On Tue, Jul 27, 2021 at 4:15 PM Steve Roth <st...@rothskeller.net> wrote:
> The implementation of io.WriteString appears to allocate a new byte slice > and copy the string into it: > > w.Write([]byte(s)) > > Only if the writer does not implement `io.StringWriter`. Avoiding this allocation where possible is exactly why `io.StringWriter` exists. > Many third party libraries avoid the allocation and copy with techniques > like: > > var b []byte > sh := (*reflect.StringHeader)(unsafe.Pointer(&s)) > bh := (*reflect.SliceHeader)(unsafe.Pointer(&b)) > bh.Data = sh.Data > bh.Len = sh.Len > bh.Cap = sh.Len > w.Write(b) > > I've seen so many different packages do this that it almost seems like a > preferred idiom. Yet, it doesn't seem to be guaranteed safe by the rules in > the "unsafe" package documentation; rule 6 comes close to allowing it but > doesn't quite get there. And the fact that the standard library doesn't > use it, in an obviously applicable place, is telling. > > So, what's the deal here? Is it safe or not? > No, that code is broken. It makes assumptions about the implementation of `Write`, which is *documented*, but not enforced by the compiler - namely, that `Write` may not retain a reference to the `[]byte` and may not modify its contents. If such an incorrect `io.Writer` is used with a library like this, it might break the program in strange and unforseen ways. Can I use it in my own code? > There are occasions where it is safe. For example, strings.Builder does a similar thing in a safe way. So, as with all unsafe: If you know it's safe, it's fine to use. Otherwise, stay away. Must I shun libraries that use it? (Must I read the source code of every > library I use, to see whether it uses it?) > Unfortunately there is no way to guarantee that a dependency contains good code. This particular issue should be reasonably easy to find by grepping for `unsafe`, which is a good practice if you want to avoid potentially unsafe code anyway. > > -- > You received this message because you are subscribed to the Google Groups > "golang-nuts" group. > To unsubscribe from this group and stop receiving emails from it, send an > email to golang-nuts+unsubscr...@googlegroups.com. > To view this discussion on the web visit > https://groups.google.com/d/msgid/golang-nuts/CAAnpqKHoA74DfM5765vUfrH4V6RpBxg1DTkfn3ScN6MhjQwRTQ%40mail.gmail.com > <https://groups.google.com/d/msgid/golang-nuts/CAAnpqKHoA74DfM5765vUfrH4V6RpBxg1DTkfn3ScN6MhjQwRTQ%40mail.gmail.com?utm_medium=email&utm_source=footer> > . > -- You received this message because you are subscribed to the Google Groups "golang-nuts" group. To unsubscribe from this group and stop receiving emails from it, send an email to golang-nuts+unsubscr...@googlegroups.com. To view this discussion on the web visit https://groups.google.com/d/msgid/golang-nuts/CAEkBMfEf6NbkAvDRs6QFO7LDggh-n-6mT3aBnG3gNh678Z6Z1g%40mail.gmail.com.
