.go文件，有关channel的一切底层操作都在这个文件，我们首先来看下数据结构：
type hchan struct { qcount uint // total data in the queue；chan中的元素总数 dataqsiz uint // size of the circular queue；底层循环数组的size buf unsafe.Pointer // points to an array of dataqsiz elements，指向底层循环数组的指针，只针对有缓冲的channel elemsize uint16 //chan中元素的大小 closed uint32 //chan是否关闭 elemtype *_type // element type；元素类型 sendx uint // send index；已发送元素在循环数组中的索引 recvx uint // receive index；已接收元素在循环数组中的索引 recvq waitq // list of recv waiters，等待接收消息的goroutine队列 sendq waitq // list of send waiters，等待发送消息的goroutine队列 // 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}创建一个底层数组容量为5，元素类型为int，那么channel的数据结构如下图所示：
3、创建
首先我们先来了解一下 Channel 在 Go 语言中是如何创建的，Go 语言 Channel 的创建都是由 make 关键字完成的，我们在前面介绍slice和map的创建时都介绍了使用 make 关键字初始化数据结构，那么一个问题，那么Go语言是如何实现通过make方式来创建不同的数据结构的呢？
Golang 中所有形如 make(chan int, 10) 在编译期间会先被转换成 OMAKE 类型的节点，随后的类型检查阶段在发现 make 的第一个参数是 Channel 类型时会将 OMAKE 类型的节点转换成 OMAKECHAN：
func typecheck1(n *Node, top int) (res *Node) { switch n.Op { case OMAKE: // ... switch t.Etype { case TCHAN: l = nil if i < len(args) { l = args[i] i++ l = typecheck(l, ctxExpr) l = defaultlit(l, types.Types[TINT]) if l.Type == nil { n.Type = nil return n } if !checkmake(t, "buffer", l) { n.Type = nil return n } n.Left = l } else { n.Left = nodintconst(0) } n.Op = OMAKECHAN } }
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makechan 或者 makechan64 的函数调用：
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)) }}创建channel的时候，其实底层是调用makechan方法，我们来看下源码：
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, "; elemalg=", elem.alg, "; dataqsiz=", size, "\n") } return c}
可以看出makechan中其实主要的代码就是一个switch，针对不同的情况
1、case mem == 0代表无缓冲型channel，只分配hchan本身结构体大小的内存 2、case elem.ptrdata==0 代表元素类型不含指针，只分配hchan本身结构体大小+元素大小*个数的内存，是连续的内存空间 3、default元素类型包括指针，两次分配内存的操作
然后将buf指向对应的地址，然后是hchan中其他变量的赋值。
4、接收
接下来我们来讲channel的接收和发送，我们使用一段程序来进行讲解
func goroutineA(a <-chan int) { val := fmt.Println("G1 received data: ", val) return}func goroutineB(b <-chan int) { val := fmt.Println("G2 received data: ", val) return}func main() { ch := make(chan int) go goroutineA(ch) go goroutineB(ch) ch 3 time.Sleep(time.Second)}首先创建了一个无缓冲型的channel，然后启动两个goroutine去消费channel的数据，紧接着向channel中发送数据。我们一步一步来分析channel是如何接收和发送数据的，首先来看接收，golang中接收channel数据有两种方式：
i i, ok这两种不同的方法经过编译器的处理都会变成 ORECV 类型的节点，但是后者会在类型检查阶段被转换成 OAS2RECV 节点，我们可以简单看一下这里转换的路线图：
// entry points for //go:nosplitfunc chanrecv1(c *hchan, elem unsafe.Pointer) { chanrecv(c, elem, true)}//go:nosplitfunc chanrecv2(c *hchan, elem unsafe.Pointer) (received bool) { _, received = chanrecv(c, elem, true) return}两者用法不同，chanrecv2可以返回channel是否关闭，但是最终调用方法都是chanrecv，我们来看下源码：
func chanrecv(c *hchan, ep unsafe.Pointer, block bool) (selected, received bool) { if debugChan { print("chanrecv: chan=", c, "\n") } //##################step1#################### if c == nil { if !block { return } gopark(nil, nil, waitReasonChanReceiveNilChan, traceEvGoStop, 2) throw("unreachable") } //##################step2#################### 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) 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 } 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 } 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 } if !block { unlock(&c.lock) return false, false } // no sender available: block on this channel. 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 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}由于源码较多，我们逐个step进行讲解;
