Blocking behavior in Go - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this Go program with channels?

Consider the following Go code. What will it print when run?

Go

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

A 42

B 0

C Compilation error

D Deadlock panic

💡 Hint

Think about how channels block until data is sent or received.

The goroutine sends 42 into the channel. The main goroutine waits to receive from the channel, so it blocks until the value is sent. Then it prints 42.

❓ Predict Output

intermediate

What happens when reading from a closed channel?

What will this Go program print?

Go

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

A 0 true

B 0 false

C Compilation error

D Deadlock panic

💡 Hint

Reading from a closed channel returns zero value and false.

When a channel is closed, receiving from it returns the zero value of the channel type and false for the second value.

❓ Predict Output

advanced

What is the output of this select with blocking channels?

Analyze the following Go code and determine its output.

Go

package main
import (
	"fmt"
	"time"
)
func main() {
	ch1 := make(chan string)
	ch2 := make(chan string)
	go func() {
		time.Sleep(100 * time.Millisecond)
		ch1 <- "first"
	}()
	go func() {
		time.Sleep(50 * time.Millisecond)
		ch2 <- "second"
	}()
	select {
	case msg1 := <-ch1:
		fmt.Println(msg1)
	case msg2 := <-ch2:
		fmt.Println(msg2)
	}
}

A first

B Compilation error

C second

D Deadlock panic

💡 Hint

Which channel sends first after the sleep?

ch2 sends after 50ms, ch1 after 100ms. The select receives from the first ready channel, so it prints "second".

❓ Predict Output

advanced

What error does this Go program produce?

What happens when you run this Go code?

Go

package main
func main() {
	ch := make(chan int)
	ch <- 1
}

A Deadlock panic

B Compilation error

C Program prints 1

D No output, program exits normally

💡 Hint

Sending on an unbuffered channel blocks until another goroutine receives.

The main goroutine tries to send on ch but no other goroutine receives, so it blocks forever and causes a deadlock panic.

🧠 Conceptual

expert

How many items are in the buffered channel after this code runs?

Given the following Go code, how many items remain in the channel after execution?

Go

package main
import "fmt"
func main() {
	ch := make(chan int, 3)
	ch <- 10
	ch <- 20
	<-ch
	ch <- 30
	fmt.Println(len(ch))
}

A 3

B 0

C 1

D 2

💡 Hint

Remember that reading from the channel removes an item.

Two items remain because one was removed by the receive operation, so the channel holds 2 items.