Cases to know when using selection statements in Golang

In Go language, select statement is like switch statement , but in select statement, case statement refers to communication, i.e. sending or receiving operation on channel.

Syntax:

select{

case SendOrReceive1: // Lệnh
case SendOrReceive2: // Lệnh
case SendOrReceive3: // Lệnh
.......
default: // Lệnh

In this article, we will learn how to use default case to avoid deadlock. But first, we will learn what is deadlock case when using select command in Golang ?

Things to know about deadlock case in Golang

Deadlock: When you try to read or write data from a channel but the channel is not available. So it blocks the current execution of the goroutine and transfers control to other goroutines, but if no other goroutine is available or other goroutines are idle due to this situation, the program will crash. This phenomenon is called deadlock. As shown in the example below:

For example:

// Chương trình Go minh hoạc
// cách deadlock phát sinh
package main 

// Hàm chính
func main() { 

	// Tạo một kênh
	// Deadlock phát sinh bởi
	// không có goroutine đang viết
	// vào kênh này, vì thế
	// lệnh select đã bị khóa mãi mãi
	c := make(chan int) 
	select { 
	case <-c: }="" }="">

Result:

fatal error: all goroutines are asleep - deadlock!

goroutine 1 [chan receive]:
main.main()

To avoid this situation, we use default case in select statement. In other words, when deadlock occurs in the program, default case of select statement is executed to avoid deadlock. As in the example below, we use default case in select statement to avoid deadlock.

For example:

// Chương trình Go minh họa cách giải quyết
// vấn đề deadlock bằng case mặc định
package main 

import "fmt"

// Hàm chính
func main() { 

	// Tạo 1 kênh
	c := make(chan int) 
	select { 
	case <-c: default:="" fmt.println("!..="" default="" case..!")="" }="" }="">

Result:

!.. Default case..!

You are also allowed to use the default case when the select statement has only nil channel . As shown in the example below, channel c is nil, so the default case is executed if the default case here is not available, then the program will block forever and deadlock arises.

For example:

// Chương trình Go minh họa
// chạy case mặc định
package main 

import "fmt"

// Hàm chính
func main() { 

	// Tạo một kênh
	var c chan int

	select { 
	case x1 := <-c: fmt.println("value:="" ",="" x1)="" default:="" fmt.println("default="" case..!")="" }="" }="">

Result:

Default case..!