Channel closing behavior in Go

Introduction

Channels let goroutines talk to each other. Closing a channel tells others no more values will come.

When you finish sending data and want to tell receivers to stop waiting.

When you want to signal that a task is done.

When you want to avoid deadlocks by closing channels after use.

Syntax

Go

close(channelName)

You only close a channel from the sender side.

Closing a channel twice causes a panic (runtime error).

Examples

This creates a channel and then closes it immediately.

Go

ch := make(chan int)
close(ch)

Looping over a channel stops automatically when the channel is closed.

Go

for v := range ch {
    fmt.Println(v)
}

Reading from a closed channel returns zero value and ok=false.

Go

v, ok := <-ch
if !ok {
    fmt.Println("Channel closed, no more data")
}

Sample Program

This program sends numbers 1 to 3 into a channel, then closes it. The main goroutine reads and prints all values until the channel closes. Then it tries to read once more to show the zero value and ok=false after closing.

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.Println(v)
    }

    v, ok := <-ch
    fmt.Printf("After close: value=%d, ok=%v\n", v, ok)
}

Important Notes

Do not send on a closed channel; it causes a panic.

Receivers can detect channel closure by checking the second value from receive.

Closing a channel is optional but useful to signal no more data.

Summary

Use close(channel) to tell receivers no more values will come.

Reading from a closed channel returns zero value and ok=false.

Looping with for v := range channel stops when channel closes.

Practice

1. What happens when you close a channel in Go?

easy

A. The channel is deleted from memory immediately

B. Senders can still send values without error

C. Receivers know no more values will be sent

D. The channel blocks all operations permanently

Solution

1. Step 1: Understand channel closing purpose

   Closing a channel signals receivers that no more values will be sent.

2. Step 2: Effects on senders and receivers

   Senders cannot send after closing; receivers get zero value and ok=false.

3. Final Answer:

   Receivers know no more values will be sent -> Option C

4. Quick Check:

   close(channel) signals receivers [OK]

Hint: Closing signals receivers no more data comes [OK]

Common Mistakes:

• Thinking senders can send after close
• Believing channel is deleted immediately
• Assuming channel blocks forever after close

2. Which of the following is the correct way to close a channel named ch in Go?

easy

A. ch.Close()

B. ch.close()

C. close channel ch

D. close(ch)

Solution

1. Step 1: Recall Go syntax for closing channels

   The built-in function close() is used with the channel as argument.

2. Step 2: Identify correct syntax

   Only close(ch) is valid; methods like ch.close() or ch.Close() do not exist.

3. Final Answer:

   close(ch) -> Option D

4. Quick Check:

   Use close(channel) syntax [OK]

Hint: Use built-in close(channel) function [OK]

Common Mistakes:

• Trying to call close as a method
• Using incorrect keywords like 'close channel'
• Capitalizing close as Close

3. What is the output of this Go program?

package main
import "fmt"
func main() {
  ch := make(chan int, 2)
  ch <- 10
  ch <- 20
  close(ch)
  for v := range ch {
    fmt.Print(v, " ")
  }
}

medium

A. 10 20

B. 10 20 0

C. Runtime panic: send on closed channel

D. No output

Solution

1. Step 1: Understand channel buffering and closing

   Channel has buffer size 2, two values sent, then channel closed.

2. Step 2: Loop over channel with range

   Range reads all values until channel is closed and drained, printing 10 and 20.

3. Final Answer:

   10 20 -> Option A

4. Quick Check:

   Range over closed channel prints buffered values [OK]

Hint: Range prints all buffered values before stopping [OK]

Common Mistakes:

• Expecting panic on reading closed channel
• Thinking zero value prints after buffered values
• Assuming no output because channel is closed

4. Identify the error in this Go code snippet:

ch := make(chan int)
go func() {
  for i := 0; i < 3; i++ {
    ch <- i
  }
  close(ch)
}()
for {
  v, ok := <-ch
  if !ok {
    break
  }
  fmt.Println(v)
}

medium

A. Channel is unbuffered but sender is in goroutine, no deadlock

B. No error, code runs and prints 0 1 2

C. Closing channel inside goroutine causes panic

D. Receiver loop never ends, causing infinite loop

Solution

1. Step 1: Analyze channel usage and goroutine

   Unbuffered channel with sender in goroutine sends 3 values then closes channel.

2. Step 2: Check receiver loop behavior

   Receiver reads values until channel closes, then breaks loop correctly.

3. Final Answer:

   No error, code runs and prints 0 1 2 -> Option B

4. Quick Check:

   Proper close and ok check ends loop [OK]

Hint: Use ok to detect closed channel and break loop [OK]

Common Mistakes:

• Thinking closing in goroutine causes panic
• Assuming infinite loop without break
• Ignoring need to check ok when receiving

5. Consider this Go program:

package main
import "fmt"
func main() {
  ch := make(chan int)
  go func() {
    for i := 1; i <= 3; i++ {
      ch <- i
    }
    close(ch)
  }()
  for {
    v, ok := <-ch
    if !ok {
      break
    }
    if v%2 == 0 {
      fmt.Println(v)
    }
  }
}

What is the output and why?

hard

A. 2
Only even numbers printed before channel closes

B. 1
2
3
All numbers printed despite channel closing

C. No output because channel is closed before reading

D. Runtime panic due to sending on closed channel

Solution

1. Step 1: Understand sending and closing in goroutine

   Values 1, 2, 3 sent then channel closed properly.

2. Step 2: Receiver loop filters even numbers

   Loop reads until closed; prints only values where v%2 == 0, which is 2.

3. Final Answer:

   2 Only even numbers printed before channel closes -> Option A

4. Quick Check:

   Filter with condition inside loop after close [OK]

Hint: Filter values after reading until channel closes [OK]

Common Mistakes:

• Expecting all values printed
• Thinking channel closes before sending
• Assuming panic on close with goroutine sender