Convert **T to *unsafe.Pointer in Go

Question

How do I convert a variable of type **T to *unsafe.Pointer?

The example below will give the compilation error:

cannot convert &ptr (type **s) to type *unsafe.Pointer

package main

import (
    "sync/atomic"
    "unsafe"
)

type s struct {
    value int
}

func main(){
    var ptr *s
    a := &s{42}

    old := ptr
    atomic.CompareAndSwapPointer(
        (*unsafe.Pointer)(&ptr), // &unsafe.Pointer(ptr)
        unsafe.Pointer(old),
        unsafe.Pointer(a))
}

If I switch (*unsafe.Pointer)(&ptr) to &unsafe.Pointer(ptr), I will get this compilation error:

cannot take the address of unsafe.Pointer(ptr)

Ps. I choose to make an example with sync/atomic because that is one situation where you actually have to do such a conversion.

Edit

One incorrect solution would be to use a temporary variable:

up := unsafe.Pointer(ptr)
atomic.CompareAndSwapPointer(&up, ...

While it compiles, the CAS will only swap what is stored in up and not in ptr. This is not the desired result, as zeebo@#go-nuts pointed out.

Answer 1

mcef@#go-nuts posted the answer how to convert **T:

(*unsafe.Pointer)(unsafe.Pointer(ptr)), where ptr is of type **T.

zeebo@#go-nuts provided a working example (posted here with permission):

package main

import (
    "fmt"
    "sync/atomic"
    "unsafe"
)

type T struct {
    value int
}

func Swap(dest **T, old, new *T) bool {
    udest := (*unsafe.Pointer)(unsafe.Pointer(dest))
    return atomic.CompareAndSwapPointer(udest,
        unsafe.Pointer(old),
        unsafe.Pointer(new),
    )
}

func main() {
    x := &T{42}
    n := &T{50}
    fmt.Println(*x, *n)

    p := x
    Swap(&x, p, n)
    fmt.Println(*x, *n)
}