On Mon, Feb 27, 2017 at 11:57 AM, Bill Katz <[email protected]> wrote:
> I thought that's handled by the inhdr.Cap % 8 == 0 check. Isn't inhdr.Data
> essentially a pointer into memory with inhdr.Cap showing the size of the
> allocated region?
Yes, but that is not sufficient. Consider
b := make([]byte, 9)
u, err := byteToUint64(b[1:])
Here the len and cap of b are 8, but the data field will not be aligned.
I think your code is safe but there is a simpler way to do it:
if len(b) % 8 != 0 || cap(b) % 8 != 0 ||
uintptr(unsafe.Pointer(&b[0])) % 8 != 0 {
return errors.New("bad len or cap or alignment")
}
return (*[1<<27]uint64)(unsafe.Pointer(&b[0]))[:len(b)/8:cap(b)/8]
Ian
> On Monday, February 27, 2017 at 2:14:46 PM UTC-5, xingtao zhao wrote:
>>
>> I think you need to check if inhdr.Data is aligned with 8 bytes as well.
>>
>> On Sunday, February 26, 2017 at 2:54:00 PM UTC-8, Bill Katz wrote:
>>>
>>> Hi,
>>>
>>> We'd like to do a zero-copy conversion of []byte to []uint64 where the
>>> underlying memory for the byte slice is properly aligned for N uint64
>>> values. After looking at (6) in the unsafe package documentation, I'm
>>> wondering if the code below is valid because I only use reflect.SliceHeader
>>> as pointers, and it seems to fit pattern 6.
>>>
>>> https://play.golang.org/p/FxxKIK08pX
>>>
>>> package main
>>>
>>> import (
>>> "fmt"
>>> "reflect"
>>> "unsafe"
>>> )
>>>
>>> func byteToUint64(b []byte) (out []uint64, err error) {
>>> inhdr := *(*reflect.SliceHeader)(unsafe.Pointer(&b))
>>> if inhdr.Len % 8 != 0 {
>>> err = fmt.Errorf("cannot convert non-aligned []byte length (%d) to
>>> []uint64", inhdr.Len)
>>> return
>>> }
>>> if inhdr.Cap % 8 != 0 {
>>> err = fmt.Errorf("cannot convert non-aligned []byte capacity (%d) to
>>> []uint64", inhdr.Cap)
>>> return
>>> }
>>> outhdr := *(*reflect.SliceHeader)(unsafe.Pointer(&out))
>>> outhdr.Len = inhdr.Len / 8
>>> outhdr.Cap = inhdr.Cap / 8
>>> outhdr.Data = inhdr.Data
>>> out = *(*[]uint64)(unsafe.Pointer(&outhdr))
>>> return
>>> }
>>>
>>> func main() {
>>> b := make([]byte, 24)
>>> u, err := byteToUint64(b)
>>> if err != nil {
>>> fmt.Println(err)
>>> } else {
>>> fmt.Printf("b: %v\n", b)
>>> fmt.Printf("u: %v\n", u)
>>> fmt.Printf("len(u) = %d, cap(u) = %d\n", len(u), cap(u))
>>> u[0] = 15
>>> fmt.Printf("Set u[0] to 15\n")
>>> fmt.Printf("b: %v\n", b)
>>> fmt.Printf("u: %v\n", u)
>>> u[2] = 255
>>> fmt.Printf("Set u[2] to 255\n")
>>> fmt.Printf("b: %v\n", b)
>>> fmt.Printf("u: %v\n", u)
>>> }
>>> }
>>>
>>> Outputs:
>>>
>>> b: [0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0]
>>> u: [0 0 0]
>>> len(u) = 3, cap(u) = 3
>>> Set u[0] to 15
>>> b: [15 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0]
>>> u: [15 0 0]
>>> Set u[2] to 255
>>> b: [15 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 255 0 0 0 0 0 0 0]
>>> u: [15 0 255]
>>>
>>>
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