In Go, all goroutines can read and write the same global variables, or objects shared across threads, so data races are possible.

example.go

package main

import (
    "fmt"
    "time"
)

var k int

func say(s string) {
    for i := 0; i < 5; i++ {
        time.Sleep(100 * time.Millisecond)
        fmt.Println(s, k)
        k++
    }
}

func main() {
    go say("world")
    say("hello")
}

example2.go

package main

import (
    "fmt"
    "time"
)

type Counter struct {
    X int
}

func say(s string, c *Counter) {
    for i := 0; i < 5; i++ {
        time.Sleep(100 * time.Millisecond)
        fmt.Println(s, c.X)
        c.X++
    }
}

func main() {
    c := &Counter{0}
    go say("world", c)
    say("hello", c)
}