goroutines deadlock when using channels

Question

Im new to go lang and I am trying to write a simple recursive algorithm which will use goroutines. I am using channels to receive output from goroutine, but when I try to do so I receive "fatal error: all goroutines are asleep - deadlock!" error. If I comment out channels code everything runs fine. This is my code:

package main

import (
    "fmt"
    "sync"
)

func main() {
    numbers := []int{2, -1, 10, 4, 3, 6, 22}

    ch := make(chan []int)

    wg := &sync.WaitGroup{}
    wg.Add(1)
    go testFunc(numbers, ch, wg)
    wg.Wait()

    result := <-ch

    fmt.Println("Result: ", result)
}

func testFunc(numbers []int, ch chan []int, wg *sync.WaitGroup) {
    defer wg.Done()
    ch <- numbers
}

What I am doing wrong ? I am assigning value to channel in goroutine and reading it in main. Isn't that enough to communicate?

You're waiting for the goroutine before receiving from the channel. You have an unbuffered channel here, so you don't need the wait group, nor do you need the goroutine at all. — JimB

Cerise Limón Cerise Limón · Accepted Answer · 2017-12-04T20:38:06

Communication on the unbuffered channel ch succeeds only when both a sender and receiver are ready.

The main function waits on the wait group before receiving the value. The goroutine attempts to send on the channel before call wg.Done. It's a deadlock.

One fix is to use a buffered channel:

     ch := make(chan []int, 1)

Another fix is to receive on the channel before calling wg.Wait().

result := <-ch
wg.Wait()

Yet another fix is to delete all lines using the wait group. It's not needed in this specific example.

goroutines deadlock when using channels

1 Answers