cgo sample code

add.c

#include "add.h"

int add_two_numbers(int x, int y) {
  return x + y;
}

int add_fzed_two_numbers(int x, int y) {
  return add_two_numbers(FuzzNumber(x),
			FuzzNumber(y));
}

struct hello return_person(void) {
  struct hello p;
  return p;
}

union quantity return_quantity(void) {
  union quantity q;
  q.weight = 3.14;
  return q;
}

add.h

/* adds two numbers */
int add_two_numbers(int, int);

/* export to go */
int add_fzed_two_numbers(int, int);

struct hello return_person(void);

union quantity return_quantity(void);

/* importing from go is also possible */
extern int FuzzNumber(int);

struct hello
{
  int age;
};

union quantity {
  float weight;
  int count;
};

cgotest.go

// +build linux,cgo darwin,cgo

package main

import (
	"fmt"
	"unsafe"
)

/*
#include "add.h"
*/
import "C"

//export FuzzNumber
func FuzzNumber(i int32) int32 {
	return i + 1
}

func main() {
	fmt.Printf("Return from c func: %d\n", C.add_fzed_two_numbers(2, 4))

	// func returning c struct
	p := C.return_person()
	p.age = 12

	// declare and use
	var param C.struct_hello
	param.age = 12
	fmt.Print("Struct repr: ")
	fmt.Println(param)

	// func returing union
	q := C.return_quantity()
	fmt.Print("Union repr: ")
	fmt.Println(q)
	ptr := (*float32)(unsafe.Pointer(&q))
	*ptr = 3.14

	// declare a union
	var uparam C.union_quantity
	uptr := (*int32)(unsafe.Pointer(&uparam))
	fmt.Print("pointer to union: ")
	fmt.Println(uptr)

	*uptr = 2
	fmt.Println(*uptr)

}