goroutines order of execution

Question

I'm trying to understand this piece of code, not sure why the 2nd go is executed before the 1st one. It'd be great if someone can really help me out with this!

func sum(a []int, c chan int) {
   fmt.Println("summing: ", a)
   total := 0
   for _, v := range a {
      total += v
   }
   //fmt.Println("send to c",total)
   c <- total  // send total to c
}
func main() {
    //a := []int{7, 2, 8,134,23,23,1,23,1234,143, -9, 4, 0, 1234}

    c := make(chan int)

    go sum([]int{1,2,3}, c)
    go sum([]int{4,5,6}, c)

    x := <-c
    fmt.Println(x)
    x = <-c
    fmt.Println(x)
}

OUTPUT:

summing:  [4 5 6]
15
summing:  [1 2 3]
6

Answer 1

You have nothing explicitly synchronizing the order of the two goroutines. If you run this enough times, you will see the calls to fmt.Println print in different sequences. When executing goroutines, as they are concurrent operations, you have no guarantees when they will execute and/or complete. You need to use various standard library packages, or channels themselves to synchronize the execution of concurrently running goroutines.

For example (by leveraging the blocking nature of channels, you could do something like):

func main() {

    c := make(chan int)

    go sum([]int{1, 2, 3}, c)

    //use the channel to block until it receives a send
    x := <-c
    fmt.Println(x)

    //then execute the next routine
    go sum([]int{4, 5, 6}, c)

    x = <-c
    fmt.Println(x)
}

Another example (significantly less practical, but here to look at other common go synchronization features) you could introduce a wait group, and a range over a channel:

func sum(a []int, c chan int, wg *sync.WaitGroup) {
    defer wg.Done()
    fmt.Println("summing: ", a)
    total := 0
    for _, v := range a {
        total += v
    }
    //fmt.Println("send to c",total)
    c <- total // send total to c
}

func main() {

    c := make(chan int)
    wg := new(sync.WaitGroup)

    //concurrently call the concurrent calls to sum, allowing execution to continue to the range of the channel 
    go func() {
        //increment the wait group, and pass it to the sum func to decrement it when it is complete
        wg.Add(1)
        go sum([]int{1, 2, 3}, c, wg)
        //wait for the above call to sum to complete
        wg.Wait()
        //and repeat...
        wg.Add(1)
        go sum([]int{4, 5, 6}, c, wg)
        wg.Wait()
        //all calls are complete, close the channel to allow the program to exit cleanly 
        close(c)
    }()

    //range of the channel
    for theSum := range c {
        x := theSum
        fmt.Println(x)
    }

}