Deadlock between goroutines

Issue

I’m new to Go. When I comment out the second goroutine, there is a fatal error. I don’t understand what causes this error to occur. Can you explain it to me?

package main

import (
    "fmt"
    "time"
)

func main() {
    ch := make(chan int)
    go func() { 
        for i := 0; i < 10; i++ {
            ch <- i
        }
    } ()
    // go func() { 
        for {
            if num, ok := <-ch; !ok {
                break
            } else {
                fmt.Printf("%d\n", num)
            }
        }
    // } ()
    time.Sleep(2 * time.Second)
    close(ch)
}

This prints the following code:

0
1
2
3
4
5
6
7
8
9
fatal error: all goroutines are asleep - deadlock!

goroutine 1 [chan receive]:
main.main()
    /tmp/sandbox169127128/main.go:17 +0xa0

Program exited.

Solution

The receiving for loop blocks on receive from ch after receiving all values from the sending goroutine. The runtime detects that the program is stuck and panics.

The fix is to close the channel after sending all values:

go func() { 
    for i := 0; i < 10; i++ {
        ch <- i
    }
    close(ch)
} ()

Receive on the closed channel yields the value 0, false. The receiving for loop breaks on the false value.

Remove close(ch) from the end of the program.

Run it on the playground.

Answered By – Bayta Darell

Answer Checked By – Willingham (GoLangFix Volunteer)

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