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Golang / GoLang Concurrency Mastery Interview Questions

What happens when you send to, receive from, or close a nil or closed channel?

This table is essential interview knowledge. Mistakes here produce panics and deadlocks that are notoriously difficult to debug.

Channel Operation Reference Table
OperationNil channelOpen, empty channelOpen, has dataClosed channel
Send (ch <- v)Blocks foreverBlocksSends OKPANIC
Receive (<-ch)Blocks foreverBlocksReturns value, ok=trueReturns zero value, ok=false
Close (close(ch))PANICCloses successfullyCloses; remaining data still readablePANIC
// Nil channel: always blocks (send or receive)
var ch chan int
// ch <- 1    // deadlock
// <-ch       // deadlock
// close(ch)  // PANIC: close of nil channel

// Closed channel: reads drain buffered data then return zero
done := make(chan struct{})
close(done)
v, ok := <-done
fmt.Println(v, ok)      // {} false
// done <- struct{}{} // PANIC: send on closed channel

// Comma-ok idiom: detect channel closure
ch2 := make(chan int, 2)
ch2 <- 1; ch2 <- 2
close(ch2)
for {
    v, ok := <-ch2
    if !ok { break }   // channel closed and drained
    fmt.Println(v)
}

// Idiomatic: range handles closure automatically
ch3 := make(chan int)
go func() { ch3 <- 1; ch3 <- 2; close(ch3) }()
for v := range ch3 { fmt.Println(v) } // exits on close

// GOLDEN RULE: only the SENDER should close a channel
// A receiver cannot know when the sender is done

What does receiving from a closed, empty buffered channel return?
Who should call close() on a channel, and why?

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