unsafe.Pointer to byte slice

[Santhosh Ram Manohar]

hello,

This piece of code prints [10 0 0 0] as expected..

func main() {

        i := 10

        p := unsafe.Pointer(&i)

        intPtr := (*[4]byte)(p)

        fmt.Println(*intPtr)

}

But if I convert the unsafe.Pointer to pointer to a byte slice rather than an array of 4 bytes it prints an empty slice, []

func main() {

        i := 10

        p := unsafe.Pointer(&i)

        intPtr := (*[]byte)(p)

        fmt.Println(*intPtr)

}

Can someone explain why its so ? thanks.

-Santhosh.

[Dave Cheney]

An array is a vector of values in memory. A slice is a small struct that describes an array stored elsewhere in memory.

https://blog.golang.org/go-slices-usage-and-internals

[Santhosh Ram Manohar]

Dave,

On Saturday, July 8, 2017 at 4:09:32 PM UTC-7, Dave Cheney wrote:

An array is a vector of values in memory. A slice is a small struct that describes an array stored elsewhere in memory.

I understand the slice vs array difference. But in this statement,

intPtr := (*[]byte)(p)

my interpretation was intPtr is a pointer to a slice of bytes and slice's underlying array reference is the 4 bytes of the integer, isn't it ?

 

https://blog.golang.org/go-slices-usage-and-internals

[Ian Lance Taylor]

On Sat, Jul 8, 2017 at 6:59 PM, Santhosh Ram Manohar
<santho...@gmail.com> wrote:
>
> On Saturday, July 8, 2017 at 4:09:32 PM UTC-7, Dave Cheney wrote:
>>
>> An array is a vector of values in memory. A slice is a small struct that
>> describes an array stored elsewhere in memory.
>
>
> I understand the slice vs array difference. But in this statement,
>
> intPtr := (*[]byte)(p)
>
> my interpretation was intPtr is a pointer to a slice of bytes and slice's
> underlying array reference is the 4 bytes of the integer, isn't it ?

A slice is not a backing array. A pointer to a slice is not a pointer
to the slice's backing array.

Ian

[Santhosh Ram Manohar]

So in this case, a pointer to 4 bytes of memory is being interpreted as pointer to a slice. When the slice is referenced, runtime sees its not actually a slice and hence treats it as if its a nil slice ?

thanks,

Santhosh.

 

Ian

[Dave Cheney]

As your using the unsafe package your doing things which are not considered safe by the lanaguge so only imprecise interpretations are possible. My interpretation is, assuming that next words after p in memory are zeroed the you have effectively a slice that points to a backing array at *p, and a length and capacity of zero.

If the words following p were not see, you'd have some fun memory corruption bugs to chase down.

[T L]

The current internal slice structure used in the official Go compiler is like:

// slice
struct {
	elements unsafe.Pointer
	len      int // number of elements
	cap      int // capacity
}

If happens that the len and cap fields are both 0 for your unsafe slice.

 

[Santhosh Ram Manohar]

On Sun, Jul 9, 2017 at 12:30 AM, Dave Cheney <da...@cheney.net> wrote:

As your using the unsafe package your doing things which are not considered safe by the lanaguge so only imprecise interpretations are possible. My interpretation is, assuming that next words after p in memory are zeroed the you have effectively a slice that points to a backing array at *p, and a length and capacity of zero.

ah, that makes sense. Thanks, Dave.

 

If the words following p were not see, you'd have some fun memory corruption bugs to chase down.

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