passing function pointer to the C code using cgo

Question

Starting from Go v1.6 cgo changed the rules of passing pointers to the C code golang/go#12416. The example of invoking a dynamic Go callback from C code from the wiki doesn\'t

Answer 1

Starting from Go 1.6 cgo has new rules.

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

[source]

These rules are checked during the runtime, and if violated program crashes. At the moment it is possible to disable checks using GODEBUG=cgocheck=0 environment variable. But in the future, that might stop working.

So it is not possible anymore to pass a pointer to C code, if the memory to which it is pointing stores a Go function/method pointer. There are several ways to overcome this limitations, but I guess in most of them you should store a synchronized data structure which represents the correspondence between a certain id and the actual pointer. This way you can pass an id to the C code, not a pointer.

The code solving this problem might look like this:

package gocallback

import (
    "fmt"
    "sync"
)

/*
extern void go_callback_int(int foo, int p1);

// normally you will have to define function or variables
// in another separate C file to avoid the multiple definition
// errors, however, using "static inline" is a nice workaround
// for simple functions like this one.
static inline void CallMyFunction(int foo) {
    go_callback_int(foo, 5);
}
*/
import "C"

//export go_callback_int
func go_callback_int(foo C.int, p1 C.int) {
    fn := lookup(int(foo))
    fn(p1)
}

func MyCallback(x C.int) {
    fmt.Println("callback with", x)
}

func Example() {
    i := register(MyCallback)
    C.CallMyFunction(C.int(i))
    unregister(i)
}

var mu sync.Mutex
var index int
var fns = make(map[int]func(C.int))

func register(fn func(C.int)) int {
    mu.Lock()
    defer mu.Unlock()
    index++
    for fns[index] != nil {
        index++
    }
    fns[index] = fn
    return index
}

func lookup(i int) func(C.int) {
    mu.Lock()
    defer mu.Unlock()
    return fns[i]
}

func unregister(i int) {
    mu.Lock()
    defer mu.Unlock()
    delete(fns, i)
}

This code comes from the (updated) wiki page.