Comment 1:

You can certainly do unsafe things already to get at the string memory. If you must, do
that. I don't believe that we will support this more easily, because a C++ string is
mutable and a Go string is not.

Status changed to WorkingAsIntended.

Comment 2 by james@jamesh.id.au:

I'm not after a way to alter the Go string.  I've worked with enough language bindings
to know not to screw around with the runtime like that.
Rather, if I'm going to have to make a copy of the string to call my API, it seems
wasteful to create an intermediate copy with C.GoString rather than just using the
original Go string as source for the copy.
Similarly, for API calls that can deal with constant string data plus a length, it'd be
nice to be able to do so without a copy.
If there are existing ways to handle this kind of thing, would it be possible to
document them?

I think this is a reasonable thing to do. We should provide a way for the C code to get a pair of a const char * and length, to make it straightforward to pass a Go string to C code without a copy. Yes, they can break things horribly, but there are many ways to do that.

I agree this is reasonable, but it's too late in the cycle to do it now.

Change https://golang.org/cl/70890 mentions this issue: cmd/cgo: permit passing string and []byte directly between Go and C

We should provide a way for the C code to get a pair of a const char * and length, to make it straightforward to pass a Go string to C code without a copy.

It occurs to me that the combination of runtime.KeepAlive and reflect.StringHeader already makes that possible today (provided that one is willing to ignore the “cannot be used safely or portably” caveat of reflect.StringHeader):

func asPtrAndLength(s string) (*C.char, int) {
	addr := &s
	hdr := (*reflect.StringHeader)(unsafe.Pointer(addr))

	p := (*C.char)(unsafe.Pointer(hdr.Data))
	n := hdr.Len

	// reflect.StringHeader stores the Data field as a uintptr, not a pointer,
	// so ensure that the string remains reachable until the uintptr is converted.
	runtime.KeepAlive(addr)

	return p, n
}