default and non-blocking

Learn non-blocking send and receive with default.

default and non-blocking

If you add default to select it runs when no case is ready, so select does not block. This enables non-blocking send and receive.

Non-blocking receive: select { case v := <-ch: ... default: }. If the channel has no data default runs. This is used for take-if-ready, else do other work.

Non-blocking pattern

The non-blocking pattern lets a goroutine check a channel and do other work if it is empty. This is an efficient alternative to busy waiting.

The pattern select { case x := <-ch: handle(x) default: doOther() } is common. If the channel has data it handles it; otherwise it moves on. This is the basis of event loops.

Exercise Non-blocking select

Inside main, add a select { case ... default: ... } block and print non-blocking with fmt.Println.

Show example solution

package main

import "fmt"

func main() {
    ch := make(chan string)
    select {
    case m := <-ch:
        fmt.Println(m)
    default:
        fmt.Println("non-blocking")
    }
}

Test yourself

When does the default case run?
- When no case is ready
- Always
- When the first case is not ready
- Only on error
Show answer

When no case is ready — when no case is ready. If even one case is ready default does not run.

default keyword

select {
case v := <-ch:
    fmt.Println(v)
____:
    fmt.Println("boş")
}

Show answer

select {
case v := <-ch:
    fmt.Println(v)
default:
    fmt.Println("boş")
}

Match default concepts

default

When none ready
non-blocking

No wait
case

Channel op
select

Multi-wait

Next lesson: Select with timeout

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