Are the two unsafe uses safe?

[tapi...@gmail.com]

1.

func String2ByteSlice(s string) []byte {
    return (*[^uint(0) >> 1]byte)(unsafe.Pointer(&s))[:len(s):len(s)]
}

2.

func String2ByteSlice(s string) []byte {
    return *(*[]byte)(unsafe.Pointer(&struct{string; int}{s, len(s)}))
}

[tapi...@gmail.com]

Sorry, the first one will get a "type [9223372036854775807]byte larger than address space" error.

The following code will get the same error

    var x *[^uint(0) >> 1]byte

Is this error essential?

[Axel Wagner]

On Sun, Feb 28, 2021 at 9:09 AM tapi...@gmail.com <tapi...@gmail.com> wrote:

Is this error essential?

Yes. You need to be able to do arithmetic on addresses to use an array, but for that they need to have a known type and size. By exceeding the address space, you are creating a type that the compiler can't work with - that is, it can't allow you to create that type, because it wouldn't know how to use it.

 

On Saturday, February 27, 2021 at 11:00:26 PM UTC-5 tapi...@gmail.com wrote:

1.

func String2ByteSlice(s string) []byte {
    return (*[^uint(0) >> 1]byte)(unsafe.Pointer(&s))[:len(s):len(s)]
}

2.

func String2ByteSlice(s string) []byte {
    return *(*[]byte)(unsafe.Pointer(&struct{string; int}{s, len(s)}))
}

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[Ian Lance Taylor]

On Sat, Feb 27, 2021 at 8:00 PM tapi...@gmail.com <tapi...@gmail.com> wrote:
>
> 1.
>
> func String2ByteSlice(s string) []byte {
> return (*[^uint(0) >> 1]byte)(unsafe.Pointer(&s))[:len(s):len(s)]
> }

This doesn't do what you probably want, as &s is not the address of
the bytes in the string.

> 2.
>
> func String2ByteSlice(s string) []byte {
> return *(*[]byte)(unsafe.Pointer(&struct{string; int}{s, len(s)}))
> }

This is more likely to work but it's not safe.

Ian

[tapi...@gmail.com]

On Sunday, February 28, 2021 at 8:29:51 AM UTC-5 Ian Lance Taylor wrote:

On Sat, Feb 27, 2021 at 8:00 PM tapi...@gmail.com <tapi...@gmail.com> wrote:
>
> 1.
>
> func String2ByteSlice(s string) []byte {
> return (*[^uint(0) >> 1]byte)(unsafe.Pointer(&s))[:len(s):len(s)]
> }

This doesn't do what you probably want, as &s is not the address of
the bytes in the string.

 

Aha, I forgot this point.

> 2.
>
> func String2ByteSlice(s string) []byte {
> return *(*[]byte)(unsafe.Pointer(&struct{string; int}{s, len(s)}))
> }

This is more likely to work but it's not safe.

Why? For slice field order is undefined?

 

Ian

[Ian Lance Taylor]

Yes: the language does not define what a slice looks like in memory.
It may be implemented differently by different implementations,
including by different releases of the same implementation.

Ian