Go Locks in a slice of struct

Question

I am new to GO. I am specifically trying to add values to an array in parallel using locks (i do not want to use channels). But somehow my answer is not correct. I have tried both approaches. Passing a pointer to slice and Passing the slice itself. I am not looking for a global lock variable.

Method 1 (Passing pointer)

type locks_block struct {
    population int
    mux sync.Mutex
}

func incr(ar *[] locks_block){

    for i:=0;i<len(*ar);i++ {

        (*ar)[i].mux.Lock()
        (*ar)[i].population = (*ar)[i].population+1;
        (*ar)[i].mux.Unlock()

    }
}

func main() {

    arr := make([]locks_block,5);

    go incr(&arr);
    go incr(&arr);
    go incr(&arr);
    go incr(&arr);


    fmt.Println(arr);
}

Method 2 (Passing slice)

type locks_block struct {
    population int
    mux sync.Mutex
}

func incr(ar [] locks_block){

    for i:=0;i<len(ar);i++ {

        ar[i].mux.Lock()
        ar[i].population = ar[i].population+1;
        ar[i].mux.Unlock()

    }
}

func main() {

    arr := make([]locks_block,5);

    go incr(arr);
    go incr(arr);
    go incr(arr);
    go incr(arr);


    fmt.Println(arr);
}

The output is not correct in either case.

Answer 1

It appears you are using the lock correctly, but are not waiting for the goroutines to "finish" before printing arr. Try adding a small <-time.After(time.Second), or using WaitGroup, or using select to wait for all goroutines to finish, or placing the fmt.Println(ar[i].population) inside the goroutines to see the results you want to see!

Same thing happens if you just start a bunch of goroutines without waiting for them to finish.

Here is a complete working example, with an extra 'id' for each goroutine, for clarity. Notice that the ordering of goroutines is not consistent!

package main

import (
    "fmt"
    "sync"
    "time"
)

type locks_block struct {
    population int
    mux        sync.Mutex
}

func incr(id int, ar []locks_block) {
    for i := 0; i < len(ar); i++ {
        ar[i].mux.Lock()
        ar[i].population = ar[i].population + 1
        fmt.Printf("goroutine #%v, population   %v\n", id, ar[i].population)
        ar[i].mux.Unlock()
    }
}

func main() {
    arr := make([]locks_block, 5)
    go incr(1, arr)
    go incr(2, arr)
    go incr(3, arr)
    go incr(4, arr)

    // this should give the goroutines enough time
    <-time.After(time.Millisecond * 500)
    fmt.Println(arr)
}