Calling a C function with a double pointer output parameter using CGo

Question

I am trying to figure out th right way to call this function:

size_t
fz_buffer_storage(fz_context *ctx, fz_buffer *buf, unsigned char **datap)
{
    if (datap)
        *datap = (buf ? buf->data : NULL);
    return (buf ? buf->len : 0);
}

using CGo to get the underlying string and its length as a byte array in Go.

Is this the right way to do it?

var bufferContents *C.uchar
length := C.fz_buffer_storage(ctx, buf, &bufferContents)
bytes := C.GoBytes(unsafe.Pointer(bufferContents), C.int(length))

Since the C code overwrites *datap, I am not sure if the garbage collector will still do the right thing.

I saw an answer here suggesting something along the lines of

var tempUcharPtr *C.uchar
bufferContents := C.malloc(C.size_t(unsafe.Sizeof(tempUcharPtr)))
defer C.free(bufferContents)
length := C.fz_buffer_storage(ctx, buf, (**C.uchar)(bufferContents))
bytes := C.GoBytes(unsafe.Pointer(*(**C.uchar)(bufferContents)), C.int(length))

which also seems to work, but it's much more convoluted and I'm wondering if it's better / safer than the previous version.

Answer 1

Apparently, the first version is fine. Quoting the docs:

Go code may pass a Go pointer to C provided the Go memory to which it points does not contain any Go pointers.

From what I understand, since var bufferContents *C.uchar will be initialised to nil, it does not count as a "Go pointer" for the above rule. The following simplified code examples confirm this:

package main

// void F(char **p) {}
import "C"

func main() {
    var p *C.char = new(C.char)
    C.F(&p)
}

will trigger "panic: runtime error: cgo argument has Go pointer to Go pointer"

package main

// void F(char **p) {}
import "C"

func main() {
    var p *C.char
    C.F(&p)
}

works just fine, even when setting GODEBUG=cgocheck=2.

Thanks to the folks on the #cgo channel on the Gophers Slack community for helping me understand this!