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The default case in a Go select statement prevents blocking by executing immediately when no channel operations can proceed, enabling non-blocking communication; it is useful for checking channel availability without waiting, performing other tasks during channel inactivity, avoiding deadlocks, and implementing polling or background work; for example, a non-blocking read or send will execute the default case if the channel is empty or full, respectively; common patterns include using default in loops to periodically check channels while doing other work or sleeping briefly, thus turning select into a "try-send" or "try-receive" operation that enhances concurrency and responsiveness.
The default case in a select statement in Go is used to avoid blocking when none of the communication operations (on channels) can proceed immediately.
In other words, if all the channel operations in the select are not ready (e.g., sending to a full channel or receiving from an empty one), and there's a default case, it will execute right away instead of waiting.
Why is this useful?
By default, select blocks until one of the channel operations can proceed. But sometimes you want non-blocking behavior — to either:
- Check if data is available without waiting
- Perform other work while channels are busy
- Avoid deadlocks in certain concurrent patterns
That’s where default comes in.
Example: Non-blocking channel read
ch := make(chan string, 1)
select {
case msg := <-ch:
fmt.Println("Received:", msg)
default:
fmt.Println("No message available")
}Even if ch is empty, this won’t block — it prints "No message available" immediately.
Example: Non-blocking send
ch := make(chan int, 1)
ch <- 42 // fill the buffer
select {
case ch <- 43:
fmt.Println("Sent 43")
default:
fmt.Println("Channel full, not blocking")
}Since the channel is full (buffer size 1), sending would block — but default runs instead.
Common use cases
- Polling channels in a loop without blocking
- Implementing timeouts (though
time.After()is often better) - Background tasks that check channel status periodically
- Preventing deadlock in tight loops with unbuffered channels
for {
select {
case msg := <-ch:
fmt.Println("Got:", msg)
default:
// Do other work or sleep briefly
time.Sleep(100 * time.Millisecond)
}
}This loop keeps running and only reads from ch when data is available — otherwise, it continues immediately.
So, to sum up:
The default case makes a select statement non-blocking. It runs immediately if no other case is ready, allowing your program to continue doing useful work instead of waiting on channels.
Basically, it turns select into a "try-send" or "try-receive" operation.
The above is the detailed content of What is the select statement's default case used for in Golang?. For more information, please follow other related articles on the PHP Chinese website!
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