Channel closing behavior in Go - Cheat Sheet & Quick Revision

Recall & Review

beginner

What happens when you close a channel in Go?

Closing a channel means no more values can be sent on it. Receivers can still receive remaining values until the channel is empty, then they get the zero value.

beginner

Can you send values on a closed channel?

No. Sending on a closed channel causes a runtime panic (error).

intermediate

What does a receive operation return when the channel is closed and empty?

It returns the zero value of the channel's type and false for the second value if using the two-value receive form.

intermediate

How can you check if a channel is closed when receiving?

Use the two-value receive: v, ok := <-ch. If ok is false, the channel is closed and empty.

intermediate

Is it safe to close a channel from multiple goroutines?

No. Closing a channel multiple times causes a panic. Only one goroutine should close the channel.

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

A The program panics (runtime error).

B The value is silently ignored.

C The value is buffered until the channel reopens.

D The send blocks forever.

Sending on a closed channel causes a runtime panic in Go.

When receiving from a closed channel, what value do you get?

A The receive blocks forever.

B The last sent value again.

C An error is returned.

D The zero value of the channel's type.

After a channel is closed and emptied, receives return the zero value of the channel's type.

How do you detect if a channel is closed when receiving?

A Use a select statement with default case.

B Use the two-value receive: v, ok := <-ch.

C Check if the channel is nil.

D You cannot detect if a channel is closed.

The two-value receive form returns ok=false when the channel is closed and empty.

Who should close a channel?

A Any goroutine at any time.

B The receiver, when done receiving.

C The sender, when no more values will be sent.

D Channels do not need to be closed.

The sender should close the channel to signal no more values will be sent.

What happens if multiple goroutines close the same channel?

A The program panics at the second close.

B The channel closes safely once.

C The channel reopens automatically.

D The closes are ignored after the first.

Closing a channel more than once causes a panic.

Explain what happens when a Go channel is closed and how receivers behave afterwards.

Think about what sending and receiving do after closing.

Describe best practices for closing channels in concurrent Go programs.

Who should close and why?

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