Why are addresses of pointers to a WaitGroup different in this golang snippet?

Question

Code snippet on playground

I am printing the pointer address for sync.WaitGroup and they are all different. Why?

func Run() *sync.WaitGroup {
    var wg sync.WaitGroup
    wg.Add(1)
    go func() {
        defer wg.Done()
        fmt.Printf("goroutine %p\n", &wg)

        time.Sleep(5 * time.Second)
        fmt.Println("wokeup")
    }()

    fmt.Printf("returning %p\n", &wg)
    return &wg
}

func main() {
    runtime.GOMAXPROCS(3)
    wg := Run()

    fmt.Printf("     main %p\n", &wg)
    wg.Wait()
}

Typical output shows that the address is diff between that inside the function Run and main. I'd expect that to be the same, n'est-ce pas ?

returning 0xc0840045c0
     main 0xc084000038
goroutine 0xc0840045c0
wokeup

peterSO · Accepted Answer · 2014-04-08 06:43:58Z

1

For example,

package main

import "fmt"
import "sync"
import "time"
import "runtime"

func Run() *sync.WaitGroup {
    var wg sync.WaitGroup
    wg.Add(1)
    go func() {
        defer wg.Done()
        fmt.Printf("goroutine %p\n", &wg)
        fmt.Println("sleep for 5s")
        time.Sleep(5 * time.Second)
        fmt.Println("wokeup")
    }()
    fmt.Printf("returning %p\n", &wg)
    return &wg
}

func main() {
    runtime.GOMAXPROCS(3)
    wg := Run()
    fmt.Printf("     main %p\n", wg)
    wg.Wait()
}

Output:

returning 0x1052e2c0
     main 0x1052e2c0
goroutine 0x1052e2c0
sleep for 5s

Change

fmt.Printf("     main %p\n", &wg)

to

fmt.Printf("     main %p\n", wg)

In main, the variable wg contains the address you want. You are printing the address of the variable that contains the address.

answered Apr 8, 2014 at 6:43

peterSO

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3 Comments

canadadry Over a year ago

After ruminating over this, I think I am going to annotate my variable so that I know they are pointers. e.g. p_wg

fuz Over a year ago

@canadadry Do as you like, but usually you have types for this. If you're unsure what type a variable has, try explicitly declaring it with var foo *Type or something to that effect.

canadadry Over a year ago

Thanks @FUZxxl that's another possibility.

photoionized · Accepted Answer · 2014-04-08 06:43:48Z

1

Well, if you notice, the "returning" print statement and the "goroutine" print statement give the same address. Your "main" print is what is off. Look at the line:

fmt.Printf("     main %p\n", &wg)

What do you notice? You're printing &wg, the address of whatever wg is assigned to.

Now look at the return value of your Run() function, you're returning &wg. A pointer to your first declaration of wg as a variable.

So basically your "main" print is printing the the address of the wg pointer address rather than the pointer address itself... hope that makes sense...

But yeah, changing the line in "main" to

fmt.Printf("     main %p\n", wg)

should work.

answered Apr 8, 2014 at 6:43

photoionized

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