How to Pass Function Pointers to C Code Using Cgo in Go 1.6 and Later?

Passing Function Pointer to C Code Using Cgo

Changes in Cgo Function Pointer Passing

In Go 1.6 and later, Cgo has stricter rules for passing pointers to C code. It is no longer allowed to pass a Go pointer that points to Go memory containing any Go pointers.

Code Example

The following Go code demonstrates how to pass a function pointer to C code:

<code class="go">import (
    "fmt"
    "unsafe"
)

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

   static inline void CallMyFunction(void* pfoo) {
       go_callback_int(pfoo, 5);
       }
*/
import "C"

//export go_callback_int
func go_callback_int(pfoo unsafe.Pointer, p1 C.int) {
    foo := *(*func(C.int))(pfoo)
    foo(p1)
}

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

// we store it in a global variable so that the garbage collector
// doesn't clean up the memory for any temporary variables created.
var MyCallbackFunc = MyCallback

func Example() {
    C.CallMyFunction(unsafe.Pointer(&MyCallbackFunc))
}

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

Error and Solution

When running this code, it produces an error because the Go memory pointed to by MyCallbackFunc contains a Go function pointer (MyCallback).

To address this, we need to find a way to pass the function pointer to C code without violating the new rules.

Using an ID Instead of a Pointer

One approach is to store the function pointer in a synchronized data structure and pass an ID to the C code instead of a direct pointer. This way, the C code can use the ID to access the function pointer through the data structure.

Code with ID-Based Function Pointer Passing

<code class="go">package gocallback

import (
    "fmt"
    "sync"
)

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

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)
}</code>

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