(1)step1
如果channel是nil：如果是非阻塞模式，直接返回(false，false)；如果是阻塞模式，调用goprak挂起goroutine，会阻塞下去。
//##################step1#################### if c == nil { if !block { return } gopark(nil, nil, waitReasonChanReceiveNilChan, traceEvGoStop, 2) throw("unreachable") }(2)step2
快速操作(不用获取锁，快速返回)，三组条件全部满足，快速返(false，false)
条件1：首先是在非阻塞模式下 条件2：如果是非缓冲型(datasiz=0)并且等待发送goroutine队列为空(sendq.first=nil，就是没人往channel写数据)，或者缓冲型channel(datasiz>0)并且buf中没有数据； 条件3：channel未关闭
//##################step2#################### if !block && (c.dataqsiz == 0 && c.sendq.first == nil || c.dataqsiz > 0 && atomic.Loaduint(&c.qcount) == 0) && atomic.Load(&c.closed) == 0 { return }(3)step3
首先加锁，如果channel已经关闭，并且buf中没有元素，返回对应类型的0值，但是received为false；两种情况
情形1：非缓冲型，channel已关闭
情形2：缓冲型，channel已关闭，并且buf无元素
//##################step3#################### lock(&c.lock) 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 }(4)step4
如果等待发送队列中有元素，证明channel已经满了，两种情形
情形1：非缓冲型，无buf
情形2：缓冲型，buf满了
//##################step4####################if sg := c.sendq.dequeue(); sg != nil { recv(c, sg, ep, func() { unlock(&c.lock) }, 3) return true, true }两种情形都正常进入recv方法，我们来看下源码：
func recv(c *hchan, sg *sudog, ep unsafe.Pointer, unlockf func(), skip int) { //##################step4-1#################### if c.dataqsiz == 0 { if raceenabled { racesync(c, sg) } if ep != nil { // copy data from sender recvDirect(c.elemtype, sg, ep) } } else { //##################step4-2#################### // 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 { typedmemmove(c.elemtype, ep, qp) } // copy data from sender to queue typedmemmove(c.elemtype, qp, sg.elem) c.recvx++ 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() } goready(gp, skip+1)}step4-1：如果是非缓冲型，那么直接从发送者的栈copy到接收者的栈
step4-2：缓冲型的，但是buf已经满了，此时recvx和sendx是重合的，如下图
首先将recvx处的元素0拷贝到接收地址，然后将下一个元素5拷贝到sendx，然后recvx和sendx分别加1。
step4-3：然后唤醒等待发送队列中的goroutine，等待调度器调度。
(5)step5
没有等待发送的队列，并且buf中有元素，直接把接收游标处的数据copy到接收数据的地址，然后改变hchan中元素数据。
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 }(6)step6
如果是非阻塞，那么直接返回；如果是阻塞的，构造sudog，保存各种值；将sudog保存到channel的recvq中，调用goparkunlock将goroutine挂起
if !block { unlock(&c.lock) return false, false }// no sender available: block on this channel. 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 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我们用本节一开始的例子来讲解下，再贴一遍程序
func goroutineA(a <-chan int) { val := fmt.Println("G1 received data: ", val) return}func goroutineB(b <-chan int) { val := fmt.Println("G2 received data: ", val) return}func main() { ch := make(chan int) go goroutineA(ch) go goroutineB(ch) ch 3 time.Sleep(time.Second)}
由于我们创建的channel是无缓冲型的，所以两个goroutine启动的G1和G2会被阻塞，G1和G2被加入到recvq中，状态为waiting，等待被唤醒。此时此刻ch如下图：
问题：当一个channel关闭后，我们是否还能从中读出数据？
package mainimport "fmt"func main() { ch := make(chan int, 6) ch 1 ch 2 close(ch) a := fmt.Println(a) b := fmt.Println(b) c := fmt.Println(c)}输出会是什么？
120
我们可以看出，当一个channel关闭后，我们依然可以从中读出数据，如果chan的buf中有元素，则读出的是chan中buf的数据，如果buf为空，则输出对应元素类型的零值。那么我们来看下如下的一段程序：
package mainimport ( "fmt" "os" "os/signal" "syscall" "time")var exit1 = make(chan struct{}, 1)func main() { go dealSignal1() count := 0 t := time.Tick(time.Second) for { select { case count++ fmt.Printf("main run %d\n", count) case fmt.Println("main exit begin") } } fmt.Println("main exit over")}func dealSignal1() { c := make(chan os.Signal, 2) signal.Notify(c, os.Interrupt, syscall.SIGTERM) go func() { close(exit1) }()}上面这段程序会有什么问题？
