What is channel buffer size?

Question

I'm trying to create an asynchronous channel and I've been looking at http://golang.org/ref/spec#Making_slices_maps_and_channels.

c := make(chan int, 10)         // channel with a buffer size of 10 

What does it mean that the buffer size is 10? What specifically does the buffer size represent/limit?

Answer 1

The buffer size is the number of elements that can be sent to the channel without the send blocking. By default, a channel has a buffer size of 0 (you get this with make(chan int)). This means that every single send will block until another goroutine receives from the channel. A channel of buffer size 1 can hold 1 element until sending blocks, so you'd get

c := make(chan int, 1) c <- 1 // doesn't block c <- 2 // blocks until another goroutine receives from the channel 

Answer 2

The following code illustrates the blocking of unbuffered channel:

// to see the diff, change 0 to 1 c := make(chan struct{}, 0) go func() {     time.Sleep(2 * time.Second)     <-c }() start := time.Now() c <- struct{}{} // block, if channel size is 0 elapsed := time.Since(start) fmt.Printf("Elapsed: %v\n", elapsed) 

You may play with the code here.