how to do callbacks in Go. implements a 'filter' function. modified from the web example.

cfuncs.go

package main
/*

#include <stdio.h>

// The gateway function
int callOnMeGo_cgo(int* in)
{
        return callOnMeGo(in);
}
*/
import "C"

clibrary.c

#include <stdio.h>

#include "clibrary.h"

int* some_c_func(callback_fcn callback, int* arr, size_t n)
{
  size_t i, j;
  int* rv;
  rv = calloc(n, sizeof(int));
  for(i=0, j=0; i < n; ++i) {
    if(callback(&arr[i])) {
      rv[j++] = arr[i];
    }
  }
  return rv;
}

clibrary.h

#ifndef CLIBRARY_H
#define CLIBRARY_H
#include <stdlib.h>
typedef int (*callback_fcn)(int*);
int* some_c_func(callback_fcn, int*, size_t);
#endif

goprog.go

package main

/*
#cgo CFLAGS: -I .
#cgo LDFLAGS: -L . -lclibrary

#include "clibrary.h"

int callOnMeGo_cgo(int in); // Forward declaration.
static inline void c_assign(int* x, int i) { x[i] = i; }
static inline int c_lookup(int* x, size_t idx) { return x[idx]; }
*/
import "C"

import (
        "fmt"
        "unsafe"
)

//export callOnMeGo
func callOnMeGo(in *C.int) int {
  fmt.Printf("Go.callOnMeGo(): called with arg = %d\n", (*in))
  if (*in) > 4 {
    return 1
  }
  return 0
}

func main() {
  fmt.Printf("Go.main(): calling C function with callback to us.\n")
  cb := (C.callback_fcn)(unsafe.Pointer(C.callOnMeGo_cgo))
  arr := (*C.int)(C.malloc(10*4))
  for i:=0; i < 10; i++ { C.c_assign(arr, (C.int)(i)) }
  lst := C.some_c_func(cb, arr, 10)
  for i:=0; i < 10; i++ {
    fmt.Printf("%d: %d\n", i, C.c_lookup(lst, (C.size_t)(i)))
  }
  C.free(unsafe.Pointer(arr))
}

makefile

all:
        gcc -Wall -Wextra -c clibrary.c
        ar cru libclibrary.a clibrary.o