5、发送
我们继续往下走，G1、G2被挂起后，往channel中发送一个数据3，其实调用的是chansend方法，我们还是逐步的去讲解
(1)step1
如果channel=nil，当前goroutine会被挂起
if c == nil { if !block { return false } gopark(nil, nil, waitReasonChanSendNilChan, traceEvGoStop, 2) throw("unreachable") }(2)step2
依然是一个不加锁的快速操作，三组条件
条件1：非阻塞
条件2：channel未关闭
条件3：channel是非缓冲型，并且等待接收队列为空；或者缓冲型，并且循环数组已经满了
if !block && c.closed == 0 && ((c.dataqsiz == 0 && c.recvq.first == nil) || (c.dataqsiz > 0 && c.qcount == c.dataqsiz)) { return false }(3)step3
加锁，如果channel已经关闭，直接panic
lock(&c.lock)if c.closed != 0 { unlock(&c.lock) panic(plainError("send on closed channel"))}(4)step4
如果等待接收队列不为空，说明什么？
情形1：非缓冲型，等待接收队列不为空
情形2：缓冲型，等待接收队列不为空(说明buf为空)
两种情形，都是直接将待发送数据直接copy到接收处
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)//直接从ep copy到sg return true}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 { sendDirect(c.elemtype, sg, ep) sg.elem = nil } gp := sg.g unlockf() gp.param = unsafe.Pointer(sg) if sg.releasetime != 0 { sg.releasetime = cputicks() } goready(gp, skip+1)}两种情形，都直接从一个用一个goroutine操作另一个goroutine的栈，因此在sendDirect方法中会有一次写屏障
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 typeBitsBulkBarrier(t, uintptr(dst), uintptr(src), t.size) // No need for cgo write barrier checks because dst is always // Go memory. memmove(dst, src, t.size)}(5)step5
如果等待队列为空，并且缓冲区未满，肯定是缓冲型的channel
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 }将元素放在sendx处，然后sendx加1，channel总量加1
(6)step6
如果以上情况都没有命中，说明什么？说明channel已经满了，如果是非阻塞的直接返回，否则需要调用gopack将这个goroutine挂起，等待被唤醒。
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) 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. KeepAlive(ep)我们对照程序分析下，在前一个小节G1、G2被挂起来了，等待sender的解救；这时候往ch中发送了一个3，(step4)这时sender发现ch的等待接收队列recvq中有receiver，就会出队一个sudog，然后将元素直接copy到sudog的elem处，然后调用goready将G1唤醒，继续执行G1原来的代码，打印出结果。如下图：
6、关闭
close一个channel会调用closechan方法，比较简单，我们也来看下
(1)step1
如果channel为nil，会直接panic
if c == nil { panic(plainError("close of nil channel")) }(2)step2
加锁，如果channel已经关闭，再次关闭会panic
lock(&c.lock) if c.closed != 0 { unlock(&c.lock) panic(plainError("close of closed channel")) }(3)step3
首选将hchan对应close标志置为1，然后声明一个链表；将等待接收队列中的所有sudog加入到链表，并将其elem赋予一个相应类型的0值；
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) }(4)step4
将所有等待发送队列的sudog加入链表
// 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)(5)step5
唤醒sudog所有goroutine
for !glist.empty() { gp := glist.pop() gp.schedlink = 0 goready(gp, 3) }三、问题
问题1：哪些操作会使channel发生panic？
三种情况
情况1：往一个已经关闭的channel写数据情况2：关闭一个nil的channel情况3：关闭一个已经关闭的channel
问题2：channel是并发安全的吗？
是
问题3：当一个channel关闭后，我们是否还能从channel读到数据？
可以，只不过接收的是无效数据
问题4：channel发送和接收元素的本质是什么？
值的拷贝
看一段示例
package mainimport ( "fmt" "time")func print(u chan int) { time.Sleep(2 * time.Second) fmt.Println("print int", }func main() { c := make(chan int, 5) a := 0 c fmt.Println(a) // modify g a = 1 go print(c) time.Sleep(5 * time.Second) fmt.Println(a)}
再看一段复杂一点的
package mainimport ( "fmt" "time")type people struct { name string}var u = people{name: "A"}func printPeople(u chan *people) { time.Sleep(2 * time.Second) fmt.Println("printPeople", }func main() { c := make(chan *people, 5) var a = &u c fmt.Println(a) // modify g a = &people{name:"B"} go printPeople(c) time.Sleep(5 * time.Second) fmt.Println(a)}
输出会是什么
&{A}printPeople &{A}&{B}因为chan中保存的是u的地址的值的拷贝，这个地址未发生改变，虽然调用a = &people{name:"B"}重新赋予了a新的地址，但是channel中的未改变。
