pass c function pointer to go

I have a C++ program that uses some go code compiled to a c library through cgo. I'd like to provide some C delegates / function pointers to the cgo library so that the go code can invoke some code provided by my C++ program. I find many examples how to call go functions from within C++ but nothing vice versa.

My go package looks like this

package main

import (
    "fmt"
    "unsafe"
)

/*
    #include <stdlib.h>
    typedef char* (*Callback)(char* args);
*/
import "C"

//export RegisterCallback
func RegisterCallback(funcptr unsafe.Pointer) {
    cstr := C.CString("hello from go")
    defer C.free(unsafe.Pointer(cstr))

    foo := *(*func(*C.char) *C.char)(funcptr)
    result := foo(cstr)

    fmt.Println(C.GoString((*C.char)(result)))
}

func main() {
}

and I am calling it from C++ like this

char* foo(char* args)
{
    std::cout << args;
    return "hello from C++";
}

//...
RegisterCallback((void*) &foo);

This, however, crashes badly within go.