How to Pass Function Pointers to C Code Using cgo After Go v1.6?

Passing Function Pointers to C Code Using cgo

Prior to Go v1.6, cgo allowed the passing of Go pointers to C code. However, after the changes implemented in v1.6, the rules for passing pointers changed significantly. This alteration rendered the previously used method for invoking dynamic Go callbacks from C code ineffective.

Problem Description

The code sample, once a functional example of invoking a Go callback from C, now results in a runtime error:

panic: runtime error: cgo argument has Go pointer to Go pointer

Solution

Under the new cgo rules, Go code may pass a Go pointer to C only if the Go memory it points to does not contain any Go pointers. This restriction prevents passing pointers to C code if the pointed memory stores Go function/method pointers.

To overcome this limitation, several approaches can be considered. A common solution involves storing a synchronized data structure that establishes a correspondence between specific IDs and the actual pointers. By passing the ID to C code instead of the pointer, the restriction is bypassed.

An updated code sample showcasing this solution:

<code class="go">package gocallback

import (
    "fmt"
    "sync"
)

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

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)
}

//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.go_callback_int(C.int(i), 5)
    unregister(i)
}

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

This solution follows the updated wiki page, providing a robust approach to passing function pointers to C code within the current cgo constraints.

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