doufangxie0203
2012-08-14 21:07
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在Go中将** T转换为* unsafe.Pointer

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.

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1条回答 默认 最新

  • dongyuluan7494 2012-08-14 22:14
    已采纳

    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)
    }
    
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