Go v1.6 发生更改后，如何使用 cgo 将函数指针传递给 Go 中的 C 代码？

<code class="go">package main

import (
    "fmt"
    "sync"
)

// Declare a function to be invoked by C code.
func MyCallback(x int) {
    fmt.Println("callback with", x)
}

func Example() {
    i := register(MyCallback) // Register the callback function and obtain its ID.
    C.CallMyFunction(C.int(i)) // Pass the ID to the C function.
    unregister(i) // Deregister the callback function.
}

// Data structures for registering and deregistering callbacks.
var mu sync.Mutex
var index int
var fns = make(map[int]func(int))

// Register a callback function and return its ID.
func register(fn func(int)) int {
    mu.Lock()
    defer mu.Unlock()
    index++
    for fns[index] != nil {
        index++
    }
    fns[index] = fn
    return index
}

// Return the callback function associated with a given ID.
func lookup(i int) func(int) {
    mu.Lock()
    defer mu.Unlock()
    return fns[i]
}

// Deregister a callback function using its ID.
func unregister(i int) {
    mu.Lock()
    defer mu.Unlock()
    delete(fns, i)
}

func main() {
    Example()
}</code>