Golang: Global vars scope and routines-CodePudding

I'm trying to understand a problem I have using a global var and a routine. I created a simplifed test case to demonstrate:

var _i int = 5

func main() {
    fmt.Println("a _i", _i)

    go func() {
        fmt.Println("b _i", _i)
        update()
    }()
    ...
}
    
func update() {
    fmt.Println("c _i", _i)
}

It correctly returns

a _i 5
b _i 5
c _i 5

But if I initialise my var inside main:

var _i int

func main() {
    _i := 5
    fmt.Println("a _i", _i)

    go func() {
        fmt.Println("b _i", _i)
        update()
    }()
    ...
}
    
func update() {
    fmt.Println("c _i", _i)
}

It returns

a _i 5
b _i 5
c _i 0

Anybody can explain what I'm missing here? Thanks

CodePudding user response：

The second program declares local variable _i in main() using a short variable declaration.

The print in the anonymous function uses the local variable in main(). The local variable has value 5. The print in the update() function uses the package-level variable. The package-level variable has value 0.

Fix by assigning to _i in main():

func main() {
    _i = 5 // <-- colon removed from this line.
    fmt.Println("a _i", _i)

    go func() {
        fmt.Println("b _i", _i)
        update()
    }()
    ...
}

CodePudding user response：

In the last version you initialized _i with _i := 5 the := creates a new _i that shadows the global one. This is why update() prints 0, the global _i was never set to anything. You can change your initialization to _i = 5 to fix this.