I'm trying to understand why my code in Go doesn't work the way I thought it would. When I execute this test, it fails:
func TestConversion(t *testing.T) {
type myType struct {
a uint8
value uint64
}
myVar1 := myType{a: 1, value: 12345}
var copyFrom []byte
copyFromHeader := (*reflect.SliceHeader)(unsafe.Pointer(©From))
copyFromHeader.Data = uintptr(unsafe.Pointer(&myVar1))
copyFromHeader.Cap = 9
copyFromHeader.Len = 9
copyTo := make([]byte, len(copyFrom))
for i := range copyFrom {
copyTo[i] = copyFrom[i]
}
myVar2 := (*myType)(unsafe.Pointer(©From[0]))
myVar3 := (*myType)(unsafe.Pointer(©To[0]))
if myVar2.value != myVar3.value {
t.Fatalf("Expected myVar3.value to be %d, but it is %d", myVar2.value, myVar3.value)
}
}
The output will be:
slab_test.go:67: Expected myVar3.value to be 12345, but it is 57
However, if I increase copyFromHeader.Data
by 1
before the copying of the data, then it all works fine. Like this:
copyFromHeader.Data = uintptr(unsafe.Pointer(&myVar1)) + 1
I don't understand why it seems to shift the underlying data by one byte.