2.6 Select

1. What Is select?

select lets one goroutine wait on multiple channel send/receive operations.

select {
case v := <-c1:
    fmt.Println("c1", v)
case c2 <- value:
    fmt.Println("sent")
default:
    fmt.Println("nothing ready")
}

At a high level:

• if one communication can proceed, that case runs;
• if multiple cases can proceed, one ready case is selected using runtime pseudo-randomized selection rather than source-code priority;
• if none can proceed and default exists, default runs;
• if none can proceed and there is no default, the goroutine blocks.

2. Nil Channels

Sending to or receiving from a nil channel blocks forever. This property is useful inside select: setting a channel variable to nil dynamically disables that case.

3. Empty Select

select {}

An empty select blocks forever. If every goroutine in a program becomes permanently blocked and the runtime detects that no progress is possible, the program can terminate with a deadlock error.

4. Timeouts

For one-off timeouts:

select {
case result := <-resultCh:
    _ = result
case <-time.After(5 * time.Second):
    return
}

For repeated loops, prefer a reusable time.Timer/time.Ticker where appropriate instead of repeatedly allocating timers through time.After.

For request-scoped cancellation, context.Context is usually the better abstraction.

5. Exiting a for-select

A plain break inside a select exits the select, not the surrounding for. Common choices are:

• return;
• a labeled break;
• cancellation that causes the surrounding function to return.

6. Runtime Internals

Compiler/runtime implementation differs by case shape. Complex selects ultimately use runtime selection machinery (historically runtime.selectgo). Treat those symbols as version-specific; the language semantics above are the stable part.