Best way of using sync.WaitGroup with external function

Question

I have some issues with the following code:

package main

import (
\"fmt\"
\"sync\"
)
// This program should go to 11, but sometimes it only prints 1 to 10.
func         


        

Answer 1

Well, first your actual error is that you're giving the Print method a copy of the sync.WaitGroup, so it doesn't call the Done() method on the one you're Wait()ing on.

Try this instead:

package main

import (
    "fmt"
    "sync"
)

func main() {    
    ch := make(chan int)

    var wg sync.WaitGroup
    wg.Add(2)    

    go Print(ch, &wg)

    go func() {  
        for i := 1; i <= 11; i++ {
            ch <- i
        }
        close(ch)
        defer wg.Done()
    }()          

    wg.Wait() //deadlock here
}                

func Print(ch <-chan int, wg *sync.WaitGroup) {
    for n := range ch { // reads from channel until it's closed
        fmt.Println(n)
    }            
    defer wg.Done()
}

Now, changing your Print method to remove the WaitGroup of it is a generally good idea: the method doesn't need to know something is waiting for it to finish its job.