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 > <https://golang.org/pkg/unsafe/#Pointer>, 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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