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# Channels

如果说goroutine是Go语音程序的并发体的话，那么channels它们之间的通信机制。一个channels是一个通信机制，它可以让一个goroutine通过它给另一个goroutine发送值信息。每个channel都有一个特殊的类型，也就是channels可发送数据的类型。一个可以发送int类型数据的channel一般写为chan int。

使用内置的make函数，我们可以创建一个channel：

```go
ch := make(chan int) // ch has type 'chan int'
```

和map类似，channel也一个对应make创建的底层数据结构的引用。当我们复制一个channel或用于函数参数传递时，我们只是拷贝了一个channel引用，因此调用者何被调用者将引用同一个channel对象。和其它的引用类型一样，channel的零值也是nil。

两个相同类型的channel可以使用==运算符比较。如果两个channel引用的是相通的对象，那么比较的结果为真。一个channel也可以和nil进行比较。

一个channel有发送和接受两个主要操作，都是通信行为。一个发送语句将一个值从一个goroutine通过channel发送到另一个执行接收操作的goroutine。发送和接收两个操作都是用`<-`运算符。在发送语句中，`<-`运算符分割channel和要发送的值。在接收语句中，`<-`运算符写在channel对象之前。一个不使用接收结果的接收操作也是合法的。

```go
ch <- x  // a send statement
x = <-ch // a receive expression in an assignment statement
<-ch     // a receive statement; result is discarded
```

Channel还支持close操作，用于关闭channel，随后对基于该channel的任何发送操作都将导致panic异常。对一个已经被close过的channel之行接收操作依然可以接受到之前已经成功发送的数据；如果channel中已经没有数据的话讲产生一个零值的数据。

使用内置的close函数就可以关闭一个channel：

```go
close(ch)
```

以最简单方式调用make函数创建的时一个无缓冲的channel，但是我们也可以指定第二个整形参数，对应channel的容量。如果channel的容量大于零，那么该channel就是带缓冲的channel。

```go
ch = make(chan int)    // unbuffered channel
ch = make(chan int, 0) // unbuffered channel
ch = make(chan int, 3) // buffered channel with capacity 3
```

我们将先讨论无缓冲的channel，然后在8.4.4节讨论带缓冲的channel。


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