Channel closing behavior in Go - Mini Project: Build & Apply

Channel closing behavior

📖 Scenario: Imagine you are working with a Go program that uses channels to communicate between goroutines. You want to understand how closing a channel affects sending and receiving data.

🎯 Goal: You will create a channel, send some data, close the channel, and then receive data from it to observe the behavior of a closed channel.

📋 What You'll Learn

Create a channel of type int called ch

Send exactly two integers 10 and 20 into ch

Close the channel ch

Use a for loop with range to receive and print all values from ch

💡 Why This Matters

🌍 Real World

Channels are used in Go programs to safely pass data between concurrent tasks, like workers processing jobs.

💼 Career

Understanding channel closing behavior is essential for writing clean, deadlock-free concurrent Go programs in many software engineering roles.

1

Create a channel

Create a channel called ch of type chan int using make.

Go

package main

func main() {
    // Create a channel called ch of type chan int
    // Your code here
}

Hint

Use make(chan int) to create the channel.

2

Send data and close the channel

Send the integers 10 and 20 into the channel ch using the go keyword and a goroutine. Then close the channel ch after sending the data.

Go

package main

func main() {
    ch := make(chan int)
    // Start a goroutine to send 10 and 20 into ch
    // Then close ch
    // Your code here
}

Hint

Use a goroutine with go func() { ... }() to send data and then close the channel.

3

Receive data from the channel

Use a for loop with range to receive all values from the channel ch. Inside the loop, print each received value using fmt.Println. Import the fmt package at the top.

Go

package main

import "fmt"

func main() {
    ch := make(chan int)
    go func() {
        ch <- 10
        ch <- 20
        close(ch)
    }()
    // Use a for loop with range to receive and print values from ch
    // Your code here
}

Hint

The range loop automatically stops when the channel is closed.

4

Print the output

Run the program and observe the output. It should print the numbers 10 and 20 each on its own line.

Go

package main

import "fmt"

func main() {
    ch := make(chan int)
    go func() {
        ch <- 10
        ch <- 20
        close(ch)
    }()
    for val := range ch {
        fmt.Println(val)
    }
    // Program output should show 10 and 20
    // Your code here
}

Hint

The program prints the values sent before the channel was closed.

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