Go version

1.22.6

Output of go env in your module/workspace:

N/A

What did you do?

N/A

What did you see happen?

The documentation for cmd/cgo#Passing pointers states that strings and slices always contain Go pointers:

Note that values of some Go types, other than the type's zero value, always include Go pointers. This is true of string, slice, interface, channel, map, and function types.

This seems to imply that the following is UB, as it constructs a slice from a C pointer rather than a Go pointer:

//export processData
func processData(ptr unsafe.Pointer, len C.size_t) {
	s := unsafe.Slice((*byte)(ptr), len)
	fmt.Println("got data:", string(s))
}

However, discussion on the Gophers Slack suggests that this may actually be perfectly fine, as Go should still know whether a pointer points to memory managed by Go. syscall.Mmap was mentioned as an example of a slice not backed by Go-managed memory.

What did you expect to see?

Accurate information in the Cgo documentation regarding the safety of strings and slices backed by C-managed memory.

In either case, an explicit description of the safety or non-safety of passing a C pointer would be appreciated in the documentation for unsafe.String and unsafe.Slice.

As an aside, it would also be good to document any other invariants that must be upheld by the data passed to unsafe.String, if applicable. In particular, whether the string must be valid UTF-8, and whether it may contain null bytes.