Channel closing behavior in Go - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of reading from a closed channel?

Consider the following Go code. What will be printed when reading from a closed channel?

Go

package main
import "fmt"
func main() {
    ch := make(chan int)
    go func() {
        ch <- 42
        close(ch)
    }()
    val, ok := <-ch
    fmt.Println(val, ok)
    val, ok = <-ch
    fmt.Println(val, ok)
}

A

42 true
0 false

B

42 true
42 true

C

0 false
0 false

D

42 false
0 false

💡 Hint

Think about what happens when you read from a closed channel in Go.

When a channel is closed, reading from it returns the zero value and false for the second value. The first read gets the sent value 42 and ok=true. The second read gets zero value 0 and ok=false.

❓ Predict Output

intermediate

What happens when closing a nil channel?

What will happen if you try to close a nil channel in Go?

Go

package main
func main() {
    var ch chan int
    close(ch)
}

A Deadlock at runtime

B Program exits normally

C Compilation error: cannot close nil channel

D Panic: close of nil channel

💡 Hint

Think about what nil means for channels and what close expects.

Closing a nil channel causes a runtime panic because nil channels are not initialized and cannot be closed.

❓ Predict Output

advanced

What is the output when closing a channel twice?

What happens when you close a channel twice in Go? Consider this code:

Go

package main
import "fmt"
func main() {
    ch := make(chan int)
    close(ch)
    close(ch)
    fmt.Println("Done")
}

A Panic: close of closed channel

B Prints "Done" and exits normally

C Compilation error: cannot close channel twice

D Deadlock at runtime

💡 Hint

Think about what Go does if you close a channel that is already closed.

Closing a channel twice causes a runtime panic with message 'close of closed channel'.

❓ Predict Output

advanced

What is the output of range over a closed channel?

What will this program print?

Go

package main
import "fmt"
func main() {
    ch := make(chan int)
    go func() {
        for i := 1; i <= 3; i++ {
            ch <- i
        }
        close(ch)
    }()
    for v := range ch {
        fmt.Print(v, " ")
    }
}

A 1 2 3 0

B 1 2 3

C 1 2

D Deadlock at runtime

💡 Hint

Remember how ranging over a channel works when it is closed.

The range loop reads all values sent before the channel is closed, then exits cleanly.

🧠 Conceptual

expert

What is the behavior of sending on a closed channel?

What happens if you try to send a value on a closed channel in Go?

A The send succeeds and the value is lost

B The send blocks forever causing a deadlock

C The program panics at runtime with 'send on closed channel'

D The value is silently discarded

💡 Hint

Think about what Go does to prevent sending on closed channels.

Sending on a closed channel causes a runtime panic with message 'send on closed channel'.