Getting Goroutine deadlock when using for loop to traverse through the channel

Question

I am tring to practise goroutine and channels, where I am facing issue where I am calling gorouting and passing channel. The goroutine will push data into into channel and then main thread will print the elements.

I have used for loop to print the contents but getting.

fatal error: all goroutines are asleep - deadlock!

2
1
goroutine 1 [chan receive]:
main.main()

package main

import "fmt"


func smallThread(a int, c chan int) {
    c <- a
}

func main() {
    c := make(chan int)
    go smallThread(1, c)
    go smallThread(2, c)
    for {
        fmt.Println(<-c)
    }
}

EDIT : Using waitgroup:

func smallThread(a int, c chan int, w *sync.WaitGroup) {
    c <- a
    w.Done()
}

func main() {
    c := make(chan int)
    var w sync.WaitGroup
    w.Add(2)
    go smallThread(1, c, &w)
    go smallThread(2, c, &w)
    //w.Wait()
    for i := range c {
        fmt.Println(i)
    }
    w.Wait()
}

EDIT2 : Working Code

func smallThread(a int, c chan int, w *sync.WaitGroup) {
    //defer w.Done()
    c <- a
    w.Done()
}

func main() {
    c := make(chan int)
    var w sync.WaitGroup
    w.Add(1)
    go smallThread(1, c, &w)
    w.Add(1)
    go smallThread(2, c, &w)
    go func(c chan int) {
        for i := range c {
            fmt.Println(i)
        }
    }(c)
    w.Wait()
}

Answer 1

When the goroutines are done, close the channel to indicate that no more values will be added. The for loop will break when after all values are received.

c := make(chan int)
var w sync.WaitGroup
w.Add(2)
go smallThread(1, c, &w)
go smallThread(2, c, &w)
go func() {
    w.Wait()
    close(c)
}()

for i := range c {
    fmt.Println(i)
}

Answer 2

Not sure what your question is, but I'll tell you what happens. Your two side goroutines push their numbers to the channel and exit. Then the main goroutine (the only one left at this point) will block forever, waiting for another element to come out of the channel.