test.go

// C CODE
#include "_cgo_export.h"
void ACFunction() {
        printf("ACFunction()\n");
        AGoFunction();
} 

// GO Code

package gocallback

import "fmt"

/*
#include <stdio.h>
extern void ACFunction();
*/
import "C"

//export AGoFunction
func AGoFunction() {
        fmt.Println("AGoFunction()")
}

func Example() {
        C.ACFunction()
}